The relationship between respiratory variation in the pulmonary arterial pressure tracing and intra-thoracic pressure changes: a pilot study.

Respiratory variation in the systemic arterial blood pressure tracing is predictive of the change in cardiac output following volume administration. However, significant inter-individual variability exists. Animal data suggest that systemic arterial respiratory variation loses its predictive ability in the setting of right ventricular failure. Human studies suggest that patients with elevated pulmonary artery pressures are relatively unresponsive to volume administration. The current respiratory variation paradigm neglects the haemodynamic contribution of the right ventricular system. The goal of this study was to determine whether respiratory variation in the pulmonary arterial system could be attributed to intra-thoracic pressure changes. Systemic (radial) and pulmonary arterial pressure waveforms as well as all ventilatory variables were extracted from a haemodynamic database. Using a fast Fourier transformation, the ratio of respiratory (0.15-0.5 Hz) to cardiac (0.67-3 Hz) power was calculated every minute for both arterial waveforms, yielding two 'spectral power ratios' (SPoR). SPoR was compared to all available ventilatory parameters using a linear regression technique. From 19 patients 3422 min of data was analysed. There were statistically significant (p < 0.05) but clinically insignificant correlations between SPoR(radial) and mean airway pressure (r = -0.0462) and minute ventilation (r = -0.130). All tested correlations between SPoR(pulmonary) and ventilatory parameters were statistically significant, with correlations ranging from 0.112-0.224 (median 0.194). The correlation between SPoR(pulmonary) and SPoR(radial) was stronger than the correlation between either measure of respiratory variation and any of the ventilatory parameters (r = 0.342, p < 0.001). Respiratory variation in the pulmonary arterial tracing does not appear to be closely related to ventilatory parameters and is more closely related to systemic arterial respiratory variation, a well-validated estimate of volume status. The observed respiratory variation in the pulmonary arterial pressure tracing may be related to cyclical changes in pre-load or after-load that occur with mechanical ventilation and may affect the predictive power of systemic arterial respiratory variation.