Assessment of pulse transit time to indicate cardiovascular changes during obstetric spinal anaesthesia

Background. Pulse transit time (PTT) measurement may provide rapidly available beat-to-beat cardiovascular information when conditions change quickly and routine invasive arterial pressure measurement is not justified, for example during obstetric spinal anaesthesia. Method. We obtained ethics approval for an observational study of PTT during the onset of spinal anaesthesia in patients having elective or urgent Caesarean section. PTT was measured as the difference in time between the peak of the ECG R wave and the upstroke of the toe plethysmograph. Arterial pressure was measured by non-invasive sphygmomanometry. Results. We analysed data from 58 normotensive patients and 15 patients with pregnancy-induced hypertension (PIH). PTT increased with the onset of spinal anaesthesia as arterial pressure decreased. An increase of 20% in PTT was 74% sensitive and 70% specific in indicating a decrease in mean arterial pressure of more than 10%. Changes in PTT were related to changes in mean arterial pressure (r(2)=0.55, P < 0.0001). Arterial pressure changes were greater and PTT increased significantly more quickly in the normotensive patients than in the patients with hypertension [median, quartiles: 32 (14, 56) ms min(-1) compared with 7 (6, 18) ms min(-1); P < 0.01, Mann-Whitney U-test]. However, the relationship between PTT and arterial pressure was similar for the normotensive patients and the patients with PIH. Conclusion. PTT measurement gave a beat-to-beat indication of arterial pressure during spinal anaesthesia, and could be developed to allow prediction of the onset of hypotension.