Journal: Acta Anaesthesiol Scand 51(10):1297-1304, 2007
Reprint: Dept of Anaesthesiology and Intensive Care Medicine, University Hospital Schleswig-Holstein, Campus Kiel, Schwanenweg 21, 24105 Kiel, Germany (R Hanss, MD)
Faculty Disclosure: Abstracted by J. Joyce, who has nothing to disclose.
Spinal anesthesia (SA) for elective cesarean section is generally well tolerated. Nevertheless, hypotension is a common adverse effect of SA in some patients. A non-invasive method to analyze the activity of the autonomic nervous system is heart rate variability (HRV). This tool can be easily established under routine conditions because it is based on regular anesthesia monitoring, the electrocardiogram, and analysis can be easily performed using standard software. Several studies have reported on the predictive value of HRV for determination of patients at high risk of hypotension. In patients scheduled for SA, the high preoperative low-to-high frequency ratio (LF/HF) correlated with severe hypotension, implicating that an increased LF/HF of HRV may indicate increased sympathetic activity.
The authors hypothesized that LF/HF as well as LF may indirectly reflect sympathetic activity. Thus they will be attenuated in the course of SA in parallel to sympatholysis. The degree of hypotension as the clinical hemodynamic side-effect of sympatholysis is correlated with the degree of LF/HF and LF decrease. Changes in HRV parameters in the course of SA depend on baseline values of LF/HF and LF. High baseline LF/HF and LF reflecting high sympathetic activity will decrease considerably, whereas low baseline values reflecting low baseline sympathetic activity will not be significantly impaired by sympatholysis in the course of SA.
In this prospective trial, patients with low baseline LF/HF were compared with patients with high baseline LF/HF. An increased LF/HF and LF at baseline were attenuated in the course of SA, whereas HF increased. These changes correlated with a signi¬ficant decrease in systolic BP. Thus, hypotension, the clinical sign of sympatholysis due to SA, correlated with a decrease in LF/HF and LF indicating that these parameters reflect sympathetic activity.
Hemodynamic impairment was demonstrated in patients under SA at a high thoracic block and was compared with changes in HRV. Spread of neuraxial blockade between thoracic dermatome of T6 to cervical dermatome of C6 provoked a decrease in LF and a small increase in HF resulting in a decrease in LF/HF, indicating that LF/HF as well as LF may be indirect indices of sympathetic activity. Changes of LF/HF were accompanied by severe hypotension.
There is evidence in the literature, which is supported by this study, that LF/HF as well as LF may reflect sympathetic to parasympathetic balance and sympa¬thetic activity, respectively. Blockade of pregan¬glionic sympathetic fibers and blockade of sympa¬thetic innervations of the heart are associated with a decrease in LF/HF and LF if it is elevated prior to SA. In contrast, HF is well accepted as an indicator of parasympathetic activity in the literature. HF increased in the course of SA. Parasympathetic activity is not directly influenced by subarachnoid block but the balance of autonomic regulation may shift to parasympathetic activity as a result of attenuation of sympathetic activity.
HRV data of all patients were correlated with BP without group assignment of patients. High preopera¬tive HRV parameters which may reflect sympathetic activity were diminished by SA accompanied by severe hypotension. The degree of sympatholysis, reflected by a decrease in LF/HF as well as LF, correlated significantly with the degree of systolic BP decrease. Interestingly, correlation of HRV and hemodynamic data of all patients did not show significant results.
Changes in HRV parameters in the course of SA may reflect decrease in sympathetic activity and a relative increase in parasympathetic activity as a result of the block. The more pronounced changes in HRV in the course of SA, the more distinct was the hypotension. These findings may reveal the physiologic block of sympathetic preganglionic fibers as a result of SA via HRV analysis. |
