Shivering following cardiac surgery: predictive factors, consequences, and characteristics.

BACKGROUND: Shivering is common after cardiac surgery and may evoke harmful hemodynamic changes. Neither those changes nor factors increasing probability of shivering are well defined. OBJECTIVES: (1) To identify factors linked with risk of shivering by comparing age, weight, body surface area, gender, intraoperative details, anesthetics, postoperative temperatures, hemodynamics, and therapeutics in shivering vs nonshivering patients. (2) To describe temperatures, hemodynamics, therapeutics, myocardial oxygen consumption correlates (rate-pressure product, heart rate, systemic vascular resistance) in shivering and nonshivering groups, and shivering and nonshivering periods. (3) To characterize the electromyogram to determine whether the tremor is cold-induced. METHODS: A descriptive design with a time series component was used to study a convenience sample of 10 shivering and 10 nonshivering adults for 4 hours during early recovery from cardiac surgery. Pulmonary artery and skin (facial, calf, trunk) temperature were measured every 60 seconds; heart rate and arterial pressure, every 15 minutes; cardiac output, 3 times. Electromyogram was recorded intermittently. Medications and treatments were noted. RESULTS: Lower skin temperature was significantly related to shivering risk. Heart rate was significantly higher initially in shiverers and remained higher by 13.6 beats per minute. Significantly more nitroprusside was used to control arterial pressure before than after shivering. No significant differences were noted between groups in core temperature, age, weight, body surface area, anesthesia type, intraoperative temperature; or surgery, circulatory bypass, or cardiac cross-clamp duration. The electromyogram pattern during shivering was typical of that produced by cold. CONCLUSIONS: These results suggest that true shivering occurs after cardiac surgery. Skin, but not core, temperature and elevated heart rate predict shivering. Shivering may be more likely in hemodynamically unstable patients.

Suppression of shivering by breath holding, relaxation, mental arithmetic, and warm water ingestion.

Four methods of suppressing cold-induced shivering were evaluated in 26 young male volunteers exposed to 0.0 +/- 1.0 degrees C air for 135 min. Voluntary relaxation of musculature (R), breath holding (BH), warm (50 degrees C) water ingestion (W), and performance of a mental arithmetic task (MA) were applied in a counterbalanced order following 2 h of cold exposure. Surface electromyograms of seven muscles were recorded and converted to root mean square voltage (RMS) as a measure of shivering intensity. Mean skin and rectal temperatures decreased significantly, 4.9 degrees C and 0.3 degrees C, respectively (p < or = 0.01). Mean reduction of EMG activity was 35% during R, 24% during BH, 18% during MA, and 5% during W. R was significantly more effective than BH, MA, and W, and BH and MA were significantly more effective than W in reducing shivering. These results indicate that, at small decreases in rectal temperature, shivering can be voluntarily suppressed to some extent during relaxation, breath holding, and mental arithmetic.

Analysis of multiple EMG and acceleration signals of various record lengths as a means to study pathological and physiological oscillations.

Quantitation of pathological and physiological oscillations are of interest to both the clinician and researcher. Power spectral analysis is a technique which allows for quantitation of both signal amplitude and frequency characteristics. For example, computer derived spectra of limb acceleration and demodulated electromyograms are used to define the properties of an oscillation. In addition, cross-correlation analysis (resultant cross-spectra, phase spectra, and coherence values) provides detailed information concerning the correlation between multiple signals: e.g., as to which muscle or muscles are driving an oscillation. We describe these techniques for analysis of pathological and physiological oscillation of the human motor system and suggest ways in which very short segments of data can be analyzed.

Effects of topical anesthesia on essential tremor.

In a single-blinded study, the effects of skin desensitization by topical anesthesia (4% xylocaine) on six patients who had essential tremor were investigated. Resting hand tremor was monitored using an accelerometer and multiple surface electromyograms. The amplitude and frequency characteristics of the oscillations were determined using spectral analysis. Topical anesthesia significantly suppressed the tremor amplitude and the associated electrical activity in all patients: the mean tremor amplitude was reduced by 40%. No changes in the frequencies of the essential tremors were detected.

Shivering and pathological and physiological clonic oscillations of the human ankle.

