Describe the use of esophageal pressure to estimate pleural pressure during mechanical ventilation.
Describe the use of esophageal manometry to adjust mechanical ventilator settings.
Discuss the use of intra-abdominal pressure measurements during mechanical ventilation.
Explain how lung volume can be measured during mechanical ventilation.
Discuss how ultrasonography and electrical impedance tomography (EIT) can be used to assess lung function during mechanical ventilation.
In some mechanically ventilated patients, assessing pleural pressure and abdominal pressure is useful. In addition, a number of bedside imaging techniques have become available. This chapter covers esophageal manometry, intra-abdominal pressure assessment, measurement of end-expiratory lung volume (EELV), chest ultrasonography, and electrical impedance tomography (EIT).
Esophageal pressure is a surrogate for pleural pressure. In critically ill patients in a supine position, the absolute value of the esophageal pressure estimates the true pleural pressure due to the weight of the mediastinal viscera, the body weight of the patient, and the superimposed pressure of a diseased lung. However, a properly placed esophageal balloon will accurately reflect changes in pleural pressure regardless of patient position. The esophageal balloon is placed in the lower third of the esophagus and inflated with a small amount of air (usually 0.5-1 mL, or more based on the design of the balloon).
In the spontaneously breathing patient, proper placement can be evaluated using the Baydur maneuver, in which airway and esophageal pressures are evaluated during airway occlusion. In intubated patients, this maneuver is performed by occluding the airway (end-expiratory pause) and pushing on the chest or abdomen. If the balloon is properly placed, equal changes will be noted for esophageal and airway pressure during airway occlusion. Proper position of the esophageal catheter can also be assessed by the observation of cardiac oscillations on the esophageal pressure waveform.
Transmission of Pressure to the Pleural Space
Esophageal pressure measurements can be used to estimate the amount of airway pressure transmitted to the pleural space during passive positive pressure ventilation. The pleural pressure (Ppl) change produced during passive inflation depends on tidal volume and chest wall compliance (Figure 33-1). Thus, chest wall compliance can be calculated as the tidal volume divided by the change in esophageal pressure.
Use of esophageal pressure to calculate chest wall compliance.
Pleural pressure is determined by airway pressure, compliance of the lungs, and compliance of the chest wall. It can be calculated from airway pressure (Paw), respiratory system compliance (CRS), and chest wall compliance (CCW):
Thus, pleural pressure at end-inspiration is: