Hemodynamic Measurement Techniques in Pregnancy
The acquisition of hemodynamic data in the obstetric population has proven challenging. Although heart rate, heart rhythm and blood pressure can serve as imprecise indicators of cardiac output, the desire for more direct measurement has resulted in the application of a number of devices and modalities during pregnancy. Once performed for the sake of research alone, invasive hemodynamic monitoring, such as central venous, pulmonary artery[19,20] and cardiac catheterization in normal healthy parturients, is no longer considered acceptable. As a consequence, the studies referencing invasive hemodynamic data from a flow-directed, thermistor-tipped pulmonary artery catheter are unlikely to be repeated or directly compared with more contemporary measurement techniques. A recent editorial describes the advantages and disadvantages of the various noninvasive CO measurement techniques available. With these limitations, the CO data from parturients undergoing CD with neuraxial techniques are presented (Table 2 & Table 3).
In 1915, measurement of CO during pregnancy was performed utilizing a gas-based, dye-dilution technique with nitrous oxide or acetylene. These measurements were believed to be inaccurate due to the wide variations (primarily increases ranging from 45 to 85%) in CO. Pulmonary artery (PA) catheterization, which provided measurements based on the Fick principle, were performed during the 1940 and 1950s and provided accurate, albeit invasive, CO measurements. In the 1960s, the dye dilution technique improved with the use of photometric dyes; dye concentrations are now determined by comparing a radial arterial blood sample with another measurement obtained via transillumination of the capillary bed of the ear. From these measurements, CO is calculated and these measurements are considered relatively accurate. Table 2 & Table 3 illustrate CO during CD with neuraxial anesthesia including data obtained with this photometric dye dilution technique.
Introduced in the 1960s, echocardiography can reliably calculate CO, although the results are based on, and therefore may be affected by, a number of assumptions. Stroke volume (SV) can be calculated by 2D and 3D echocardiography (2DE and 3DE) or by Doppler methodology. Several modalities exist to determine SV with 2DE. The area/length method assumes that the left ventricle (LV) is a cylindrical structure. LV volume is then calculated in systole and diastole by measuring the area (cm2) and the longitude (cm) of the cylinder. The obtained SV is then multiplied by the recorded HR to calculate the CO. Because pregnancy is associated with dilation of the LV, assumptions regarding the shape of the LV may be inaccurate and 2DE CO calculations may overestimate SV in late pregnancy. A more robust 2DE modality is the 'method of disks', in which 20 'disk-shaped' measurements from the four- and two-chamber views of the left ventricle in systole and diastole are obtained and added together.
An additional method for SV and CO calculation is based on the Doppler principle. It also assumes the 'cylinder' hypothesis; in this case the cylinder is the left ventricular outflow tract (LVOT). This method involves measuring the cross-sectional area of the LVOT (cm2) and the distance traveled by the blood during a heart beat (cm), and is obtained by measuring the velocity of systolic blood flow in the LVOT (cm). The ideal signal must be obtained to provide accurate measurement and is thus operator-dependent. SV is then calculated as a function of velocity over time, and multiplied by the recorded HR to calculate the CO. Measurements with this technique have been compared with the gold standard of pulmonary artery catheter thermodilution and have been validated in the pregnant state. The data displayed in Table 2 & Table 3 that were obtained with this technique are considered quite accurate.
Newer 3DE modalities are rapidly replacing 2DE in research and clinical applications for their superior accuracy and reproducibility.[26–33] Furthermore, 3DE can mitigate the errors inherent to 2DE by eliminating foreshortening and avoiding geometric assumptions. An additional advantage of 3DE is that it is not reliant on correct image orientation at acquisition and the associated geometric assumptions. The risk of foreshortening the right ventricle can be reduced, which may be of particular benefit to the parturient with congenital heart disease or whose right ventricle is strained secondary to increased CO or increased pulmonary vascular resistance. 3DE speckle tracking algorithms, which track the motion of speckles within the scan volume, further improve the visual geometric border detection of the ventricle, an inherent limitation in determining the shape and size of the ventricle. This modality shows good intra- and inter-observer reliability, as well as test–retest reliability for routine evaluation of ejection fraction and left ventricular volumes. 3DE direct volumetric and speckle-tracking methods appear to provide comparable and reproducible results of ventricular volume and function, and are both feasible options for routine clinical practice.[32,33]
In 1966, impedance cardiography was introduced in which changes in the electrical impedance across the thorax or whole body throughout the cardiac cycle were analyzed; from these measurements, CO is calculated. However, CO obtained by impedance cardiography correlates poorly with thermodilution measurements during CD and during pregnancy in the presence of hypertensive diseases. The overall accuracy of this technique in healthy term pregnancies has therefore been questioned. Despite this, the method may be valuable for producing noninvasive, continuous data to detect CO trends.[37,38] Consequently, in Table 2 & Table 3, the data obtained through this technique[39,40] may not be as accurate as those obtained through other techniques.
Modern devices have been developed that continuously measure CO using information derived from an arterial line such as the FloTrac (Flo Trac/Vigileo™, Edwards Lifesciences, CA, USA), PiCCO® (Pulsion Medical Systems AG, Munich, Germany) and LiDCO (PulseCO system, LiDCO™ Ltd, Cambridge, UK). The LiDCO device can be used with a peripheral venous and a radial arterial line and has the advantage of measuring beat-to-beat CO measurements, making it an excellent tool for measuring trends. A study employing bioimpedance and LiDCO measurements in the same patient found similar trends in changes in CO in response to spinal anesthesia and vasopressor administration. A recent study also found acceptable agreement between thermodilution and LiDCO measurement immediately postpartum in pre-eclamptic patients. Although studies have been performed that evaluate the clinical application of these devices, further validation in pregnancy is necessary.
Expert Rev of Obstet Gynecol. 2012;7(1):59-75. © 2012 Expert Reviews Ltd.