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

Background. Pulse transit time (PTT) measurement may providerapidly available beat-to-beat cardiovascular information whenconditions change quickly and routine invasive arterial pressuremeasurement is not justified, for example during obstetric spinalanaesthesia. Method. We obtained ethics approval for an observational studyof PTT during the onset of spinal anaesthesia in patients havingelective or urgent Caesarean section. PTT was measured as thedifference in time between the peak of the ECG R wave and theupstroke of the toe plethysmograph. Arterial pressure was measuredby non-invasive sphygmomanometry. Results. We analysed data from 58 normotensive patients and15 patients with pregnancy-induced hypertension (PIH). PTT increasedwith the onset of spinal anaesthesia as arterial pressure decreased.An increase of 20% in PTT was 74% sensitive and 70% specificin indicating a decrease in mean arterial pressure of more than10%. Changes in PTT were related to changes in mean arterialpressure (r²=0.55, P<0.0001). Arterial pressure changes weregreater and PTT increased significantly more quickly in thenormotensive patients than in the patients with hypertension[median, quartiles: 32 (14, 56) ms min^–1 compared with7 (6, 18) ms min^–1; P<0.01, Mann–Whitney U-test].However, the relationship between PTT and arterial pressurewas similar for the normotensive patients and the patients withPIH. Conclusion. PTT measurement gave a beat-to-beat indication ofarterial pressure during spinal anaesthesia, and could be developedto allow prediction of the onset of hypotension. Data from this study were presented in part at the ObstetricAnaesthetists Association Meeting at Nottingham, UK, on May10, 2002 and at the 13th World Congress of the InternationalSociety for the Study of Hypertension in Pregnancy at Toronto,Canada, on June 2, 2002.