Episode 81: Bacterial meningitis with Casey Albin

We talk about diagnosis, treatment, and subsequent care of the patient with bacterial meningitis, with Emory neurointensivist Casey Albin, MD (@caseyalbin).

Learn more at the Intensive Care Academy!

Find us on Patreon here!

Buy your merch here!

Takeaway lessons

  1. Many septic patients have altered mental status, but suspicion should be raised for CNS infection when there is also: headache, photophobia, vomiting, or any possibility of seizure activity.
  2. Meningitis and encephalitis are separate entities usually involving different organisms, different imaging findings, and with different prognostic implications and downstream complications. However, at the early diagnostic stage, they can be largely lumped together.
  3. Empiric antimicrobials must consider CNS penetration. Piperacillin/tazobactam (ie Zosyn) has very little. Ceftriaxone is better. Cefepime is fine, although the prospect of cefepime neurotoxicity may make neurologists leery; ceftazidime is fine too. Add vancomycin (not necessarily for MRSA but for resistant Strep pneumo), acyclovir (for HSV), and a liberal approach to adding ampicillin for Listenia for anybody older, immunocompromised, or in the midst of an outbreak.
  4. Dexamethasone has been shown to reduce hearing loss after Strep pneumo meningitis. If suspicion for meningitis is strong early, it’s reasonable to give early (before or concurrent with antibiotics). It’s probably not worth giving >24 hours later.
  5. The main benefit of lumbar puncture is to allow stopping or narrowing antimicrobials without treating with the entire empiric cocktail for a full two weeks. (There is also the chance of identifying a resistance organism.)
  6. Ideally, LP is done before antimicrobials. However, if non-culture-based diagnostics are available such as PCR panels, successful diagnosis can often occur even after antibiotic administration. It’s worth doing the LP even if late and no PCR is available, as the signature of protein, glucose, etc will often still be useful. (At least, up front in a patient who might have CNS infection, avoid creating new obstacles like loading them with anticoagulation, antiplatelets, low molecular weight heparin, etc.)
  7. Most patients will already have a CT head performed before LP is considered, making the question of whether this is necessary (to assess risk of downward herniation) fairly moot. However, if not, it should probably be done prior to LP in anyone with an altered level of consciousness.
  8. Order from all CSF: Gram stain and culture, cell counts (first and last tubes), glucose, protein, and HSV PCR. (VZV generally does not cause clinical meningitis per se, usually causing a meningitis vasculitis, e.g. in someone with small-vessel strokes.) If available, order PCR arrays too, although some centers may not run it unless the CSF WBC count is elevated (e.g. >5). In a patient with any immunocompromise, test for cryptococcus as well. Other immunosuppressed testing is case-specific.
  9. Always measure opening pressure. This is not accurate in a patient sitting up. While technically possible to puncture a patient sitting up, then rotate them with assistance to lay flat, it’s not easy or elegant. In a sick patient, just do the LP laying down.
  10. Remember that opening pressure is measured at the bedside in centimeters of water, but should be converted to millimeters of mercury to be clinically applicable.
  11. Draw at least 20 cc of CSF in all cases. If opening pressure is high (and CT not concerning), fill the four tubes (~36 cc) and measure the closing pressure. Few patients are harmed by draining <40 cc. Draining >40-50 can create some risk for herniation or hemorrhage (eg small subdural hemorrhage) and should not be done thoughtlessly.
  12. Meaningfully elevated CSF protein should not just be “high,” but should exceed the patient’s age.
  13. Any meningitis patient with an altered mental status should at least have a spot EEG, and possibly long-term EEG depending on the findings.
  14. Any meningitis patient with a high opening pressure on LP, who is sufficiently obtunded to be intubated, should be considered for invasive ICP monitoring (e.g. EVD), if available. Otherwise, close monitoring for ICP crisis with neuro and pupil checks and serial CT scans.
  15. Treating high ICP in meningitis with EVD or lumbar drain is often appropriate.
  16. Any neurologic deterioration after antibiotics and other initial care is very likely either seizure or ICP crisis. These are fixable patients; diagnose and treat these complications aggressively.
  17. Transcranial doppler may be a useful non-invasive screen for elevated ICP, by revealing a high-resistance waveform (high pulsatility index) as ICP increases.

Resources

Episode 80: Implementing the A-F bundle with Kali Dayton

We discuss the practical barriers to implementing the A-F ICU liberation bundle, with Kali Dayton, ACNP-BC (@daytonicu), host of the Walking Home from the ICU podcast, and consultant to ICUs working on these issues.

Learn more at the Intensive Care Academy!

Find us on Patreon here!

Buy your merch here!

