The Influence of Intravascular Fluid Administration on Aortic Velocity Time Integral in Obstetric Patients Undergoing Cesarean Section

Pregnancy is associated with a myriad of physiologic changes, including expansion of blood volume, decrease in oncotic pressure, and increased cardiac output. Comorbidities including maternal hypertension, renal or cardiac disease may further complicate the assessment of volume status and are associated with increased risks related to fluid administration (c). In addition, the majority of obstetric patients receive neuraxial analgesia or anesthesia to facilitate both vaginal and operative delivery. These techniques are associated with hemodynamic changes including maternal hypotension secondary to a decrease in SVR and CO. These changes may in turn be associated with a reduction in placental blood flow affecting fetal outcome, and may also cause maternal nausea, vomiting and dizziness. Further, the obstetric population is high risk for intrapartum hemorrhage. As such, it is critical to have an accurate method to assess fluid status in intrapartum patients.

The use of invasive volume assessment tools including arterial lines and central venous catheters is limited given the brevity of obstetric procedures, the morbidity of these invasive techniques for the awake patient, and the associated time and costs (b). Non-invasive methods that assess volume status in the obstetric population include techniques such as direct measurement of blood loss (g), carotid dopplers (c), bio-impedence devices (f); However these methods have been shown to be imperfect and are not widely used.

Echocardiography is a non-invasive, widely-available tool that can provide rapid information for experienced operators such as anesthesiologists. Respiratory variations in inferior vena cava diameter (IVCd) and variation in aortic velocity time integral (∆VTI) following passive leg raise have been extensively studied in spontaneously breathing patients. IVCd has been shown to predict fluid responsiveness, however requires a high threshold (>40-50% collapsibility) to detect a difference. IVCd may also change with diaphragmatic motion, thus results may be confounded if the patient becomes tachypneic (d) or is spontaneously breathing (g). In contrast, aortic velocity time integral (∆VTI) may be a more sensitive indicator of volume status. Lamia et al showed a 12% change in VTI as 77% sensitive and 100% specific for a 15% change in cardiac output following 500 mL volume expansion in ICU patients with shock (both spontaneously breathing and mechanically ventilated, e). While there are some studies of IVC collapsibility in the obstetric population, there are limited data on VTI as a measure of volume status in laboring patients.

The purpose of this study is to determine whether the aortic velocity time integral (VTI) is influenced by intravascular fluid administration during cesarean section. Secondary aims include determining whether inferior vena cava diameter (IVCd), blood pressure, and heart rate are influenced by intravascular fluid administration during cesarean section. Additionally, total amount of vasopressor agents administered during the procedure will be recorded.