Shivering and physiological and pathological clonus of the ankle were compared using power spectral and cross-correlation analysis of their respective electromyographic and acceleration waveforms. The major spectral peaks from each type of involuntary oscillation possessed similar frequencies (5-7 Hz). Soleus electromyographic activity was significantly correlated with the motion signal, whereas no correlation was observed between motion and tibialis anterior electromyographic signal. These data suggest that although shivering and physiological and pathological clonus are activated by different stimuli, these overt ankle oscillations may be an expression of a common spinal neuronal network.

Experimental hypothermia and cold perception.

Twelve subjects clothed in flotation suits were immersed in 10 degrees C cold water and their surface temperatures at the back and groin, as well as core temperatures, were continuously monitored. Subjects were unable to reliably assess how cold they were, with the highest correlation observed between perceived temperature and actual temperature reaching only 0.51. This was felt to be partially due to the uneven distribution of surface temperatures seen in this experiment and in most cold water immersions. Rapid cooling in cold water also produced the perceptual phenomenon of "overshooting" previously observed in cold air studies, characterized by sudden temperature drops being perceived as cold sensations of greater magnitude. The results suggest that subjects who are rapidly cooled in water may have considerable difficulty separating feelings of cold from feelings of pain and discomfort, which can have serious implications in survival situations and highlights the subjective and highly variable nature of cold perception. Perceived cold sensation may be a very poor, and possibly dangerous, predictor in cold water immersion situations.

Core temperature response to immersed bicycle ergometer exercise at water temperatures of 21 degrees, 25 degrees, and 29 degrees C.

A bicycle ergometer modified for aquatic exercise was used to determine the effects of immersion on core temperature during submaximal exercise at different water temperatures. An exercise intensity (60% of maximal oxygen consumption) and duration (30 minutes) considered appropriate for cardiovascular conditioning were used. These data will be useful in cardiovascular and leg-strengthening hydrotherapy programs. Rectal temperature, skin temperature, and a rating of thermal comfort were studied in five normal men (14.8% +/- 5.6% fat) during headout immersion at water temperatures of 21.1 degrees, 25.3 degrees, and 29.4 degrees C and exercise in air of 21.1 degrees C. Subjects were immersed for 30 minutes during static and exercise (63% +/- 0.6% maximal oxygen consumption) conditions. Data were collected every 5 minutes and analyzed by repeated measured analysis of variance. At water temperatures, rectal temperature fell from control during static immersion (p less than or equal to 0.05) and was lower than control at the end of the 30-minute recovery period (p less than or equal to 0.05). During exercise there was no change in rectal temperature at water temperatures of 21.1 degrees and 25.3 degrees C; however, rectal temperature rose at water temperatures of 29.4 degrees (p less than or equal to 0.05) and air 21.1 degrees C (p less than or equal to 0.05). At the end of recovery rectal temperature was lower than control at water temperatures 21.1 degrees C (p less than or equal to 0.05) and greater than control at water temperatures 29.4 degrees C (p less than or equal to 0.05). There was no change from control in rectal temperatures at water temperatures 25.3 degrees C and air at 21.1 degrees C. These results indicate that immersion in 25.3 degrees and 21.1 degrees C water effectively attenuates the rise in rectal temperature during exercise at 63% of maximal oxygen consumption, whereas immersion in 29.4 degrees C water does not. In addition, both skin and rectal temperatures affect perception of thermal state but do not give the subjects an accurate estimation of thermal balance.

Synchronized slow-amplitude modulations in the electromyograms of shivering muscles.

The electromyograms (EMG) of shivering human subjects exposed to 0 degrees C air in an environmental chamber were analyzed to detect slow-amplitude modulations (SAMs, less than 1 Hz) in the EMGs of widely separated muscles and to study the relationship of these SAMs to respiration rate and skin temperature. Distinct amplitude modulations were observed in the raw EMGs during shivering. The peaks in EMG activity occurred simultaneously in the majority of the monitored muscles in all subjects. Pearson correlations between the average rectified EMGs of 93% of the muscles were significant (P less than 0.05). Visual analysis of the EMG and respiration signals indicated that the peaks in muscular activity occurred 6-12 times/min, whereas respiration ranged from 10 to 23 cycles/min. For all subjects respiration was at a higher frequency than amplitude modulation in the EMG. Comparison of EMG records with expiratory flow rate traces in shivering subjects indicated no one-to-one correlation between the occurrence of respiration and EMG amplitude modulations. Respiratory flow rate and average rectified EMG showed significant correlation in only 33% of the cases. In addition, skin temperature changes could not be correlated with the SAMS.

