Interpretation of radial pulse contour during fentanyl/nitrous oxide anesthesia and mechanical ventilation

Background: Peripheral arterial blood pressure is not a reliable substitute for proximal aortic pressure. Recognition of this phenomenon is important for correct appreciation of cardiac afterload. Our aim was to evaluate the utility of the radial pulse wave to better understand ventriculo-vascular coupling during anesthesia. Methods: We observed the differences between aortic systolic pressure (AoSAP, tipmanometry) and radial systolic pressure in 15 patients, (including two women) aged 53-78years, before coronary artery bypass surgery. We studied the induction of anesthesia with fentanyl (20 μg kg^-1), moderate volume loading, and thereafter the addition of 70% nitrous oxide. The circulatory effects of mechanical ventilation were studied by doubling the tidal volumes. Pulse wave contours were assessed by calculation of radical and aortic augmentation indices (AI), which measure the second systolic pressure peak. Results: Radial systolic pressure was higher than AoSAP in the control situation (8±2mmHg), and this SAP gradient increased further with fentanyl (12±2mmHg). The gradient persisted throughout the study, but was partially reduced by volume loading and nitrous oxide, respectively. Radial augmentation index was the only parameter remaining in a stepwise multivariate model to explain the variance in the SAP gradient (r²=0.48). Radial augmentation index also correlated with aortic pulse pressure (r²=0.71). Mechanical ventilation had significant and similar effects on pulse wave augmentation both in the aorta and in the radial artery, and did not affect the radial to aortic SAP gradient. Conclusion: These elderly coronary patients had stiff vasculature (high aortic AI) and considerable pulse wave reflection, which was beneficially delayed by fentanyl. Changes in the radial pulse wave augmentation during mechanical ventilation were mainly a result of cyclic changes in the stroke volume, and were seldom associated with an increased systolic pressure gradient from the aorta to the radial artery.