Introduction
Pre-ejection period (PEP) is defined as the time interval from the R spike of the ECG to the beginning of the upstroke of the arterial pressure wave. The ventilation-induced changes in PEP have been proposed as an alternative to Pulse Pressure Variation for the estimation of intravascular volume status. In a first step to investigate the ability to measure this parameter with plethysmography, we determined the correlation between PEP’s measured with an invasive arterial line (PEPa) and with plethysmography (PEPpl).
Materials and Methods
This study is a sub analysis, based on two studies, COMPAS2 and VENTIRATE. Both studies were performed after ethical approval and informed consent was obtained. Measurements were made during a 60 seconds registration period with the ventilator set at a tidal volume of 8 ml/kg, and a mechanical ventilation rate of 12/min. ECG, invasive arterial waveforms and plethysmographic waveforms were recorded simultaneously. The individual PEP’s were synchronized and calculated offline. Two linear mixed models were calculated to predict PEPpl from PEPa. MOD1 is a repeated measure model, MOD2 adds a clustering effect per monitor.
Results
21 patients were included. Per patient, 56 [46 , 88] beats were used. Median values for PEPa and PEPpl per patient were 176 [147 , 207] msec and 254 [108 , 460] respectively. MOD1 and MOD2 differed significantly, pointing out a clustering effect of the different monitors used in the two groups. A systematic error of 199 msec was found between the two monitors. The median (range) R² per patient was 0.08 (0.0 – 0.35).
Discussion
The plethysmographic curve as we know it, is a highly processed and filtered signal. The clustering effect seen in the results, represent a difference in processing between two monitors, with different latencies. The low R² values represent the weak overall interchangeability between PEPa and PEPpl. This weak correlation can be due to the difference in data-processing, or due to the different physiologic phenomenon behind both curves.
Conclusion
Although the arterial pressure wave and the plethysmographic curve are morphologically similar, the measured PEP values drawn from both curves are not freely interchangeable.
 


04 / Do pre-ejection periods simultaneously measured with plethysmography reflect invasive arterial line measurements?