Hypothermia outcome score: development and implications.

Multiple rewarming methods have been recommended for the treatment of hypothermia in the ED. Because the hypothermic patient population is heterogenous, a method for stratifying mortality risk when comparing therapies is desired. We used univariable and multivariable statistical analyses to identify variables which discriminated between patient death or survival in the 24 h after arrival in the ED. Prehospital cardiac arrest, a low or absent presenting BP, elevated BUN, and the need for either tracheal intubation or NG tube placement in the ED were found to be significant predictors of patient demise in a large database (n = 428). The likelihood ratio was used to develop and validate an empiric hypothermia outcome score that can be used in future hypothermia treatment studies to account for differences of patient presentation.

Spontaneous post-anesthetic tremor does not resemble thermoregulatory shivering.

Spontaneous post-anesthetic tremor that resembles shivering is common during recovery from anesthesia. Risks to postoperative patients include an increase in metabolic rate of up to 400%, hypoxemia, wound dehiscence, dental damage, and disruption of delicate surgical repairs. The etiology of spontaneous post-anesthetic tremor is most commonly attributed to normal thermoregulatory shivering in response to intraoperative hypothermia. However, the mechanism of this tremor remains unknown, hampering prevention and treatment. The present study was designed to determine whether mechanisms other than thermoregulation contribute to the tremor. The electromyograms (EMGs) of eight muscles were observed in nine women during recovery from isoflurane anesthesia. Signals from each muscle were compared to those of pathologic clonus induced by plantar flexion in unanesthetized patients with spinal cord transections and to those of cold-induced shivering in normal, unanesthetized subjects. Two distinct EMG patterns were identified: 1) regular, bursting signals of 5-7 Hz similar to those produced by pathologic clonus in patients with spinal cord transections; and 2) tonic, irregular signals of 5-15 Hz which had poorly defined bursts that did not demonstrate the synchronous 4-8-cycle/min waxing and waning pattern typical of normal shivering. EMG activity occurred most often at expired isoflurane concentrations of 0.1-0.19%, and was not related to rectal temperature. During the later part of recovery when isoflurane concentrations were less than or equal to 0.1%, hypothermic patients frequently demonstrated no clinical or EMG evidence of muscular activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular responses to face immersion (the diving reflex) in human beings after alcohol consumption.

The diving reflex is a complex cardiovascular-respiratory response to immersion. This multireflex response in animals consists of apnea; bradycardia (the experimental parameter most often measured); peripheral vasoconstriction (resulting in preferential perfusion of the brain and heart); decreased cardiac output (with a maintained or increased stroke volume); and an increase in mean arterial blood pressure. These cardiovascular alterations act in a manner that conserves oxygen for the heart and brain. Because alcohol is often involved in water accidents with associated submersions, these initial experiments dealt with human volunteers who consumed alcohol to a blood level of 0.1 g% (legal level of intoxication). The diving reflex was elicited before and after alcohol consumption. Heart rate, blood pressure, and cardiac output were measured before, during, and after face immersion in 4 C water. In the sober state, immersion decreased heart rate, increased stroke volume, increased blood pressure, and increased total peripheral resistance. In comparison to the sober state, alcohol consumption increased pre-immersion heart rate, but did not significantly alter pre-immersion blood pressure or stroke volume. Following alcohol consumption, facial immersion decreased heart rate, but not to the same low rate as in the sober state. Alcohol consumption also evoked a lesser increase in blood pressure and failed to increase stroke volume significantly.

Human studies concerning thermal-induced shivering, postoperative "shivering," and cold-induced vasodilation.

Human reaction to cold stress and hypothermia involves shivering. Another form of overt shaking, postoperative shivering, has been attributed as a thermoregulatory response to postoperative hypothermia. Analysis of the normal human shivering pattern showed a synchronized, slow amplitude modulation (six to eight cycles/min) over all muscles sampled. In addition, there was a frequency of 8 to 10 Hz associated with each low-frequency amplitude modulation. EMG signals from postoperative patients revealed none of the major patterns seen in thermal-induced shivering. Cold-induced vasodilation also was studied and found to occur simultaneously in all cold-stressed fingers regardless of size or innervation. Thermal shivering and cold-induced vasodilation are considered to be manifestations of central neural oscillators.

