Journal: Can J Anesth 54(3):183-189, 2007. 30 References
Reprint: Dept. of Anesthesiology, Montreal Heart Institute, 5000 Belanger Street, Montreal, Quebec H1T 1C8, Canada (A Deschamps, MD)
Faculty Disclosure: Abstracted by R. Klotz, who has nothing to disclose.
For the first stage of labor, epidural analgesia should ideally result in a somatosensory block sufficient to inhibit the C-fibers entering the dorsal horn of the spinal cord at the levels of T10 through L1. Since laboring patients are readily accessible to measure the height of somatosensory block in response to epidural analgesia, the authors considered that it would be relevant to correlate the changes in heart rate (HR) and blood pressure (BP) variability with the progression of epidural block in this patient population. The authors tested the hypothesis that markers of autonomic outflow as assessed by HR and BP variability correlate closely with the progression of the somatosensory block following the onset of epidural analgesia for labor.
Twelve parturients were enrolled over a 5-month period. Multiparous or primiparous parturients in established labor presenting with uncomplicated pregnancies at term (38-40 weeks gestation) were considered for enrollment. Lead II of the ECG, respiratory rate, fetal HR (external monitor), and uterine contractions were recorded. Blood pressure was monitored continuously using a noninvasive measurement device. Epidural anesthesia consisted of 0.125% bupivacaine with 50 mcg of fentanyl (total volume 20 mL). Control data were recorded for 10 minutes with the patient in the left lateral position. The level of sensory block was evaluated bilaterally with ice at 2-minute intervals. Analysis of HR and BP variability was evaluated and calculated every 2 minutes for 10 minutes following establishment of the epidural block.
Changes in high frequency (HF) power of HR variability have been shown to indicate changes in parasympathetic activity, while changes in all frequencies of BP variability have been correlated with changes in sympathetic activity (the reader is referred to the article for a detailed discussion of the analysis of HR and BP variability).
This study shows that in laboring parturients, BP variability correlates with the progression of both sympathetic and somatosensory block following epidural anesthesia, while HR variability is a surrogate marker of increased parasympathetic activity. There are several observations from these findings that have important clinical implications. First, a correlation between changes in BP variability and the progression of somatosensory block after onset of epidural blockade can be observed in the absence of changes in the absolute values of BP and HR. These variables are sufficiently sensitive to detect small changes in autonomic tone during the onset of epidural analgesia. Second, analysis of BP variability appears to provide a better surrogate marker of the progression of the epidural block with a gradual decrease in power coinciding with the increase in somatosensory block.
It should not be assumed that HR variability could not be a useful marker of somatosensory blockade. A level of T11 is reached in most patients at 6 minutes post-epidural blockade, and this also corresponds with a significant increase in HF power of HR variability. This increase, which has been repeatedly associated with parasympathetic outflow, maintains a plateau thereafter.
Several studies have used analysis of HR variability to predict hypotension after spinal anesthesia, to prevent hypotension prior to spinal anesthesia, to predict morbidity and mortality from arrhythmias, as a predictor of mortality in head trauma patients, and as an index of survival in the critically ill patients. The results presented here provide an example of how analyses of HR and BP variability could be used to assess the degree of somatosensory blockade while monitoring autonomic changes that might compromise hemodynamic stability in laboring parturients.
In conclusion, both HR and BP variability show significant changes with epidural analgesia for labor, but only BP variability correlates with progression of somatosensory block. Therefore, analysis of BP variability could be a useful tool to monitor both the decrease in sympathetic activity and the progression of somatosensory block following onset of epidural analgesia in laboring parturients. |
