A seven-slide primer on ventilator physiology that cites its sources. The physics is right, the P-SILI slides argue both sides fairly, and the two viscoelasticity slides lean on a paper about something else.
| Term | Plain definition | Why it matters at the bedside |
|---|---|---|
| Time constant | Resistance × compliance: how fast a lung unit fills and empties. | Units with different time constants fill out of step with each other. |
| Pendelluft | Gas moving between lung regions within a breath, even with no flow at the airway opening. | With strong patient effort, gas shifts from non-dependent to dependent lung, stretching dependent regions more than the tidal volume suggests. Shown in injured pig lungs with EIT and dynamic CT (Yoshida 2013): matching that dependent inflation under paralysis took nearly three times the driving pressure. Invisible on the ventilator; visible on electrical impedance tomography. |
| P-SILI | Lung injury attributed to the patient's own vigorous inspiratory effort. Term proposed by Brochard, Slutsky and Pesenti in 2017, originally about spontaneously breathing, non-intubated patients with high respiratory drive. | The same concern extends to ventilated patients: a strong drive on pressure support can generate large transpulmonary pressures while the airway pressure looks reassuring. |
| Viscoelasticity | Lung tissue behaves partly like a spring (stores energy) and partly like a damper (dissipates it), so its stiffness depends on time and speed of inflation. | During an inspiratory hold, pressure falls from the first zero-flow value toward the plateau as tissue stress relaxes and gas redistributes, so hold length changes the plateau you read. Covered in Bates's Lung Mechanics, chapter 4 (tissue resistance and elastance). |
| Percolation | A threshold effect: once damaged regions connect across a network, the whole network's behavior changes abruptly. | Explains why lung function can stay near normal through early disease, then worsen quickly. The basis of slide 5's figure. |
Every PMID on the slides opened and compared with the claim it supports.
| Post says | Verdict | Note |
|---|---|---|
| Pendelluft first described by Otis et al. in the 1950s (PMID 13286206) | Verified | Otis AB et al., J Appl Physiol 1956, mechanical factors in distribution of ventilation. The source of the slow-versus-rapid rate figure. |
| Pendelluft and EIT, link to P-SILI (PMID 41920010) | Verified | Piraino T, Respir Care 2026 review: effort-related pendelluft is common in ventilated patients and may contribute to P-SILI, with real gaps in the outcome data. |
| P-SILI mechanism, lower vs higher PEEP figure (PMID 31815775) | Verified | Yoshida, Grieco, Brochard, Fujino, Curr Opin Crit Care 2020. Argues a higher PEEP strategy can make spontaneous effort safer in severe ARDS. |
| Arguments against P-SILI: largely animal and physiologic data; associations, not causation; NMB benefits don't prove the mechanism (PMID 31815775) | Verified | Fair summary of the uncertainty. |
| Viscoelasticity definition, stress relaxation, rate dependence (PMID 17575096) | Wrong source | Bates et al. 2007 is a percolation model of fibrosis and emphysema. The definitions are correct; they need a lung-mechanics source. |
| Stiff regions forming a connected network sharply raise elastance (PMID 17575096) | Verified | This is the paper's main finding, and the hexagonal network figure comes from it. |
| Viscoelasticity and ARDS: collagen–elastin changes, stress relaxation during holds, regional heterogeneity (PMID 17575096) | Wrong source | The cited paper does not study ARDS or inspiratory holds. The physiology is broadly right but unsourced as posted. |
Slide 4 mentions neuromuscular blockade. Here are the two trials behind that.
| Trial | Population | Result |
|---|---|---|
| ACURASYS (2010) | 340 patients, severe ARDS (P/F <150) within 48 h, cisatracurium vs placebo for 48 h | Adjusted 90-day mortality HR 0.68 (0.48–0.98). Crude 90-day mortality 31.6% vs 40.7%, P = 0.08, not significant unadjusted |
| ROSE (2019) | 1,006 patients, moderate-to-severe ARDS, high-PEEP strategy in both arms, lighter sedation targets in control | 90-day mortality 42.5% vs 42.8%; stopped for futility. Paralyzed patients were less physically active and had more adverse cardiovascular events |
The usual reading: when the control arm is lightly sedated on higher PEEP, routine early paralysis adds nothing, which fits Yoshida's argument that PEEP can make spontaneous effort safer. It also shows why slide 4's caution matters. Neither trial measured effort or pendelluft, so neither tests P-SILI directly. Measuring drive and effort at the bedside (airway occlusion pressure, P0.1, esophageal pressure) is the practical next step the slides point toward.
Slide 7's teaser, previewed.
Mechanical power is the energy delivered to the respiratory system per minute, combining tidal volume, driving pressure, PEEP, flow and rate in one number (Gattinoni et al., 2016). The slide's figure splits each breath into PEEP, elastic and resistive components. In Gattinoni's derivation, power rose with the square of tidal volume, driving pressure and flow, with rate to the power 1.4, and linearly with PEEP. The idea: no single setting injures the lung on its own, and rate is a multiplier people forget. The equation was validated against measured pressure-volume loops in 80 patients; no trial has yet established a safe threshold.