Multicenter hypothermia survey.

A multicenter survey evaluated the clinical presentation, treatment, and outcome of accidental hypothermia. Data were collected from 13 emergency departments, with 401 of the 428 cases presenting during a two-year study period. Core temperatures ranged from 35 C to 15.6 C (mean, 30.57 C +/- 3.53) with 272 cases (63.6%) less than or equal to 32.2 C. There were no significant differences by age in presenting temperature, rewarming strategies, or mortality. The first hour rewarming rate was significantly (P less than .05) faster in the population less than or equal to 59 years (1.08 +/- 1.39 C/hr) than in those greater than or equal to 60 years (0.75 +/- 1.16 C/hr). Male core temperatures averaged 30.27 +/- 3.44 C versus female temperatures of 31.1 +/- 3.61 C. There were no clinically significant differences in male (N = 296) versus female (N = 132) profiles. High ethanol levels (315 to 800 mg%) did not affect outcome. Nine of 27 (33%) patients who received CPR initiated in the field survived, versus six of 14 (43%) with CPR begun in the ED. The profile of the CPR versus non-CPR population differed significantly (P less than .05) in location (outdoors), initial temperature (24.8 +/- 3.77 C vs 30.94 +/- 3.12 C), third-hour rewarming rate (2.28 +/- 1.53 C vs 1.17 +/- 1.18 C/hr), and numerous laboratory parameters. Tracheal intubation was performed without incident in 117 cases, of which 97 were less than or equal to 32.2 C. There were 73 fatalities (17.1%). Of these, 84.9% (N = 62) were less than or equal to 32.2 C. Predisposing conditions in this group included "serious" illness (30), systemic infection (28), trauma (15), immersion (ten), frostbite (seven), and overdose (two). The initial pulse, hemoglobin, and first-hour rewarming rate was lower in the deceased population, while the potassium, urea nitrogen, creatinine, and phosphorus were elevated. Excluding treatment combinations, outcome with exclusive use of a single rewarming strategy was passive external rewarming, 14 deaths below 32.2 C, 13 above; active external rewarming, six deaths below 32.2 C, two above; active core rewarming, 38 deaths below 32.2 C, none above. Refinements of the American Heart Association's CPR standards in hypothermia and a Hypothermia Survival Index are proposed.

Pilot studies of vapor transfer through breathable outerwear by simulating sweating in the cold.

Claims that rainproof films allow the unimpeded evaporation of sweat have been examined by modifying a heat transfer method to include evaporation of water. This physical study simulates a casual hiker (2 mets) in 4 degrees C (40 degrees F) dry weather wearing clothing of 1.5-2.0 clo and sweating at 40 g X h-1. Goose down, wool, polyester, and polyolefin fills were combined with no protective cover, Gore-Tex, and polyurethane covers. No significant differences in insulation between dry (control) and wet (stabilized evaporation) occurred. Neither cover nor fill had significant effects on heat transfer. Total water evaporated (sweat produced) was affected only by the wool filler. A significantly greater amount of water was trapped by the natural fibers and the polyurethane cover. These data indicate that breathable rainwear is likely to have little effect on heat transfer, even though it allowed 89% of sweat to evaporate compared to 51% for the polyurethane raincoat.

Co-culture of rat trigeminal ganglion neurons and corneal epithelium.

Corneal epithelium and the trigeminal ganglion neurons which normally innervate the epithelium have been grown in adjacent chambers of a 35 mm tissue culture plate. Dissociated nerve cells from late embryonic rats were plated inside an 8 mm cloning cylinder attached to the center of the culture plate by silicone grease. In 7-10 days neurites extended out of this inner chamber by growing through the grease seal and along parallel scratches in the collagen coating of the tissue culture plate. Once this occurred, pure corneal epithelial explants were isolated from young adult rats and plated in the area surrounding the cloning cylinder, i.e. in the outer chamber. Cultures were monitored regularly with phase microscopy and, at various times, were fixed for ultrastructural examination. Within 24-48 hours of the epithelial plating, there were both individual neurites and bundles of neurites in contact with the epithelium. This interaction increased substantially over the next few days. Growth cones of the neurites could be seen to approach the microvilli-covered surface of the epithelium, travel over the surface and penetrate between the epithelial cells. This tissue culture model of the innervated ocular surface may prove valuable in the study of a variety of ocular conditions or diseases, as well as provide a means to study functional relationships and mechanisms of cellular interaction between neurons and their target cells.

