Previous Next

NEURAXIAL ANESTHESIA (also see Chapter 43 , Chapter 44 , and Chapter 45 )

Autonomic thermoregulation is impaired during regional anesthesia, and the result is typically intraoperative core hypothermia. Interestingly, this hypothermia is often not

1578


Figure 40-10 Changes in body heat content and distribution of heat within the body during the core temperature plateau. Elapsed time zero indicates the onset of arteriovenous shunt vasoconstriction, which causes the plateau. Core temperature decreased 1.3°C in 2 hours before constriction and then remained constant. The difference between core temperature and mean body temperature (which continued to decrease) indicates how much heat was retained in the core thermal compartment by thermoregulatory vasoconstriction. In this case, 22 ± 8 kcal was constrained to the core. Results are presented as means ± SD. (Redrawn with modification from Kurz A, Sessler DI, Christensen R, Dechert M: Heat balance and distribution during the core-temperature plateau in anesthetized humans. Anesthesiology 83:491–499, 1995.)

consciously perceived by patients, but it nonetheless triggers shivering. The result is frequently a potentially dangerous clinical paradox: a shivering patient who denies feeling cold.

Thermoregulation

Epidural anesthesia[61] [62] and spinal anesthesia[62] [63] each decrease the thresholds triggering vasoconstriction and shivering (above the level of the block) about 0.6°C ( Fig. 40-11 ). Presumably, this decrease does not result from recirculation of neuraxially administered local anesthetic because the impairment is similar during epidural and spinal anesthesia,[61] [62] [63] even though the amount and location of administered local anesthetic differ substantially. Furthermore, lidocaine administered intravenously in doses producing plasma concentrations similar to those occurring during epidural anesthesia has no thermoregulatory effect.[64] Finally, neuraxial administration of 2-chloroprocaine, a local anesthetic that has a plasma half-life near 20 seconds, also impairs thermoregulatory control.[65]

The vasoconstriction and shivering thresholds are comparably decreased during regional anesthesia,[63] thus


Figure 40-11 Spinal anesthesia increased the sweating threshold but reduced the thresholds for vasoconstriction and shivering. Consequently, the interthreshold range increased substantially. The vasoconstriction-to-shivering range, however, remained normal during spinal anesthesia. Results are presented as means ± SD. (Redrawn from Kurz A, Sessler DI, Schroeder M, Kurz M: Thermoregulatory response thresholds during spinal anesthesia. Anesth Analg 77:721–726, 1993.)

suggesting an alteration in central rather than peripheral control. The mechanism by which peripheral administration of a local anesthetic impairs centrally mediated thermoregulation may involve an alteration in afferent thermal input from the legs. The key factor here is that tonic cold signals dominate thermal input at leg skin temperatures in typical operating room environments.[2] [66] Regional anesthesia blocks all thermal input from blocked regions, which in the typical case is primarily cold information. The brain may then interpret decreased cold information as relative leg warming. This appears to be an unconscious process because perceived temperature does not increase. [67] Because skin temperature is an important input to the thermoregulatory control system, leg warming proportionately reduces the vasoconstriction and shivering thresholds. Consistent with this theory, a leg skin temperature near 38°C is required to produce the same reduction in cold-response thresholds in an unanesthetized subject as is produced by regional anesthesia.[68] Furthermore, the reduction in thresholds is proportional to the number of spinal segments blocked ( Fig. 40-12 ). [69] Major conduction anesthesia may thus reduce the vasoconstriction and shivering thresholds by producing an abnormal elevation in apparent (as opposed to actual) leg temperature.

Because neuraxial anesthesia prevents vasoconstriction and shivering in blocked regions, it is not surprising that epidural anesthesia decreases the maximum intensity of shivering. However, epidural anesthesia also reduces the gain of shivering, which suggests that the regulatory system is unable to compensate for lower body paralysis ( Fig. 40-13 ). [27] Thermoregulatory defenses, once triggered, are thus less effective than usual during regional anesthesia.

