Effects of body position on the ventilatory response to hypercapnia

Abstract

Effect of posture on the hypercapnic ventilatory response was studied on the anaesthetized rats by using rebreathing techniques in the supine and head-down positions. There were no statistically significant alterations in tidal volume, frequency, minute ventilation, and P(ET)CO(2)between the head-down and supine positions during breathing at rest. However, the esophageal pressure inspiratory swings were significantly greater in the head-down compared with supine position. Moreover, we found that body position did not affect the hypercapnic ventilatory response, but did affect the relationship between inspiratory driving pressure and the increase of end tidal PCO(2). Greater inspiratory pressure is required to maintain the same level of the ventilatory response to hypercapnia in the horizontal position with the head-down. We believe that the discrepancy between postural alterations in the hypercapnic ventilatory and pressure responses is presumably a result of decreased lung compliance and increased airflow impedance of respiratory system in the head-down position.

Figure 1

Regression curves describing the relationship between minute ventilation (A), tidal volume (B), respiratory frequency (C), and esophageal pressure (D), on one side, and progressive hypercapnia in the supine (solid lines) and head-down-tilt (dashed lines) positions, on the other side.

Figure 2

Slope of the ventilatory (A) and esophageal pressure (B) responses to hypercapnia at P_ETCO₂ of 65 mmHg in both supine and head-down tilt positions. (*) Significant difference between the slopes in supine and head down-tilt (HDT) positions (P < 0.05).