Episode 79: Transfusion reactions with Joe Chaffin

We discuss transfusion reactions, risks, and management, including infection, consent, TRALI, TACO, and hemolytic reactions—with Dr. Joe Chaffin (@bloodbankguy), the “Blood Bank Guy” and transfusion medicine specialist.

Learn more at the Intensive Care Academy!

Find us on Patreon here!

Buy your merch here!

Takeaway lessons

  1. The risk of transfusion-related infection (HIV, hepatitis B, and hepatitis C) is around 1 in 3 million.
  2. Acute hemolytic transfusion reactions (usually due to clerical errors or unit mix-ups) occur about 1 in every 75 or 76 thousand transfusions. Mortality is only one per million or so, however.
  3. Simple febrile transfusion reactions occur about 1/100-300 transfusions.
  4. Transfusion is always slightly immunosuppressing, perhaps increasing risk of post-op infection, cancer recurrence, etc. This effect is real, but small and not easily quantified.
  5. Urticarial reactions (hives) seem trivial to clinicians, but can be very frightening to patients, even causing them to refuse future transfusions.
  6. 80% of hemolytic reactions initially present with only fever, perhaps some chills. There is no way to differentiate from non-hemolytic febrile reaction at this stage. While the odds favor a non-hemolytic reaction, if you presume this and continue your transfusions, you are relying on luck, and you will eventually be wrong, which would be an indefensible medical error.
  7. Once a hemolytic reaction is obvious, you waited too long. The main determinant of mortality after hemolytic transfusion reaction is the volume of blood transfused.
  8. Typical workup for a febrile, possible hemolytic reaction is to confirm the labels and clerical match, then return the blood to the blood bank, where they will check patient blood for hemolysis, direct Coomb’s, and usually repeating the ABO/Rh testing. This can cause a delay in transfusion and maybe loss of the unit of blood; by typical regulations, once blood is removed from the blood bank or portable cooler, it must be transfused within 4 hours or wasted.
  9. The hallmark of ABO mismatch is severe intravascular hemolysis. Most other hemolytic reactions yield extravascular hemolysis, e.g. in the spleen. Cytokine storm will be be seen. Compared to the myoglobin released in rhabdomyolysis, the free hemoglobin released in intravascular hemolysis is not quite as nephrotoxic (the resulting AKI may be more related to shock than from direct toxicity).
  10. Hemolysis is only destructive to the transfused blood, so anemia per se generally does not develop. One exception can occur in sickle cell patients, where transfusion can induce a “hyperhemolysis” phenomenon where native red cells are also hemolyzed.
  11. Mortality from acute hemolytic reactions is fairly low in previously healthy patients. Patients already critically ill may not do as well.
  12. TRALI is mostly diagnosed by consensus criteria. “Definitive” TRALI (there is no longer a less definite category) is defined as:
    • No evidence of lung injury prior to transfusion
    • Onset within 6 hours after end of transfusion
    • P/F ratio <300 or SaO2 <92% on room air
    • Radiographic evidence of bilateral infiltrates with no evidence of left atrial dysfunction
  13. The challenge when hypoxia occurs after transfusion is usually to distinguish TRALI from TACO. The latter is mere volume overload; the former occurs when pre-existing inflammation primes neutrophils for activation in the lungs, whereupon factors in the transfused blood causes neutrophil activation as a second hit. The most common of these triggers is incompatible anti-HLA antibodies in the transfused blood.
  14. TRALI is largely a clinical diagnosis. However, if a case of possible TRALI is reported, the donor will be investigated and potentially screened for anti-HLA antibodies (something usually not done without a suspicious case). Other products from that donor will also be recalled from the bank. Report your possible TRALI cases!
  15. Now that female donors with previous pregnancies are excluded from donating plasma (without HLA screening), the old truism that plasma-rich products (e.g. FFP or platelets) are the highest risk for inducing TRALI is no longer true; the most common precipitant is PRBCs. Any product can induce TRALI, however, including HLA antibody-negative products.

Episode 78: Echoing the RV with Matt Siuba

We talk the nitty-gritty of assessing the right heart using echocardiography, with our friend Matt Siuba (@msiuba), intensivist at the Cleveland Clinic and master of zentensivism.

Learn more at the Intensive Care Academy!

Find us on Patreon here!

Buy your merch here!

