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. 2022 Oct 12;3(4):610-624.
doi: 10.34197/ats-scholar.2022-0024HT. eCollection 2022 Dec.

How I Teach Auto-PEEP: Applying the Physiology of Expiration

Affiliations

How I Teach Auto-PEEP: Applying the Physiology of Expiration

Michael Keller et al. ATS Sch. .

Abstract

Teaching complex topics in mechanical ventilation can prove challenging for clinical educators, both at the bedside and in the classroom setting. Some of these topics, such as the topic of auto-positive end-expiratory pressure (auto-PEEP), consist of complicated physiological principles that can be difficult to convey in an organized and intuitive manner. In this entry of "How I Teach," we provide an approach to teaching the concept of auto-PEEP to senior residents and fellows working in the intensive care unit. We offer a framework for educators to effectively present the concepts of auto-PEEP to learners, either at the bedside or in the classroom setting, by summarizing key concepts and including concrete examples of the educational techniques we use. This framework includes specific content we emphasize, how to present this content using a variety of educational resources, assessing learner understanding, and how to modify the topic on the basis of location, time, or resource constraints.

Keywords: How I Teach; auto-PEEP; mechanical ventilation; medical education; physiology.

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Figures

Figure 1.
Figure 1.
Illustration of the pressure–time, flow–time, and volume–time scalars with a focus on expiration. We emphasize to the learner that, at the onset of expiration, the pressure gradient will be largest and expiratory flow will be at its peak value. As air flows out of the alveoli, alveolar volume and pressure (and therefore flow) will gradually decline in an exponential fashion until alveolar pressure equals the downstream pressure (set PEEP), at which point the flow will cease. The dotted line represents alveolar pressure throughout the respiratory cycle, which is only measurable when the flow is paused. PEEP = positive end-expiratory pressure.
Figure 2.
Figure 2.
Illustration depicting how auto-PEEP may cause ineffective triggering. In the figure on the left, the trigger threshold is set at 2 cm H2O below PEEPset. Because the PEEPset is 5 cm H2O, this patient would have to lower alveolar pressure to 3 cm H2O to trigger a breath. Because this patient has developed 7 cm H2O of auto-PEEP above the PEEPset of 5 cm H2O (i.e., PEEPtotal of 12 cm H2O), the patient would have to generate at least −9 cm H2O to lower the alveolar pressure to 3 cm H20. In the figure on the right, the PEEPset has been raised to 10 cm H20 for the same patient. The trigger threshold is still set to 2 cm H2O below the PEEPset of 10 cm H20 (i.e., 8 cm H2O). Now the patient would only have to generate negative 4 cm H2O (i.e., from 12 to 8 cm H2O) to lower the alveolar pressure below the trigger threshold. auto-PEEP = auto-positive end-expiratory pressure; PEEPset = PEEP set on ventilator; PEEPtotal = Total PEEP.
Figure 3.
Figure 3.
(A) Illustration of the decline in tidal volume over time during expiration. The period of time required for volume to decrease to 37% of its initial value is equivalent to one time constant. Each additional time constant represents another 63% reduction in volume from the previous value. We emphasize that, clinically, a patient should have an exhalation time greater than at least three time constants to adequately exhale the tidal volume, as indicated by the arrow, representing a reduction to less than 5% of the initial tidal volume. (B) Illustration of two patients with different time constants (tau). We emphasize to the learner how the patient with the longer tau will require a longer expiratory time to exhale a given volume.
Figure 4.
Figure 4.
Signs of auto-PEEP. (A) Asymmetry of areas under the curve. The area under the inspiratory flow–time scalar appears larger than the expiratory flow–time scalar as a result of significant asymmetry of the curves. (B) Persistent end-expiratory flow, signifying a persistent pressure gradient between alveolar pressure and PEEPset because of auto-PEEP. (C) The end-expiratory hold maneuver. The dotted line represents alveolar pressure throughout the respiratory cycle (which is only able to be measured when the flow is paused). (D) Ineffective triggering. In this example, the patient is unable to lower alveolar pressure below the threshold for triggering because of significant auto-PEEP. auto-PEEP = auto-positive end-expiratory pressure; PEEPset = PEEP set on ventilator.

Comment in

  • doi: 10.34197/ats-scholar.2022-0122ED

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