Ventilatory changes following head-up tilt and standing in healthy subjects

Passive tilting increases ventilation in healthy subjects; however, controversy surrounds the proposed mechanism. This study is aimed to evaluate the possible mechanism for changes to ventilation following passive head-up tilt (HUT) and active standing by comparison of a range of ventilatory, metabolic and mechanical parameters. Ventilatory parameters (VT, VE, VE/VO2, VE/VCO2, f and PetCO2), functional residual capacity (FRC), respiratory mechanics with impulse oscillometry; oxygen consumption (VO2) and carbon dioxide production (VCO2) were measured in 20 healthy male subjects whilst supine, following HUT to 70° and unsupported standing. Data were analysed using a linear mixed model. HUT to 70° from supine increased minute ventilation (VE) (P<0.001), tidal volume (VT) (P=0.001), ventilatory equivalent for O2 (VE/VO2) (P=0.020) and the ventilatory equivalent for CO2 (VE/VCO2) (P<0.001) with no change in f (P=0.488). HUT also increased FRC (P<0.001) and respiratory system reactance (X5Hz) (P<0.001) with reduced respiratory system resistance (R5Hz) (P=0.004) and end-tidal carbon dioxide (PetCO2) (P<0.001) compared to supine. Standing increased VE (P<0.001), VT (P<0.001) and VE/VCO2 (P=0.020) with no change in respiratory rate (f) (P=0.065), VE/VO2 (P=0.543). Similar changes in FRC (P<0.001), R5Hz (P=0.013), X5Hz (P<0.001) and PetCO2 (P<0.001) compared to HUT were found. In contrast to HUT, standing increased VO2 (P=0.002) and VCO2 (P=0.048). The greater increase in VE in standing compared to HUT appears to be related to increased VO2 and VCO2 associated with increased muscle activity in the unsupported standing position. This has implications for exercise prescription and rehabilitation of critically ill patients who have reduced cardiovascular and respiratory reserve.

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