Takeaway lessons

  1. RV echo starts with evaluating three things: size, squeeze, and septal kinetics.
    • Size should be <2/3 the LV
    • Squeeze can be assessed in a variety of ways
    • The septum should not be bowing into the LV.
  2. Dilation is an early and somewhat compensatory finding, and can be used as a screening test (the “D-dimer of RV dysfunction”). Septal changes are probably later and more of a sign of dysfunction (i.e. not compensatory).
  3. Evaluating the RV’s ejection fraction is impractical due to its complex shape (without 3D echo or cardiac MRI or other advanced tools). So methods like TAPSE that reduce it to its longitudinal function become a more practical surrogate.
  4. TAPSE is not an isolated marker of RV contractility, but a marker of the overall RV-PA unit. However, this is probably a feature, not a failure. You don’t really want to know how the RV is contracting in the abstract, but how it’s contracting in its current loading conditions. So TAPSE will vary by afterload and preload, but not artifactually—i.e. if the loading conditions change and TAPSE improves, then contractility is better in the current conditions.
  5. s’ is similar to TAPSE, and similarly limited (mainly evaluating longitudinal function). It assesses velocity, not movement, which theoretically may represent something different (maybe a better marker of function?), although that difference is not very well studied; some studies do suggest that s’ may be more sensitive to changes after adding an inotrope, but who knows if that means anything. The most common cause for a big discrepancy between TAPSE and s’ is probably technical error, not a clinical distinction.
  6. RVSP can be useful as a marker of afterload, but says nothing about the cause of RVSP—high left sided pressures vs high PVR—and also incorporates the RV function, so separating all this out can be difficult.
  7. TAPSE/PASP (or TAPSE/RVSP) ratio might be a somewhat more accurate marker of RV/PA coupling, but not really clear if it’s clinically better than using the TAPSE alone, which is already a fair marker of RV/PA coupling. By measuring more things, it also introduces more room for technical error (usually underestimating RVSP), such as the need to estimate the TV gradient and the CVP. More tricuspid regurgitation will also tend to reduce the ratio, without necessarily indicating better RV function.
  8. CVP estimates derived from the IVC are very unreliable in the critically ill. Many chronic PH patients have chronically distended IVCs regardless of their RAP. Using a transduced CVP is probably better. You can also just trend the TV gradient as a marker of its own and ignore the CVP component.
  9. Shortening of the PA acceleration time (PAAT or PVAT) is a useful marker of pulmonary afterload. Notching of the waveform usually indicates a very high afterload, much more likely to be caused by pulmonary factors than high left heart pressures.
  10. Fractional area change of the RV is another tool for approximating the LV “EF” which may work better in chronic dysfunction, where TAPSE may be misleadingly preserved. However, it requires a good view of the RV free wall, which is not always achievable.
  11. Strain measurement has not yet penetrated point-of-care ultrasound machines reliably, but use is increasing. While still load-dependent, strain measurement is not angle dependent, which may make it helpful for right heart assessment.
  12. In the less common clinical scenario of RV infarction/ischemia, most of the above still applies, yet the pulmonary afterload will not necessarily be elevated. In almost every other case, the problem driving RV failure is usually the afterload, hence reducing the afterload is usually the easiest treatment.
  13. A proposed algorithm:
    1. Look for RV dilation
    2. Assess contractility using TAPSE and/or s’ (or other methods like eyeball gestalt, fractional area change, etc)
    3. Assess afterload using PA acceleration time and notching
    4. Compare contractility and afterload in context with the clinical scenario to understand the right heart’s function and conditions, with the understanding that your marker of contractility also incorporates the afterload to some extent.
  14. Don’t forget that invasive monitoring, from CVP to a PA catheter, is always an option as well. CVP is “for free” and rarely wrong if you know how to interpret it, including the waveform, and in the sickest patients, a Swan can be quite helpful, particularly for monitoring; multiple advanced echo studies are not always possible or reliable, particularly with rotating shift staff.
  15. If you have a Swan, wedge it. Otherwise it’s just a cardiac output monitor. Some fancy newer devices also allow measuring PA and RV pressures separately, which is an evolving science.

Episode 77: Mastering APRV with Rory Spiegel

We discuss the practicalities of using airway pressure release ventilation (APRV) with Dr. Rory Spiegel (@EMnerd_), emergency physician and intensivist at MedStar Washington Hospital Center (and EMNerd at Emcrit).

Find us on Patreon here!

Buy your merch here!

