Impact of Withdrawal of 450 ml of Blood on Respiration-Induced Oscillations of the Ear Plethysmographic Waveform

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Abstract
It has been widely appreciated that ventilation-induced variations in systolic blood pressure during mechanical ventilation correlate with changes in intravascular volume. The present study assessed whether alterations in volume status likewise can be detected with noninvasive monitoring (ear plethysmograph) in non-intubated subjects (awake volunteers). Eight healthy adults were monitored with EKG, noninvasive blood pressure, an unfiltered ear plethysmograph, and a respiratory force transduction belt before (PRE) and after (POST) withdrawal of 450 ml of blood from an antecubital vein. Spectral-domain analysis was used to determine the peak ventilatory frequency and the power of the associated variation in the ear plethysmographic tracing; Interphase differences in the respiration-induced plethysmographic variations were assessed by Wilcoxon signed rank test. In addition, the changes in the ear plethysmographic tracing were compared to changes in heart rate and blood pressure. There was a significant increase in respiratory-associated oscillations at the respiratory frequency between the PRE and POST phases (p = 0.012). These changes were detected despite lack of changes in heart rate or blood pressure. Respiration-induced changes of the ear plethysmographic waveform during spontaneous ventilation increase significantly as a consequence of withdrawal of approximately one unit of blood in healthy volunteers.

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