Maternal Hemodynamic Monitoring in Obstetric Anesthesia

THE physiologic changes of pregnancy, including an initial gradual increase in cardiac output, followed by the development of increasing aortocaval compression in the third trimester, as well as comorbidities such as preeclampsia, have generated considerable research into maternal hemodynamics. The use of the pulmonary artery catheter allowed a better understanding of the physiology of the healthy parturient and the hemodynamics of preeclampsia, including the effects of epidural analgesia in labor. Early use of dye dilution techniques gave clinicians insight into the hemodynamic changes during spinal anesthesia for cesarean delivery. In sharp contrast with these invasive measures is the use in the current issue of ANESTHESIOLOGY, by Langesæter et al., of minimally invasive pulse waveform analysis in the assessment of hemodynamic changes during this procedure. Heart rate and blood pressure are appropriately used as surrogate markers of maternal cardiac output in all routine obstetric anesthesia deliveries, and in most of the clinically valuable obstetric anesthesia research to date. Intraarterial monitoring provides a useful indicator of beat-by-beat changes in unstable patients. During regional anesthesia for cesarean delivery, maintenance of baseline maternal blood pressure, using phenylephrine, has been shown to produce the closest to zero umbilical arterial base deficit, the currently accepted short-term marker of neonatal well-being. This is despite the fact that phenylephrine, given in doses high enough to produce baroreceptor-mediated decreases in heart rate, probably depresses maternal cardiac output. The effectiveness of phenylephrine may be related to the limited susceptibility of the uterine artery to the vasoconstrictive effects of agonists in advanced pregnancy. However, the maximum change in cardiac output has been shown to correlate better with uteroplacental blood flow than with upper arm blood pressure. The maintenance of blood pressure and maternal cardiac output are therefore both important for maternal safety and comfort and for fetal well-being. For clinical management and research purposes, there has been an increasing awareness of the potential complications of invasive monitoring. In addition, the importance of the effects of fluid and vasopressor administration on flow, rather than on pressure, is now recognized in the nonobstetric population. In particular, central venous pressure and pulmonary wedge pressure are unlikely to predict the response to fluid administration, and pulse pressure variation and stroke volume variation may be better indicators of fluid resuscitation. These factors have led to a resurgence of interest in minimally invasive techniques of cardiac output monitoring. Noninvasive methods of cardiac output measurement used in obstetric anesthesia have provided valuable information on maternal and fetal well-being and hemodynamics in the critical care setting and during regional anesthesia for cesarean delivery. These techniques include transthoracic echocardiography, transesophageal echocardiography, transesophageal, suprasternal aortic, and uterine artery Doppler ultrasound techniques, and transthoracic and whole body electrical bioimpedance. All of these methods have disadvantages, including expense, the requirement for user education, movement artifact, and, in the case of bioimpedance techniques, the potential for inaccuracy in terms of absolute cardiac output values in advanced pregnancy and in the presence of increased lung water. None provide beat-by-beat data. Arterial pulse waveform analysis methods are attractive to the obstetric anesthesiologist in that they provide beat-by-beat assessment of cardiac output and could be used both in critical care monitoring (e.g., in complicated severe preeclampsia) and for research purposes (e.g., effects of fluids, vasopressors, and oxytocic drugs) in the laboring patient or during anesthesia. Of crucial importance in the acceptance of these monitors are the precision and reliability of the employed algorithm in following changes in cardiac output (including in the setting of rapidly changing systemic vascular resistance), and the ability to predict ventricular preload response, through the derivation of fluid responsive parameters. When interpreting published data, usually involving Bland and Altman’s recommendation for the use of bias and precision statistics, the reader should bear in mind that in view of the 10– 20% accuracy of thermodilution, limits of agreement of up to 30% between the new and the accepted technique are generally regarded as acceptable. Currently commercially available methods consist of calibrated devices (LiDCOplus [LiDCO, Cambridge, United Kingdom] and PiCCOplus [Pulsion Medical Systems, Munich, Germany]) and the uncalibrated Vigileo monitor (Edwards Lifesciences, Irvine, CA). The PulseCO algorithm used in the LiDCOplus monitor requires only a peripheral arterial and venous line and This Editorial View accompanies the following article: Langesæter E, Rosseland LA, Stubhaug A: Continuous invasive blood pressure and cardiac output monitoring during cesarean delivery: A randomized, double-blind comparison of low-dose versus high-dose spinal anesthesia with intravenous phenylephrine or placebo infusion. ANESTHESIOLOGY 2008; 109:856–63.