Relation between heart rate and pulse transit time during paced respiration

Pulse transit time (PTT) is a simple, non-invasive measurement, defined as the time taken from a reference time for the pulse pressure wave to travel to the periphery. PTT is influenced by heart rate, blood pressure changes a nd the compliance of the arteries, but few quantitative data are available describing the factors which influence PTT. The aim of this study was to in vestigate the relationship between the cardiac beat-to-beat interval (RR) a nd PTT, using paced respiration to generate changes in both variables. We analysed PTT and RR interval from 15 normal healthy subjects during pace d breathing, and the cross-correlation function between PTT and RR was used to quantify their relationship. Over the 15 subjects, the maximum change i n PTT ranged from 7 to 23 ms with a mean +/- standard deviation of 14 +/- 5 ms, and that in RR interval from 86 to 443 ms (241 +/- 102 ms). Examining changes over time, the best correlation (r = +0.69, p < 0.01) was obtained when PTT was advanced relative to RR, with a change in RR followed by a cor responding change in PTT 3.17 <plus/minus> 0.76 beats later. We conclude th at there is a strong relationship between PTT changes and RR interval chang es, but these changes are not in phase.