Electrical Cardiometry for Trending Maternal Cardiac Output

New technologies for the measurement of cardiac output are commonly evaluated using either validation studies or trending studies. These two types of studies have different goals and methodologies, but both types of studies compare the new technology to a reference standard. The reference standard example used in this chapter is transthoracic echocardiography. Validation studies take one-time paired measurements of cardiac output using the two technologies in many individuals and compare the absolute values obtained with the two technologies using Bland–Altman analysis to calculate a mean percentage error. Trending studies evaluate whether the new technology agrees with the reference standard regarding the direction of change of cardiac output within individual people over time. Electrical cardiometry is an emerging impedance cardiography technology for measuring cardiac output which appears to be useful as a trend indicator, and this capability should be sufficient to detect the reductions in cardiac output which identify obstruction of uterine venous return. The absolute accuracy of electrical cardiometry in validation studies in pregnant patients is in doubt and probably cannot be relied upon. This chapter describes a possible improvement in the electrical cardiometry algorithm for the determination of usefully correct absolute values of cardiac output, using patient weight, height, and the “ICON” and “Left Ventricular Ejection Time” parameters provided by electrical cardiometry, but this possible improvement in the accuracy of absolute values is not required in order to successfully use electrical cardiometry for “cardiac output-guided maternal positioning” in order to avoid obstruction of uterine venous return. Electrical cardiometry works by analyzing the changes in thoracic electrical impedance which occur during the cardiac cycle and produces estimates of mean blood velocity during systole as well as the duration of systole. The product of these two values (mean velocity and time) is the equivalent of a “stroke distance” which is then multiplied by a proprietary patient constant based largely on weight to derive an estimate of stroke volume. This chapter proposes an alternate approach to the use of electrical cardiometry data in a way which makes its methodology more closely resemble that of transthoracic echocardiography, involving the explicit estimation of three quantities: (1) the mean velocity of blood flow during systole in the left ventricular outflow tract, (2) the duration of systole, and (3) the cross-sectional area of the left ventricular outflow tract. This is a detailed and “technical” chapter regarding electrical cardiometry as one specific continuous, hands-free, and easy-to-use technology for the measurement or trending of cardiac output, but other technologies for trending cardiac output may be equally or even more useful. Further investigation is required.