Takeaway lessons

  1. The most immediate benefit of APRV is to help restore lungs to FRC (functional residual capacity). While this can be achieved with PEEP, most people don’t use enough PEEP. APRV proves a higher mean airway pressure while also reducing sedation requirements, and provides a physiologically automatic titration of “PEEP” based on lung compliance.
  2. Phigh can usually be set to equal the previous plateau pressure on a conventional mode (assuming reasonably appropriate settings there). This helps match higher Phigh to a more poorly compliant lung and vice versa. The release volume that results should be checked to give a sense of the effects; it should be more or less in the range of normal tidal volumes, although usually lower than your previous tidal volume on a conventional mode due to the intentional air trapping. (If it’s not lower, your Tlow may be set too long, allowing too much release.)
  3. Thigh can range from 1.5 seconds to infinity. Longer T-high is better for recruitment, shorter is better for ventilation. When initially flipping to APRV, shorter Thigh is usually needed; try to match the patient’s minute ventilation (from the prior mode) fairly closely, although usually you’ll need to accept a small loss of ventilation. A too-long initial T-high is a common error; patients this sick usually cannot tolerate more acidosis. Usually an initial T-high of 2 seconds is about right.
  4. Tlow should be set to terminate when the expiratory flow drops to 75% of the peak expiratory flow (so if the peak was 100 L/s, terminate Tlow when it drops to 75 L/s). This was about the point in pig models where alveolar derecruitment began to occur. Peak and end expiratory flow can be checked on most modern vents, although it may not be easy to find in the screens. Usually the right T-low is around 0.3–0.6 seconds.
  5. Occasional patients may need a shorter T-low than this for optimal recruitment. But few need longer; Tlow should rarely be lengthened, even as patients recruit. Although the amount of air trapping will usually increase as the lung becomes more compliant (e.g. the same T-low duration will terminate expiration at 85% instead of 75% of peak expiratory flow), this is usually fine; this is when you’ll start weaning and stretching your Thigh.
  6. Plow should be set to zero in almost all cases, allowing the fastest expiration (higher Plow reduces the driving pressure and substantially reduces expiratory flow). In a few vents (older Puritan Bennett, older Servos), the machine may attempt to synchronize with patient efforts by allowing the Tlow to “kick out” and extend, creating large release volumes and loss of desired air trapping. Increasing the Plow may provide some safety margin in this case, although switching from APRV altogether is probably the best solution.
  7. As the patient recruits on APRV, release volume should gradually increase despite a fixed Phigh, as the lung recruits. The expiratory flow curve will flatten and the compliance will increase. Thus, release volumes are initially small—”lung protective” in conventional thinking—and later will increase. This increase should be allowed, as it’s still associated with a normal/low driving pressure, since the “PEEP” gradually increases as trapping increases. A large release volume + low driving pressure is felt to be lung protective in APRV thinking.
  8. Driving pressure on ARPV can be checked on most vents by performing an expiratory (not inspiratory) hold to determine the effective “PEEP.”
  9. Patients can breathe spontaneously on APRV and be comfortable, but this is mostly determined by lung recruitment and how close they are to FRC. When the lungs are still tightly closed, spontaneously breathing will not be either comfortable or safe, so when initially flipped to APRV, patients should NOT be breathing; they will look uncomfortable, require very high minute ventilation, and generate high pressures. (There is also great discomfort here due to the hypercarbia usually unavoidably present.) Use a shorter Thigh and ventilate using the vent releases in this period, while using deep sedation and/or paralysis to suppress breathing.
  10. As patients stabilize and recruit, the minute ventilation needed to maintain pCO2 will drop as ventilation becomes more efficient. When MV and the CO2 approach normal physiologic ranges, sedation can be lightened and patients allowed to breathe. Ultimately, severe ARDS patients on APRV require less total sedation and need for paralysis than in other modes.
  11. Weaning occurs as thus: CO2 will gradually fall and release volumes naturally increase as the lungs recruit. Eventually they become hypocapnic, so Thigh must be increased to reduce the minute ventilation. As MV reaches normal, stretching the Thigh further causes hypercapnia, so patients should now be allowed to start breathing spontaneously to make up the difference in MV. Breathing should look comfortable, with a benign clinical appearance and gentle inspiratory flows (not sharp peaks); if not, recruitment may not yet be optimal and it may not be time for spontaneous breathing.
  12. Rory does not drop the Phigh during the weaning period, although many teach this; he finds it often causes derecruitment. He adjusts Phigh only in response to the perceived disease state; for example, it may need to be weaned as the disease improves and the initial Phigh may start to cause overdistention. He rarely touches it until the patient is ready for breathing. Once the patient is breathing spontaneously, this provides a good feedback tool to adjust Phigh; if you drop Phigh and spontaneous breathing looks worse (like a failed SBT – lower volumes, high rates), you derecruited them and Phigh should go back up. Spontaneous effort is a more sensitive and faster method of feedback than monitoring the release volumes alone.
  13. Permissive hypercarbia is okay. But severe hypercarbia before starting APRV is a marker of advanced underlying disease and lung injury which may make it difficult to tolerate APRV, and persistent hypercarbia on APRV is a marker of failure—the lung is not recruiting, and the mode is probably not totally safe as a result (persistent acidosis + persistently high driving pressures and risk of overdistention of ventilated lung).
  14. Hypotension is not necessarily a contraindication to APRV. Cardiac output is best when the lungs are at FRC, neither over- or under-distended. However, it’s true that the lungs “overdistend instantly, but recruit over time,” so until the lung recruits, intrathoracic pressure may be elevated, and delicate patients (eg hypovolemic) may not tolerate this well.
  15. Pneumothorax should not be a contraindication to APRV. The more recruited the lungs, the less strain on each individual lung unit given the same overall driving pressure.
  16. Using APRV is a skill that requires practice. However, it also helps create a general mindset of approaching the lung physiologically with the goal of restoring FRC, as well as appreciating the value of using minute ventilation as a marker of recruitment; these tools probably benefit patients even in other modes. With this approach, APRV can often be avoided, and used mainly as a rescue modality.

