Major hormonal and hemodynamic changes occur in the cardiovascular system in pregnancy that can unmask and impact underlying cardiac disease. Cardiac output can rise by up to 50% throughout gestation, with an additional 30% rise intrapartum. This increase is multifactorial, owing to an increase of 50% in maternal blood volume during pregnancy, a decrease in afterload due to a decline in systemic vascular resistance and a rise in the maternal heart rate of up to 20 beats per minute. Cardiac output has been the most extensively studied physiologic parameter of cardiac performance during pregnancy, and is dependent on both heart rate and stroke volume. Most of the changes in cardiac output on pregnancy are a result of stroke volume. However, during the mid-second trimester, stroke volume plateaus while heart rate continues to rise, becoming the major contributor to cardiac output in the latter half of pregnancy. During labor, cardiac output is further increased by the autotransfusion of blood from the uterus during a uterine contraction and maternal expulsive efforts. This can lead to an increase of up to 500cc of blood within seconds into the systemic circulation, which can significantly impact cardiac output and stroke volume.

Following delivery, many of these cardiovascular changes reverse in the first 2 weeks postpartum with further normalization toward preconception values 3-12 months later. Women without heart disease adapt well and adverse events are generally rare. However, the stress induced by these antenatal changes can cause a patient with underlying disease to decompensate during the latter half of pregnancy and more specifically, intrapartum when these changes peak. Studies on maternal morbidity attributable to cardiac disease in the United States have shown the most likely time for an adverse cardiac event is intrapartum and immediately postpartum, owing to the drastic and acute changes in hemodynamic status during these time periods. Without an invasive monitor (PAC), hemodynamic monitoring during these time relies on surrogate markers such as blood pressure or heart rate.

There is relatively scarce data on continuous hemodynamic profiles in women with both congenital and acquired heart disease in pregnancy, specifically during labor. Previous literature on the use of invasive monitoring (PAC) during pregnancy have been performed only intermittently during the labor and postpartum process. However, the hemodynamic status during the second and third labor stages is not static and therefore the need for continuous evaluation cannot be underscored.

The increased morbidity associated with invasive monitoring limits the use of pulmonary artery catheters in the pregnant population and therefore a non-invasive way to obtain hemodynamic profiles in women with heart disease is desirable. The non-invasive cardiac output monitor (NICOM) is based on bio-reactance technology and is operator-independent, allowing negligible inter-observer variability in data collection and ease of use. Measurements of cardiac output and stroke volume are not dependent on the distance between the electrodes, which can significantly increase the accuracy of the results. It involves the application of four sensors on the thorax. Changes in aortic blood flow drive phase shifts of propagating waves which are detected by the sensors as the frequency changes. These changes correlate with instantaneous changes in blood volume and blood flow in the aorta. Bio-reactance has been validated against pulmonary artery catheters in non-pregnant populations which manifest various forms of hemodynamic instability and following cardiac surgery.

Transthoracic bio-reactance, or the non-invasive cardiac output measurement \[NICOMTM, Cheetah Medical Inc., Portland, OR\] system, is a new technique that is able to measure multiple hemodynamic parameters with four transdermal electrodes placed on the patients' thorax. It is based on frequency- and phase-modulation of the voltage signal measured in response to an applied transthoracic current. Its readings have been shown in multiple studies to correlate well with PAC in the non-pregnant population. It has shown acceptable accuracy, precision and responsiveness for cardiac output monitoring in patients experiencing a wide-range of hemodynamic situations. However, it has not been validated in the pregnant, structurally abnormal heart i.e. congenital cardiac disease, as this was exclusion criteria in the aforementioned studies. Establishing normative values during the second stage of labor utilizing the NICOM in women with congenital cardiac disease has the potential to be clinically useful in developing goal-directed management therapy for these women.