Impedance cardiographic waveforms in children: Involvement in left ventricular ejection time determination during anesthesia

OBJECTIVE: The physiologic basis of the impedance cardiographic (ICG) signal is still a matter of debate. An accurate determination of left ventricular ejection time (LVET) is needed to calculate stroke volume. In children, several shapes of the ICG signal are observed during anesthesia or in critical care. The aim of this study was to determine the relationship between the shape of the ICG signal and various morphologic and hemodynamic determinants obtained with Doppler echocardiography to highlight the effect of the various shapes in the accuracy of LVET determination. DESIGN: Prospective study. SETTING: Pediatric surgery in a university hospital. PATIENTS: 103 children, ASA physical status I or II. INTERVENTIONS: General anesthesia for elective surgery. Measurements: Electrocardiography, ICG, and Doppler echocardiography were recorded simultaneously. Classic hemodynamic variables, such as heart rate, LVET, stroke volume, or systemic vascular resistance, were measured or calculated. A mathematical model of the ICG signal was used to determine the shape of the ICG signal and its potential relationship to various morphologic and hemodynamic determinants. The ICG signal shape was then modeled with the normalized amplitudes (NHA) and phase (NHP) of the three harmonics of the fundamental signal. Analysis was performed with a multiple correspondence analysis. RESULTS: ICG method LVET did not significantly correlate with LVET determined with Doppler or with heart rate. NHA and NHP of the ICG signal were not significantly related to LVET determined with Doppler or to stroke volume. NHA were linked to heart rate and the relative length of the cardiac diastolic period. CONCLUSION: The absence of relationship between LVET determined with Doppler and the main variables defining the shape of the ICG signal highlight the relative limit of stroke volume determination with ICG method. The involvement of the arterial compliance in the ICG signal shape can be discussed in regard to the links between NHA and both heart rate and the relative length of the cardiac diastolic period, respectively.