Daniel 1. Sessler, M. D. * ANESTHESIA and surgery commonly cause substantial thermal perturbations. Hypothermia, the typical alteration, results from a combination of anesthetic-induced impairment of thermoregulatory control, a cool operating room environment, and factors unique to surgery that promote excessive heat loss. Available data suggest that inhibition of normal thermoregulatory defenses contributes more to hypothermia than does cold exposure per se. Furthermore, much core hypothermia results from altered distribution of body heat rather than from a systemic imbalance between metabolic heat production and heat loss.

Perioperative Heat Balance

Administration of general anesthesia requires continuous monitoring of vital parameters of the body including body temperature. However, temperature continues to be one of the least seriously monitored parameters perioperatively. Inadvertent perioperative hypothermia is a relatively common occurrence with both general and regional anesthesia and can have significant adverse impact on patients' outcome. While guidelines for perioperative temperature management have been proposed, there are no specific guidelines regarding the best site or best modality of temperature monitoring and management intraoperatively. Various warming and cooling devices are available which help maintain perioperative normothermia. This article discusses the physiology of thermoregulation, effects of anesthesia on thermoregulation, various temperature monitoring sites and methods, perioperative warming devices, guidelines for perioperative temperature management and inadvertent temperature complications (hypothermia/hyperthermia) and measures to control it in the operating room.

Temperature management under general anesthesia: Compulsion or option.

We investigated the usefulness of peripheral flow index (PFI) measurement using a standard pulse oximetry digit probe for early prediction of successful regional blocks. Sixty-six patients scheduled for limb surgery underwent either axillary or sciatic block using a nerve stimulator technique with mepivacaine 1.5%. PFI, which is the ratio of the pulsatile versus the nonpulsatile component of the pulse oximetry signal, was recorded from 10 min before block insertion until 30 min afterwards. PFI recordings of the unblocked limb were similarly recorded. Pinprick and cold sensation were assessed at 5-min intervals until 30 min after blockade. An increase in PFI by a factor of 1.55 at 10 min after axillary block placement (P = 0.006), and 12 min after sciatic block placement (P = 0.001) was required to predict a successful block. The sensitivity and specificity of PFI was 100% for predicting axillary block outcomes at this time. Positive predictive value was 95% and negative predictive value was 93%. For sciatic blocks, sensitivity and specificity were 90% and 100%, respectively. The calculated positive predictive value at time 12 min for sciatic blocks was 94% and negative predictive value was 92%. At 15 min after block placement, cold and pinprick sensations had the same calculated values for sensitivity and specificity at 71% and 100%, respectively, for axillary blocks. For sciatic blocks, cold sensation had a sensitivity of 77% and a specificity of 100%, whereas pinprick had a sensitivity of just 20% with a specificity of 100%. We conclude that PFI provides a simple, early, and objective assessment of the success and failure of nerve blocks.

Peripheral Flow Index Is a Reliable and Early Indicator of Regional Block Success

The cardiovascular effects observed with epidural anaesthesia are complex and are predominantly related to the extent of sympathetic denervation, autonomous balance, baseline blood volume and cardiovascular function of the patient. With epidural anaesthesia the pharmacological effect of systemically absorbed local anaesthetic agents and inclusion of adrenaline to the local anaesthetic solution may play a role. Individual cardiovascular response to different levels of sympathetic blockade varies widely, depending on the degree of sympathetic tone prior to the block. Epidural anaesthesia per se has little effect on respiration in patients with pre-existing lung disease. Thoracic epidural anaesthesia appears to reverse the diaphragmatic dysfunction which is a major determinant of the decrease in lung volumes observed after upper abdominal surgery. This article summarizes cardiovascular and pulmonary responses to epidural anaesthesia.

Cardiovascular and pulmonary effects of epidural anaesthesia

Background and Objectives. Epinephrine is frequently combined with local anesthesia to prolong analgesia. Determination of the minimal concentration and the dose of epinephrine that produces prolongation of analgesia is important in the face of epinephrine9s potential for systemic and local toxicity. The authors undertook this study to determine a dose-response curve of epinephrine on duration of analgesia of both 1% lidocaine and 0.25% bupivacaine after local infiltration. In order to determine whether epinephrine-induced vasoconstriction affected duration of analgesia, the authors correlated duration of analgesia with magnitude of local vasoconstriction as measured with laser Doppler flowmetry. Methods. Six volunteers were studied in a randomized double-blind manner. Ten skin wheals of 0.2 mL solution were subcutaneously injected into both forearms of each volunteer. The solutions consisted of 1% lidocaine with epinephrine concentrations of 0, 1:50,000, 1:200,000, 1:800,000, and 1:3,200,000, and 0.25% bupivacaine with the same epinephrine concentrations. Duration of loss of sensation to pinprick at each wheal was recorded. Skin wheals with 0.2 mL of these same solutions were also subcutaneously injected into the abdomen of the same 6 volunteers, and laser Doppler flowmetry readings of skin blood flow were measured for 6 hours after injection. Results. Epinephrine prolonged duration of analgesia for both lidocaine and bupivacaine in a dose-related manner (P Conclusions. Epinephrine prolongs duration of analgesia after local infiltration in a dose-related manner. Addition of epinephrine in concentrations of 1:50,000 or 1:200,000 increases duration of analgesia after local infiltration by approximately 200%. Addition of doses as dilute as 1:3,200,000 still increases duration of analgesia by approximately 100%. Duration of analgesia appears to correlate with magnitude of epinephrine-induced vasoconstriction using laser Doppler flowmetry. Based on study data, the use of epinephrine in concentrations from 1:200,000 to 1:3,200,000 is recommended for prolongation of analgesia after local infiltration.

Epinephrine prolongs duration of subcutaneous infiltration of local anesthesia in a dose-related manner. Correlation with magnitude of vasoconstriction.

We studied the time course of sensory and sympathetic blockade in response to epidural local anesthetic test and bolus doses in 11 patients. Sympathetic activity was measured by monitoring cutaneous perfusion in the foot and the reflex vasoconstrictive response to deep inspiration (IGVR) using laser Doppler flowmetry. Sensory tests included the detection of touch, cold and painful stimuli. Following the 3-mL test dose perfusion increased to 192 +/- 38% (mean +/- SEM) of baseline (P < 0.05) in the patients with successful epidural anesthesia (9 of 11). In 8 of these patients, IGVR decreased to 73 +/- 10% of baseline (P < 0.05) within 6 min of the test dose, and preceded changes in sensation to cold, pin-prick, and light touch by 3.8 +/- 3.5 min (P < 0.2), 9.6 +/- 3.1 min (P < 0.01), and 11.6 +/- 2.7 min (P < 0.01), respectively. Five control patients who received only 60 mg of lidocaine intravenously and the two patients with failed epidurals did not show any perfusion or IGVR changes. This study confirms that sympathetic block precedes sensory block in sacral dermatomes after epidural anesthesia. Perfusion and IGVR changes are sensitive measures of sympathetic blockade and may predict successful epidural catheter placement.

Time course of sympathetic blockade during epidural anesthesia: laser Doppler flowmetry studies of regional skin perfusion.

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Time course of sympathetic blockade during epidural anesthesia: laser Doppler flowmetry studies of regional skin perfusion.