Episode 76: Rehabilitation psychology, with Megan Hosey

We discuss the field of rehabilitation psychology, and how it can help patients with persistent critical illness, with Dr. Megan Hosey (@DrMeganHoseyPhD), clinical psychologist and assistant professor at Johns Hopkins School of Medicine, where she practices in the medical ICU.

Find us on Patreon here!

Buy your merch here!

Takeaway lessons

  1. Rehabilitation psychology is a specialty of clinical psychology that generally partners with patients who have acute illness or injury, and helps them adapt to life in these new circumstances. They discuss health behaviors, values and priorities, help patients find paths back to what they love, and assess cognitive and behavioral changes that accompany new illness. In the ICU, they can assist with the psychological aspects of care, particularly in patients with a prolonged stay where psychological factors play an important role in recovery, or for treatment-refractory delirium.
  2. Delirium often dominates the patient experience of the ICU. This is primarily an experience of inattention, with relatively little awareness of their circumstances, the day, the context for events, and the presence of often-vivid hallucinations and delusions.
  3. ICU care is highly anxiety provoking, with common questions of “when,” “why,” and many other (often unanswerable) questions. The more certainty and structure you can provide, the better.
  4. Depression is common as well in longstanding inpatients, and is often better characterized as “hospital demoralization,” a fairly appropriate response to prolonged confinement and limited access to their regular life. This can lead to sensations of helplessness and hopelessness.
  5. Motivation can be improved by strategies to reduce the emotional barriers to engagement, while also strengthening their sense of meaning—i.e. what matters to them, and how will their involvement help move towards that?
  6. Effective psychological care relies on communication with the patient, and medical measures like tracheostomies and endotracheal tubes can be a barrier. Good care that minimizes sedation and delirium, close involvement from respiratory therapy and speech therapy (with tools like speaking valves), and non-verbal tools like speech boards, eye gaze, yes/nos, etc. are key.
  7. Patients with persistent/chronic critical illness appreciate having their schedule set out for the day, to give them a clear sense for what to expect and reduce anxiety.
  8. Try to build pleasurable activities into their day, aka “behavioral activation.” Doing things that are meaningful and pleasurable creates a positive feedback loop that enables more activity. Animal therapy, “sunshine therapy” (getting outside), music therapy (or just playing preferred music) are all valuable. Merely asking patients their preferred music and playing it can reduce anxiety and sedation requirement (see Linda Chlan’s work on this)
  9. Relaxation strategies can be learned, and in the ICU setting, vital sign monitoring can even be used as a form of biofeedback to appreciate changes in heart rate or respiratory rate in response to stress.
  10. Motivational interviewing emphasizes taking control over the aspects of their life that can be controlled.
  11. Normalize and validate the difficulty of being in the hospital. (“It is very common for people to feel frustrated, scared, or down in the hospital. This makes sense as you’re away from the people and things that you love, all while not feeling well. We can work together to find ways to help you feel like yourself.”)
  12. Create a schedule for the day to establish predictability and reduce anxiety. When possible, getting patient preferences (eg, morning rehab therapies, sitting up to chair for favorite tv show, evening wash up, recreate parts of bedtime routine from home, view church service remotely on Sundays via ipad, etc)
  13. Give patients choice over some activities (what time of day rehab therapies? Preferred positions for peri care? pick a length of time to do trach collar trial vs. go as long as you can?)
  14. Benzodiazepines as a treatment for anxiety in the hospitalized patient tend to be a short-term solution only, and may ultimately contribute to delirium. SSRIs or similar drugs might be a useful adjunct for patients who describe depressive symptoms like helplessness, hopelessness, worthlessness, guilt, etc.
  15. Gentle attempts at reorientation are appropriate for the delirious patient. In more agitated patients, emphatic and repetitive attempts at reorientation are usually not helpful. “Join the journey” (or as the improv comics say, “Yes, and…”) by redirecting benign delusions toward productive ends rather than disputing them. Floridly delirious patients generally cannot comprehend, even if they are able to parrot back information to please the interrogator.
  16. Consider implementing a “Get to Know Me” board; see Ognjen Gajic’s work

