Sizing the lung of mechanically ventilated patients

Abstract

Introduction: This small observational study was motivated by our belief that scaling the tidal volume in mechanically ventilated patients to the size of the injured lung is safer and more 'physiologic' than scaling it to predicted body weight, i.e. its size before it was injured. We defined Total Lung Capacity (TLC) as the thoracic gas volume at an airway pressure of 40 cm H2O and tested if TLC could be inferred from the volume of gas that enters the lungs during a brief 'recruitment' maneuver.

Methods: Lung volume at relaxed end expiration (Vrel) as well as inspiratory capacity (IC), defined as the volume of gas that enters the lung during a 5 second inflation to 40 cm H2O, were measured in 14 patients with respiratory failure. TLC was defined as the sum of IC and Vrel. The dependence of IC and Vrel on body mass index (BMI), respiratory system elastance and plateau airway pressure was assessed.

Results: TLC was reduced to 59 ± 23% of that predicted. Vrel/TLC, which averaged 0.45 ± 0.11, was no different than the 0.47 ± 0.04 predicted during health in the supine posture. The greater than expected variability in observed Vrel/TLC was largely accounted for by BMI. Vrel and IC were correlated (r = 0.76). Taking BMI into account strengthened the correlation (r = 0.92).

Conclusions: We conclude that body mass is a powerful determinant of lung volume and plateau airway pressure. Effective lung size can be easily estimated from a recruitment maneuver derived inspiratory capacity measurement and body mass index.

Figure 1

Relationship between lung volume at relaxed end-expiration (Vrel) expressed as a fraction of total lung capacity (TLC) and body mass index (BMI). Open symbols identify measurements of patients 1 to 4, in whom Vrel was measured at zero end-expiratory pressure. Except for the outlier with a BMI of 71, in the expected population BMI range, Vrel/TLC declines by 1% TLC for each 1 kg/m² increase in BMI (r = -0.81).

Figure 2

Relationship between relaxation volume, lung volume at relaxed end-expiration (Vrel), and inspiratory capacity (IC). Open symbols identify measurements of patients 1 to 4, in whom Vrel was measured at zero end-expiratory pressure. The remaining Vrel measurements were made at a positive end-expiratory pressure of 5 cm H₂O.

Figure 3

Distribution of tidal volumes (VTs) expressed as a percentage of predicted total lung capacity (TLC) (left) and as a percentage of observed TLC (right). Open symbols identify measurements of patients 1 to 4, in whom lung volume at relaxed end-expiration was measured at zero end-expiratory pressure.