Pleural mechanics and the pathophysiology of air leaks

Abstract

Pneumothorax, with or without a pleural air leak, can be a challenging clinical problem.

Keywords: pleural injury; pneumothorax; transpulmonary pressure.

FIGURE 1.

In the spontaneously breathing patient, the difference between airway pressure and pleural pressure (transpulmonary pressure) determines air flow and lung volumes. Pleural pressure is estimated by measuring esophageal pressure (Pes) with an esophageal balloon in the distal third of the esophagus; airway pressure (Pa_w) and air flow are measured at the mouth with a pressure sensor and pneumotachometer. Transpulmonary pressure is the difference between P_Aw and P_Es. In the normal person, the transpulmonary pressure varies with inspiration and expiration; increasing transpulmonary pressure results in increased lung volumes and inspiratory air flow. The cursor shows the transition from inspiratory to expiration at end-inspiratory lung volume (see text).

FIGURE 2.

Pleural mechanics during spontaneous breathing. Spontaneous breathing under normal conditions (A), spontaneous breathing with a stable pneumothorax (B), and spontaneous breathing with an air leak controlled by a chest tube (C) are shown. Pleural pressures, normally negative (A1), rise with a stable pneumothorax (B1). Pleural pressures return to normal with a chest tube even in the setting of an ongoing air leak (C1). Transpulmonary pressures, reflecting the difference between mouth (0 cm H2O) and pleural pressures, mirror the pleural pressures (A2, B2, C2). Because of the elevated pleural pressures, absolute lung volumes are lower with an undrained pneumothorax, and tidal volumes might also be lower because of increase in lung resistance and elastance (B3, gray). In contrast, inspiratory tidal volumes are higher and expiratory tidal volumes are lower than normal in patients with an ongoing air leak (C3, gray). Tidal volumes do reach baseline because of the leak volume out of the chest tube. Similarly, tidal air flow with spontaneous breathing (A4) might be lower with a stable pneumothorax (B4), but higher (with an inspiratory bias) with an air leak controlled by a chest tube (C4).

FIGURE 3.

Pleural mechanics during mechanical ventilation. Mechanical ventilation without pneumothorax or air leak (A) is compared with mechanical ventilation with a stable pneumothorax (B) and mechanical ventilation with an air leak controlled by a chest tube (C). With pressure-cycled ventilation, the airway pressures will be similar in the 3 conditions. Pleural pressures will rise with a stable pneumothorax (B2). Pleural pressures return to normal with a chest tube despite an ongoing air leak (C2). Transpulmonary pressures, reflecting the difference between airway and pleural pressures, are significantly lower with a pneumothorax (B3). Because of the compromised pleural pressures, absolute lung volumes and tidal volumes are lower with an undrained pneumothorax (B4, gray). In contrast, inspiratory tidal volumes are higher than normal in patients with a controlled air leak and do not return to baseline because of leak volume loss (C4, gray). Similarly, inspiratory air flow is higher and expiratory airflow is lower with an air leak controlled by a chest tube (C5, gray).

FIGURE 4.

A, Pressure-volume (PV) curve of the lung. At low lung volumes (analogous to a deflated balloon; V_lo), the lung is noncompliant and significant changes in transpulmonary pressures are required to increase lung volumes. Atelectasis is often associated with this range of lung volumes. In the compliant portion of the curve (V_n), changes in transpulmonary pressure result in efficient changes in lung volume. Functional residual capacity (FRC) defines the lower end of this portion of the curve. At high lung volumes (analogous to an overinflated balloon; V_hi), changes in transpulmonary pressures result in less efficient changes in lung volumes. The maximum lung volume defines total lung capacity (TLC). B and C, Campbell diagram of pleural pressure versus lung volume illustrating the changes in breathing associated with a pneumothorax. The lung’s elastic PV curve (solid line) and the PV curve of the relaxed chest wall (dashed line) are shown. B, In the spontaneous breathing patient, the resting FRC is at the intersection of the lung and relaxed chest wall curves. The loop shows the pleural pressure-volume trace during a spontaneous breath (dark gray). C, With a pneumothorax, the relaxed chest wall PV curve shifts downward, reflecting the volume of the pneumothorax. After pneumothorax, the spontaneous breathing loop (light gray) reflects the greater change in pleural pressure needed for a given change in lung volume (due to increased pulmonary resistance and elastance at a lower FRC).