Critical Care Scenarios Brandon Oto, PA-C, FCCM and Bryan Boling, DNP, ACNP, FCCM

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).







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Takeaway lessons









* 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.







* 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.)







* 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.







* 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.







* 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.







* 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.







* 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,

We learn about the training, role, and benefits of hospital chaplains, with Christine V. Davies, MDiv, MSW, Director of Chaplaincy at Robert Wood Johnson University Hospital.







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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.







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Takeaway lessons









* 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.







* 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.







* 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.







* 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.







* 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?







* 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.







* 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.







* 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)







* 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.







* Motivational interviewing emphasizes taking control over the aspects of their life that can be controlled.







* Normalize and validate the difficulty of being in the hospital.

We explore the profession of respiratory therapy in the US, including their role and training and how to optimize our clinical relationships, with Keith Lamb (@kdlamb1), RRT, RRT-ACCS, FAARC, FCCM. Keith is an RT at the University of Virginia in Charlottesville, working clinically in neuro/surgical/trauma critical care, who has been active in research and has held a variety of leadership positions.







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Responsible self-directed learning occurs in a zone between comfort and novelty.







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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.







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Takeaway lessons









* 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).







* 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.







* 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.







* 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.







* 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.







* 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.







* A swivel connector on the ETT outlet adds about 1 cm H2O of resistance. An HME adds about 3 cm H2O.







* 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.







* 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.







* 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







* Quality check your ATC by watching the tracheal pressure—the vent will display this ...