Transdiaphragmatic twitch pressure: Effects of lung volume and chest wall shape

The effect of lung volume and thoracoabdominal shape on the transdiaphragmatic twitch pressure (Pdi(t)) amplitude was evaluated in six volunteers during airway occlusion. Twitch stimulation was applied through fine wire electrodes implanted near both phrenic nerves. Stimulations were tolerated with little discomfort and constant phrenic nerve responses were maintained for hours. At FRC the group mean Pdi(t) was 31.4 cm H₂O (range, 19 to 36 cm H₂O), and its coefficient of variation ranged between 2 and 5% in individual subjects. At 1 L above FRC, the Pdi(t) decreased a mean of 7.8 cm H₂O (range, 2.8 to 11.9 cm H₂O). This change was caused primarily by a decrease in esophageal pressure amplitude. The shape of the relaxed chest wall was altered by loading the rib cage with a force of 5 to 9 kg. Load and shape had little effect on Pdi(t) independently of lung volume. Our modified technique of phrenic nerve stimulation through small wire electrodes is ideally suited for longitudinal intervention studies in patients. We conclude that the variability of Pdi(t) with shape is small compared with its expected decrease with lung volume.