Resources

  1. The Conversation Project: good questions about end of life care wishes
  2. Vital Talk: quick guides on effective communication at end of life 
  3. American Psychological Association, Division for Rehabilitation psychology: for anyone who has interest about what a rehab psychologist is and what they do
  4. PADIS Guidelines
  5. ABCDEF Bundle

Episode 75: Automatic tube compensation, with Ben Fabry

We discuss the principles and application of automatic tube compensation (ATC) on modern ventilators, with its creator Ben Fabry. Dr. Fabry is a professor and chair of biophysics at University of Erlangen-Nuremberg, originally trained as an electrical engineer, who originally developed ATC as part of his PhD program.

Find us on Patreon here!

Buy your merch here!

Takeaway lessons

  1. ATC, originally called “electronic extubation,” is meant to normalize or eliminate the resistance to flow created by the endotracheal tube. Since this resistance is always present, yet is dynamic and varies by flow (and tube size), it creates a continuous confounding variable, making the displayed pressure on the ventilator a measurement not of tracheal pressure, but of another, largely meaningless pressure (the pressure outside the patient).
  2. ATC works by increasing airway pressure during spontaneous inspiration to eliminate the pressure gradient created by the tube at the current flow, and reducing it during expiration to reverse the effect.
  3. While ATC can be used in any mode, it is mostly meant for pressure support or other spontaneous modes. It has no real role in volume control. In pressure control, it has little meaningful impact during inspiration, although it will reduce the airway pressure below the set PEEP during expiration, which may help facilitate expiration.
  4. The original ATC test ventilator could drop pressure below atmospheric pressure during expiration, but this feature is not possible on modern ventilators, so the lowest possible pressure during ATC is zero (probably not quite even, that due to expiratory valve resistance). Some modern vents will not drop pressure during expiration at all.
  5. In principal, actual tracheal pressure could be measured by a separate monitoring lumen. In practice, this is dangerous, as the lumen could be occluded by mucus, so the resistance constant is instead applied mathematically. The modifiers were derived empirically by testing a variety of tubes at different flow rates.
  6. ATC will generally ask for the tube size. Length has some effect but a fairly trivial one, as resistance is mostly influenced by turbulence, which is mainly a product of diameter. Resistance is not a constant, but increases with (roughly) the square of the flow of gas.
  7. A swivel connector on the ETT outlet adds about 1 cm H2O of resistance. An HME adds about 3 cm H2O.
  8. Changes in gas composition at different FiO2 changes resistance trivially, although a mix like Heliox would change it significantly, and would make the internal calculations incorrect.
  9. No fixed single pressure support value can accurately match tube resistance, due to its dynamic nature during and between breaths, even if you were willing to set the sort of pressure needed—which might be 50+ cm H2O in a strongly breathing patient.
  10. The main downside of ATC is that modern ventilators don’t do it very well—they can only vary flow so quickly, so when there are brisk changes in pressure, they fail to match it. They usually can match only about 50% of tube resistance, with the worst at the start of a breath as they lag behind the initial drop in pressure. (You can appreciate this by seeing the airway pressure drop below the set PEEP.) Response is even less in some of the current generation of vents with radial blowers and slower valves
  11. Quality check your ATC by watching the tracheal pressure—the vent will display this as a second pressure tracing. It should remain positive above the set PEEP for the whole breath. (The airway pressure should also remain positive, but the tracheal pressure will be a more sensitive marker.) If it becomes negative or drops to the PEEP at any point during inspiration, it implies a failure to fully support the patient. This problem can occur during vigorous spontaneous breathing in any mode, since it’s driven mostly by slow ventilator response; it is just compounded by ATC, because not only does flow have to vary to meet a set pressure, but the set pressure is varying during the breath too. The best way to solve this problem is probably better machines, with faster valves, and maybe situating them closer to the patient (shorter circuits) to speed up responsiveness.
  12. When ATC fails to fully support, you can try adding PSV with a very fast rise time, but this is a bludgeon; the best mode is probably flow-proportional assist ventilation with ATC. PAV allows controlling not just tracheal pressure to a constant value, but alveolar pressure; it will further boost the pressure to try and maintain constant alveolar pressure during inspiration. Increase proportional assist until the tracheal pressure no longer drops below the PEEP. (You could achieve a similar effect by “lying” to your ATC and setting a smaller tube size, which will also increase the initial amount of support; but this is less precise and maybe less safe.) PAV plus ATC may be the ideal mode for patients spontaneously breathing without severe lung disease.
  13. PAV could be used, by the way, in a patient with bronchospasm (or similar increase in airway resistance from physiologic factors), as it allows you to compensate for any amount of airway resistance you wish. However, it will only do this during inspiration, and will not drop pressure below the PEEP during expiration (i.e. to reduce autoPEEP and aid exhalation). This probably wouldn’t help them exhale anyway, since the resistance is in the lungs, not in the vent.
  14. PSV in a weaker patient tends to mask some of their true spontaneous breathing patterns. ATC may unmask these, showing some odd patterns, such as very shallow or rapid breaths. This is not dyssynchrony; it is the true physiologic pattern that was merely being hidden in a less supportive mode. It is also not tiring, at least if the ATC is working correctly and you’re offering adequate support, because they are not experiencing resistance to breathing.
  15. If present, a Cheyne Stokes pattern of breathing may also become amplified by the “signal boosting” properties of ATC, rather than being damped by inadequate support. If this results in phenomena like very long apneic pauses, it may (perhaps) be a problem.