Neuraxial anesthesia is frequently supplemented with sedative and analgesic medications. With the exception of midazolam,[36] all significantly impair thermoregulatory control.[22] [28] [70] Such inhibition may be severe when combined with the intrinsic impairment produced by regional anesthesia and other factors, including advanced age and preexisting illness ( Fig. 40-14 ).[11]

1579


Figure 40-12 The number of dermatomes blocked (sacral segments, 5; lumbar segments, 5; thoracic segments, 12) versus reduction in the shivering threshold (difference between the control shivering threshold and the spinal shivering threshold). The shivering threshold was reduced more by extensive spinal blocks than by less extensive ones (Δ threshold = 0.74 − 0.06 [dermatomes blocked]; r2 = .58, P < .006). The curved lines indicate the 95% confidence intervals for the slope. (Redrawn from Leslie K, Sessler DI: Reduction in the shivering threshold is proportional to spinal block height. Anesthesiology 84:1327–1331, 1996.)


Figure 40-13 Systemic oxygen consumption without (circles) and with (squares) epidural anesthesia. The horizontal standard deviation bars indicate variability in the thresholds among the volunteers; although errors bars are shown only once in each series, the same temperature variability applies to each data point. The slopes of the oxygen consumption-versus-core temperature relationships (solid lines) were determined by linear regression. These slopes defined the gain of shivering with and without epidural anesthesia. Gain was reduced 3.7-fold, from -412 mL/min/°C (r2 = .99) to -112 mL/min/°C (r2 = .96). (Redrawn from Kim J-S, Ikeda T, Sessler D, et al: Epidural anesthesia reduces the gain and maximum intensity of shivering. Anesthesiology 88:851–857, 1998.)


Figure 40-14 Fifteen patients younger than 80 years (58 ± 10 years [mean ± SD]) shivered at 36.1°C ± 0.6°C during spinal anesthesia; in contrast, 8 patients 80 years or older (89 ± 7 years) shivered at a significantly lower mean temperature, 35.2°C ± 0.8°C. The shivering thresholds in seven of the ten patients older than 80 years was less than 35.5°C, whereas the threshold equaled or exceeded this value in all the younger patients. (Redrawn from Vassilieff N, Rosencher N, Sessler DI, Conseiller C: The shivering threshold during spinal anesthesia is reduced in the elderly. Anesthesiology 83:1162–1166, 1995.)

Interestingly, core hypothermia during regional anesthesia may not trigger a perception of cold.[61] [71] The reason is that thermal perception (behavioral regulation) is largely determined by skin rather than core temperature. During regional anesthesia, core hypothermia is accompanied by a real increase in skin temperature. The result is typically a perception of continued or increased warmth accompanied by autonomic thermoregulatory responses, including shivering ( Fig. 40-15 ).[61] [71]

Taken together, these data indicate that neuraxial anesthesia inhibits numerous aspects of thermoregulatory control. The vasoconstriction and shivering thresholds


Figure 40-15 Induction of epidural anesthesia at an elapsed time of 15 minutes decreased core temperature and increased thermal comfort as determined by a 100-mm visual analog scale (VAS). Interestingly, however, maximal thermal comfort coincided with the minimum core temperature. Results are presented as means ± SD. TM, tympanic membrane. (Redrawn with modification from Sessler DI, Ponte J: Shivering during epidural anesthesia. Anesthesiology 72:816–821, 1990.)

1580
are reduced by regional anesthesia[61] [62] [63] [68] [69] and further reduced by adjuvant drugs[22] [36] and advanced age.[11] Even once triggered, the gain and maximum response intensity of shivering are about half normal.[72] Finally, behavioral thermoregulation is impaired.[71] The result is that cold defenses are triggered at a lower temperature than normal during regional anesthesia, defenses are less effective once triggered, and patients frequently do not recognize that they are hypothermic. Because core temperature monitoring remains rare during regional anesthesia, substantial hypothermia often goes undetected in these patients.[73]

Previous Next