Upper and lower extremity regional haemoglobin oxygen saturation in spinal anaesthesia

Abstract

Differential distribution of blood flow to blocked or unblocked areas in regional anesthesia may occur due to the differential vasodilatation following sympathetic efferent nerve blockade. Laser doppler flowmetry and thermography [1, 2] have been used to detect this, successfully predicting the effects of regional anaesthesia, though they are not always available in the operating theatre. Well maintained blood pressure during spinal anaesthesia may, secondary to vasoconstriction in the unblocked area, compensate for the vasodilatation of spinal anaesthesia. Vasodilatation effects measured simultaneously in blocked and unblocked areas have not been reported. Near-infrared spectroscopy (NIRS) gives real-time regional haemoglobin oxygen saturation (rSO2) [3, 4]. In this pilot study, we measured rSO2 simultaneously, above and below the spinal anaesthetic levels during block onset. After approval of the institutional human investigation committee (no. 200808010R), and with informed consent, 20 consecutive patients (29–79 years) without a history of neurological or vascular disorder, having lower extremity operations under spinal anaesthesia were recruited to the study. NIRS (INVOS Cerebral Oximeter Model 5100B; Somanetics, Troy, MI, USA) with two adhesive patches was used to monitor rSO2. One patch was applied over the biceps brachii muscle, (the body part above the anaesthetic level), and the other patch was applied over the medial side of the gastrocnemius muscle of the non-surgical leg, (the body part below the anaesthetic level). In both extremities, rSO2 was monitored continuously and recorded every minute from before intrathecal bupivacaine injection until 15 min after injection. Isobaric bupivacaine 0.5% 12–15 mg was injected intrathecally and the level of anaesthesia was examined 10 min later by loss of cold sensation to alcohol swab by an anaesthetist not involved in the study. Statistical analysis used Sigmastat v.3 by spss. Data are mean (SD). Analysis used two-way ANOVA and a p value < 0.05 was considered significant. Multiple comparisons were used to differentiate between groups. Anaesthesia between T6-T11 was achieved and blood pressure was maintained within 15% of baseline. Baseline arm and calf rSO2 were 70.8 (12.8) and 65.1 (10.4) %, respectively. As shown in Fig. 5, rSO2 in the calf increased soon after induction of spinal anesthesia (p < 0.001). rSO2 in the arm did not decrease significantly. There was a statistically significant interaction between time and site (p = 0.016). Changes in regional haemoglobin oxygen saturation (rSO2) at upper and lower extremities as measured by near-infrared spectroscopy in spinal anesthesia. Data are mean (SE). Our results showed a divergence of rSO2 between unblocked and blocked body parts with the onset of spinal anaesthesia. The slight decrease of rSO2 in the upper limb may indicate mild vasoconstriction with onset of spinal anesthesia. NIRS may serve as a potential monitor for the onset of neuroaxial blockade predicting the success of the block. It may be potentially useful in patients with hearing or speech difficulties. The rSO2 detects haemoglobin oxygen saturation at a depth of 30 and 40 mm in tissue rather than skin oxygen saturation which has been found to be unreliable for detecting sympathetic block [5]. There are limitations to this work, firstly lower extremity rSO2 increased by < 5% in four out of 20 patients although spinal anaesthesia was adequate. Secondly, the lower extremity increase in rSO2 indicated onset of spinal anesthesia but not adequacy of surgical anesthesia. We suggest that detection of rSO2 over non-anaesthetised and anaesthetised body parts could be an alternative monitor for onset of neuroaxial blockade.