Episode 74: Obstructive shock, with Sarah Lorenzini (Nurses’ Podcrawl 2024)

Our collaboration with Sarah Lorenzini of the Rapid Response RN podcast, discussing a case and general principles for diagnosing and managing obstructive shock. Check out the other episodes on shock in the Nurses’ Podcrawl 2024!

Find us on Patreon here!

Buy your merch here!

Episode 73: POCUS for nephrology, with Abhilash Koratala

We discuss the role of point-of-care ultrasound in evaluating the patient with kidney injury and assessing volume status, with Abhilash Koratala (@nephroP), nephrologist, Director of Clinical Imaging for Nephrology at the Medical College of Wisconsin, and champion of nephrology-focused ultrasound.

Find us on Patreon here!

Buy your merch here!

Takeaway lessons

  1. A quick kidney and bladder ultrasound to rule out urinary obstruction is appropriate for most significant AKIs, maybe even if it was done previously (as obstruction can develop at any time).
  2. Ultrasound of the lungs and IVC help establish the presence of elevated filling pressures; if present, the VEXUS scan can be performed to establish the presence of venous congestion that might be contributing to kidney injury.
  3. Pulmonary edema as evidenced by B-lines establishes that the patient is not fluid tolerant, and suggests that further volume loading may be harmful. It increases the chance that AKI is due to congestive nephropathy as well, although each can also occur in isolation (and of course AKI can be a cause, leading to volume retention and then pulmonary edema).
  4. Abhilash does an 8-zone lung exam (2 anterior and 2 lateral zones on each side), which is plenty for cardiogenic pulmonary edema. He does not really count B-lines; if he sees B-lines in more than one dependent zone, he takes it as evidence the patient could be decongested.
  5. IVC is a reasonable method of estimating RAP; it is not reliable to gauge fluid responsiveness or other questions. The internal jugular vein is a good fallback if the IVC is untenable or seems unreliable, such as if bandages limit access, or the presence of cirrhosis (which alters local vasculature in unpredictable ways). Look for the highest point of distention and measure roughly from the sternal angle, adding it to the right atrial depth to approximate the CVP (usually ~5 cm although this is not very reliable).
  6. A non-plethoric IVC and absence of B-lines suggests a fluid tolerant patient. He uses the ACE guidelines of IVC >2.1 and <50% collapse with deep inspiration (sniff) to equate RAP ~15 mmHg.
  7. In the presence of elevated RAP, VEXUS helps determine whether that change is likely to be affecting organ perfusion by altering flow characteristics. Higher VEXUS scores are well-associated with risk of AKI.
  8. High RAP with a low VEXUS suggests that congestive nephropathy is not actively worsening renal function, whereas a higher VEXUS suggests the opposite. Serial VEXUS scans help track the progress of decongestion to dial in a patient to an optimal fluid balance.
  9. VEXUS is a right-sided heart parameter, so the state of the left heart’s filling may differ somewhat (e.g. as evidenced by lung markers like pulmonary edema—so track your B-lines too!). It is probably more precise and reliable than other markers like peripheral edema.
  10. Right and left heart filling should generally be well-linked. Venous congestion and elevated RAP usually indicate a well-filled LA as well, unless the lungs are acting as a significant resistor. If major PH is present, consider introducing measures like pulmonary vasodilators instead of further fluid loading; overdistending the RV will not help the LV.
  11. Although portal vein pulsatility can usually move towards normal after optimal decongestion, hepatic vein waveforms may remain abnormal in some patients with TR, PH, etc. Flow chanegs in intrarenal vessels often lag behind other vessels, as renal edema takes time to resolve.
  12. Hepatic vein waveform may be permanently blunted in cirrhotics, confounding it somewhat. Distinguishing the systolic and diastolic waves can also be hard to identify without ECG synching; ECG is highly recommended when available.
  13. Portal vein is also probably unreliable in cirrhosis, due to portal hypertension and AVMs; it may be permanently pulsatile in some (although loss of pulsatility is still associated with decongestion). Since many cirrhotics have recurrent hospitalizations, you can compare against prior scans.
  14. Intrarenal vessels are technically difficult, especially when patients cannot perform a breath hold; critically ill patients have a failure rate here >20%. It is easier in stable patients. However, CKD patients may have abnormal waveforms at baseline.
  15. Overall, there should not be any disease state that falsely confounds ALL of the VEXUS vessels; while states like mechanical ventilation can increase flow changes and point to congestion, this is real congestion, not an artifact.
  16. Invasive monitoring like a CVP or PA catheter replaces some of the function of the VEXUS scan, but VEXUS helps determine the degree of organ impact at the numbers reported by these devices. High filling pressures generally are associated with congestion and low pressures are associated with its absence, but VEXUS is often helpful in the gray area. Non-invasive measurements of filling using echo, such as RVSP or E/A and E/E’ may have a supplemental role as well; the latter may help distinguish cardiogenic from non-cardiogenic pulmonary edema, but do not tell much of a story about systemic congestion.
  17. Femoral vein doppler could be a supplement to VEXUS, mainly when you cannot get or cannot trust one of the other vessels. It is farther from the heart, so may be less sensitive to changes in RAP. A normal femoral vein (continuous or mildly pulsatile) should probably not rule out venous congestion, but an abnormal femoral vein is very suggestive of it. This can sometimes be noted on routine DVT scans.
  18. Abhilash runs a cardiorenal clinic where he finds outpatient VEXUS very useful to establish and monitor volume status. It is more difficult to use in a dialysis clinic due to lack of privacy and high patient volumes; quick lung ultrasound (8 or even 4 zone) might be more useful here.

