Please see Bibliography of References for a list of abstracts, manuscripts and posters.
Gershon R, Harris R, Garrett J, Lynde G. Proceedings of the 2010 Annual Meeting of the American Society of Anesthesiologists; October 16-20, 2010; San Diego, California.
Near-infrared spectroscopy-derived StO2 is a noninvasive, continuous monitor of tissue oxygen saturation, evaluating oxygen utilization in the microcirculation, where variations in StO2 represent changes in local perfusion. Hemoglobin content crossed by the near-infrared light allows subtle detection of regional changes in oxygen delivery and consumption at the skeletal muscle tissue level. This is an initial evaluation whether the StO2 monitor has predictive value with regards to the particular physiology of the healthy parturient undergoing an uneventful elective caesarean section with an epidural anesthetic. Placement of epidural decubitus, timing of C section, intraoperative course and StO2 measurements as per Figure 1. [figure1] At 10:44am the G5P2 patient was brought to the operating room, monitors reattached, positioned in left uterine displacement, and oxygen 8 l/min administered by face mask. Skin incised 11:05am, uterine 11:13am, and delivery at 11:15am. Placental manual extraction and expulsion within 20 seconds of delivery, pitocin infused, well contracted uterus exteriorized for closure, and patient in PACU at 12:00 noon. The baseline StO2 in normal subjects at the thenar eminence was established to be 87% and 75-80% at the arch of the foot. The StO2 monitor has been used to study adequacy of resuscitation in the trauma patient; as well as quantifying vascular sensitivity, perfusion and tissue oxygenation in the septic patient. It has also been shown to respond significantly to regional changes in oxygen delivery in patients undergoing cardiopulmonary bypass. We placed the probe on the arch of the nondependent foot, as opposed to the thenar eminence, as a better proxy of regional oxygenation in the fetus. Although epidural placement occured in decubitus position, of note is the opening StO2 reading of 45. With commencement of sympathetic blockade with proper hydration and maintenance of hemodynamic stability, the StO2 reading rose to near normal levels of 80. The FHR remained unchanged throughout. The continuous and significant rise in StO2 to 85-90%, may demonstrates a combination of the importance in positioning the patient with left uterine displacement, as well as the capacity of a sympathetic block with adequate fluid replenishment to achieve supra maximal regional perfusion. The relationship between this rise in StO2 and the addition of epinephrine to the local anesthetic totaling 125 mcg is unknown but warrants further study. The StO2 peaked immediately after placental expulsion and uterine contraction. A 550 ml acute blood loss may in general adversely affect StO2; yet the autotransfusion of blood to the maternal right heart we believe caused the final spike in StO2. Although the StO2 receded as expected several hours after delivery secondary to blood loss, hemodilution and equilibration, it did not return to the pre-epidural base line of 45. This may be due to residual sympathetic blockade and concurrent volume repletion, or the now contracted uterus allowing improved regional blood flow to the lower extremities.