Measurement of systemic oxygen delivery and inotropy in healthy term neonates with the Ultrasonic Cardiac Output Monitor (USCOM)

Introduction

The aim of this study was to assess the normal values for Smith–Madigan inotropy (SMI), Smith–Madigan inotropy index (SMII), oxygen delivery (DO₂) and oxygen delivery index DO₂I in healthy term neonates on the first day of life and during circulatory adaptation over the first three days of life.

Methods

Hemodynamics of the left heart were measured non-invasively in 71 normal full-term neonates over the first three days using the Ultrasonic Cardiac Output Monitor (USCOM). This was combined with hemoglobin concentration from umbilical cord blood and pulse oximetry to calculate DO₂ and DO₂I. Blood pressure was measured using automated oscillometry and combined with the hemodynamic measures and hemoglobin concentration using the Smith–Madigan method to calculate inotropy (SMI) and inotropy index (SMII).

Results

SMI and SMII showed no significant change during the study period, ranging from 154 to 168 mW and 694 to 731 mW/m². Mean (SD) DO₂ and DO₂I showed a significant fall over three days from 131 (63) ml/min and 596 (278) ml/m²/min to 118 (46) ml/min and 517 (173) ml/m²/min (p < 0.01 and < 0.001 respectively) with a corresponding decrease in cardiac output from 758 (143) ml/min to 658 (131) ml/min, (p = 0.002). There was no significant change in stroke volume, heart rate, SMI or SMII within the first day. DO₂ and DO₂I showed small but significant decreases within the first day from 153 (46) ml/min and 699 (174) ml/min/m² to 129 (36) ml/min and 609 (141) ml/min/m² (p = 0.017 and 0.048 respectively).

Conclusions

Normal inotropy of the left heart and systemic DO₂ values in healthy full-term neonates over the first three days of life were assessed using the USCOM. Subjects showed stable myocardial contractility over the first three days with decreasing DO₂ and DO₂I in line with the decrease in cardiac output (CO). DO₂ and DO₂I showed small but significant reductions during the first 24 h. USCOM proved to be a feasible and convenient non-invasive bedside tool to assess inotropy and oxygen delivery in neonates.