References

Episode 4 with Philippe Rola on the VEXUS scan

NephroPOCUS

POCUS in AKI: Transcending boundaries: Unleashing the potential of multi-organ point-of-care ultrasound in acute kidney injury. Batool A, Chaudhry S, Koratala A. World J Nephrol 2023; 12(4): 93-103 [PMID: 37766842 DOI: 10.5527/wjn.v12.i4.93]

VEXUS for nephrologists: Koratala A, Reisinger N. Venous Excess Doppler Ultrasound for the Nephrologist: Pearls and Pitfalls. Kidney Med. 2022 May 19;4(7):100482. doi: 10.1016/j.xkme.2022.100482. PMID: 35707749; PMCID: PMC9190062.

Episode 72: CPR-induced consciousness with Jack Howard

Find us on Patreon here!

Buy your merch here!

We discuss the phenomenon of CPR-induced consciousness (i.e. patients demonstrating awakeness during resuscitation) with Jack Howard, Intensive Care Paramedic at Ambulance Victoria in the northern suburbs of Melbourne, Australia, and first author on a recent literature review and Delphi-derived expert guideline on CPRIC management.

Takeaway lessons

  1. Data is light, but perhaps 1% of cardiac arrests have some form of consciousness witnessed.
  2. It is primarily a problem because of the potential to delay or interfere with care (either due to the emotional confrontation and surprise, or from actual physical interference with medical care). However, there are also ethical questions about patient suffering.
  3. The first response in many people seeing CPRIC will be to stop CPR and assume they’ve made a mistake about loss of pulses.
  4. CPRIC is associated with better outcomes, probably as a marker of better neurologic perfusion before and/or during arrest.
  5. There was general agreement by the panel that ketamine should be used as first-line for CPRIC. If unavailable or if it fails, the group was unable to agree on a best second line; fentanyl, midazolam, or a paralytic are all options. In CPRIC that physically interferes with care, larger doses are appropriate.
  6. Paralytics as a first line (without sedation) are never recommended.
  7. There is minimal data on the effect on outcomes when CPRIC is treated. One small Ambulance Victoria study had a trend towards lower rates of ROSC when sedation was used.
  8. Speak to patients as though they can hear and understand you.
  9. It is not clear but very possible that a larger number of patients than those who demonstrate external awareness may have a degree of subclinical consciousness; interviews of survivors and EEG analysis has supported this.
  10. Many CPRIC patients will have ROSC, but if they don’t, they are probably excellent candidates for ECPR/ECMO or other rescue interventions. A minimum of 45 minutes of resuscitation should be offered.

References