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. 2018 May;6(10):e13712.
doi: 10.14814/phy2.13712.

Pressure-decay Testing of Pleural Air Leaks in Intact Murine Lungs: Evidence for Peripheral Airway Regulation

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Free PMC article

Pressure-decay Testing of Pleural Air Leaks in Intact Murine Lungs: Evidence for Peripheral Airway Regulation

Andrew B Servais et al. Physiol Rep. .
Free PMC article

Abstract

The critical care management of pleural air leaks can be challenging in all patients, but particularly in patients on mechanical ventilation. To investigate the effect of central airway pressure and pleural pressure on pulmonary air leaks, we studied orotracheally intubated mice with pleural injuries. We used clinically relevant variables - namely, airway pressure and pleural pressure - to investigate flow through peripheral air leaks. The model studied the pleural injuries using a pressure-decay maneuver. The pressure-decay maneuver involved a 3 sec ramp to 30 cmH2 0 followed by a 3 sec breath hold. After pleural injury, the pressure-decay maneuver demonstrated a distinctive airway pressure time history. Peak inflation was followed by a rapid decrease to a lower plateau phase. The decay phase of the inflation maneuver was influenced by the injury area. The rate of pressure decline with multiple injuries (28 ± 8 cmH2 0/sec) was significantly greater than a single injury (12 ± 3 cmH2 O/sec) (P < 0.05). In contrast, the plateau phase pressure was independent of injury surface area, but dependent upon transpulmonary pressure. The mean plateau transpulmonary pressure was 18 ± 0.7 cm H2 O. Finally, analysis of the inflation ramp demonstrated that nearly all volume loss occurred at the end of inflation (P < 0.001). We conclude that the air flow through peripheral lung injuries was greatest at increased lung volumes and limited by peripheral airway closure. In addition to suggesting an intrinsic mechanism for limiting flow through peripheral air leaks, these findings suggest the utility of positive end-expiratory pressure and negative pleural pressure to maintain lung volumes in patients with pleural injuries.

Keywords: Air leak; injury; lung; ventilation.

Figures

Figure 1
Figure 1
Pressure‐volume loops in a mouse before and after pleural injury with air leak. (A) A standard pressure‐volume loop was performed by the Flexivent ventilator provided an assessment of the mechanical properties of the lung. (B) After injury with a 25 g needle, the pressure‐volume loop is distorted by the air leak.
Figure 2
Figure 2
Standard inflation maneuver in simulacrum and in vivo. A baseline maneuver was performed followed by an identical maneuver with air leak. (A) An intact polyvinylidene chloride balloon was used to demonstrate the inflation maneuver to 30 cmH20 followed by a standard pressure plateau. (B) The same polyvinylidene balloon with an air leak demonstrates exponential decay to 0 cmH20. (C) In vivo, the baseline tracheal airway pressures (solid line) demonstrated a similar ramp and plateau. The cumulative volume delivered by the ventilator is shown (gray dashed line). The modest plateau pressure decline reflected the expected stress‐relaxation; cardiac movement is also more obvious during the pressure plateau. (D) After pleural injury with a 25 g needle, the ramp pressures approached the 30 cmH20 peak pressure, but rapidly declined to a plateau pressure. The cumulative volume in the pleural injury is higher than in the uninjured tracing. Tracings from representative mice are shown.
Figure 3
Figure 3
Pressure decline of standard inflation maneuver after single and multiple pleural injuries. (A) Tracheal pressures during an inflation maneuver in baseline condition (solid line), after a single pleural injury (dotted line), and after 5 injuries (dashed line). Representative tracings are shown. In both injury conditions, a decline in pressures (1) is followed by a plateau phase (2). (B) The rate of decay (decay rate = (peak pressure−plateau pressure)/time) plotted as a function of the peak pressure. Single pleural injuries (open circles) are compared to 5 injuries (closed circles); each point represents one mouse with error bars reflecting the mean of three estimates of the rate of decay ± 1 SD.
Figure 4
Figure 4
Transpulmonary pressure plateau after single pleural injury. Tracheal pressure plateau of standard inflation maneuver in mice with variable “pleural” pressures; that is, body box pressures outside of the lung after chest wall removal. (A) Inflation maneuver in a mouse with no pleural injury and a pleural pressure of 0 cmH20. Plateau pressures are indistinguishable from baseline tracings. (B) A single pleural injury and a pleural pressure of −10 cmH20 demonstrated decreased peak and plateau pressures. Similarly, pleural pressures of −20 cmH20 (C), −30 cmH20 (D), and −40 cmH20 (E) demonstrated a progressive decrease in peak pressures and the plateau phase. Representative tracings of N = 3 mice are overlaid in each condition. (F) A summary of plateau transpulmonary pressure (PTP , dark gray), defined as tracheal pressure minus body box pressure, compared to plateau tracheal pressure (P aw, light gray). There was no significant difference between mean PTP in the 4 conditions (P > 0.05). In contrast, mean transpulmonary pressures were significantly different from P aw in each condition (P < 0.001). N = 3 mice; error bars reflect mean ± 1 SD.
Figure 5
Figure 5
Air leak volume associated with single and multiple pleural injuries. (A) Baseline inflation maneuver (no injury) demonstrating the ventilator‐delivered air flow (gray line) and corresponding tracheal pressures (black line). As expected, the delivered air volume preceded the rise in tracheal pressures. (B) Representative inflation maneuver after a pleural injury (single) demonstrated a second peak of air delivery near the tracheal pressure peak (arrow). (C) The total air volume delivered by the ventilator during the inflation maneuver in three conditions was summarized in three time intervals (N = 4 mice). In the initial 1 sec of the inflation maneuver, less volume was delivered in the multiple injury condition, possibly related to compliance changes after injury (asterisk, P < 0.05). The major difference was the increased volume delivered in both the single and multiple injury (injury × 5) conditions during the 3rd second of the maneuver – the difference between all three conditions was highly significant during this interval (asterisks, P < 0.001). (D) When negative pressure outside the lung was applied during the inflation maneuver, the fractional air volumes delivered by the ventilator relative to baseline (no leak) controls increased linearly with decreasing pressure (N = 4 mice; P < 0.01 for each condition). Linear trendline is shown (gray dashes) (R 2 = 0.9914). Error bars reflect mean ± 1 SD.

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