The effect of ethanol and temperature on calcium-dependent sensory neuron action potentials.

The ability of ethanol to decrease the duration of sensory neuron action potentials was evaluated at different temperatures. Intracellular stimulation and recordings were made as the drug was perfused onto dorsal root ganglion neurons in vitro. The cells were bathed in a medium which caused the evoked action potentials to have long durations which were calcium dependent. Ethanol decreased the duration of the action potentials in a dose-dependent manner, without altering amplitude, rate of rise, or resting potential. Furthermore, the potency of the drug varied directly with the temperature of the bathing medium. The recovery time for the reversal of ethanol's effect was also found to be temperature dependent. However, moderate temperature changes alone did not significantly alter the action potential waveform. These data are compatible with a boundary lipid or direct effect on the calcium channel for the mechanism of ethanol's action.

The effects of ethanol on the electrophysiology of calcium channels.

Acute ethanol intoxication affects many systems in the body, especially the central nervous system. Because early experiments using axonal preparations required very high concentrations of ethanol to produce ionic current alterations, researchers turned their attention away from specific effects on electrogenesis and looked for effects at the synapse. The role of Ca2+ in the release of neurotransmitters was well known and was considered a possible site of action for ethanol. Indeed, several studies demonstrated that ethanol alters Ca2+ binding or transport in synaptosomes and neural tissue. The purpose of this chapter is to present electrophysiological evidence for the acute effects of ethanol on calcium channels. It is necessary first to define the relevant ethanol concentrations and to describe the characteristics of tissue preparations that may best help to determine the effects of ethanol. A discussion of these two points along with a brief synopsis of the role of Ca2+ in excitable tissues is presented. This is followed by a discussion of the effects of ethanol on Ca2+ and Ca2+-activated conductances in both nonmammalian and mammalian cells, and a model is presented in an attempt to unify the experimental evidence of the acute effects of ethanol.

A decrease in clonus amplitude by topical anesthesia.

Skin desensitization by topical anesthetic was studied for its effects on the frequency and amplitude of 3 overt involuntary oscillations of the ankle: pathological clonus, physiological clonus, and physiological action tremor. Pathological clonus was studied in two spinal cord patients, physiological clonus in 3 normal subjects following prolonged exercise, and physiological action tremor in 7 normal subjects performing voluntary movements. Topical anesthetic applied to the skin of the leg, ankle and proximal half of the foot resulted in a significant decrease in the amplitude of all 3 oscillations with no significant change in frequency. The effect of topical anesthetic on pathological clonus amplitude was so profound that neither patient could initiate clonus for approximately 30 min, despite repeated attempts to do so. While the mechanism determining the frequencies of these oscillations is a matter of considerable debate, the consistency in their response to topical anesthetic suggests that a common mechanism may exist. A possible clinical application is suggested. Topical anesthetic may be a non-invasive means of attenuating undesirable large amplitude tremors and clonus.

Characterization of rat corneal epithelium maintained in tissue culture.

Using Dispase-treated rat corneas, primary cultures of corneal epithelium have been established which are free of contaminating cell types. Cultures were maintained for periods up to 36 days and were monitored with morphological and electrophysiological methods. Phase and scanning electron microscopy revealed a mosaic of polygonal epithelial cells which migrated over the culture plate. Actively migrating cells at the periphery of the culture presented a complex border of ruffles and filopodia. Surface specializations of the epithelial cells, i.e. microvilli, reflected those seen in vivo. Transmission electron microscopy revealed many cytological features common to the intact cornea: bundles of tonofilaments, desmosomes between adjacent cells and glycocalyx-covered microvilli on all free surfaces. These features developed over time in culture. Membrane potentials of the cultured epithelial cells could be recorded intracellularly. Therefore, it appears that pure cultures of rat corneal epithelium which maintain the morphological characteristics of their in vivo counterparts can be grown for considerable periods of time and that their electrophysiological properties can be monitored. This system offers the possibility of studying, under the controlled conditions of tissue culture, a wide variety of factors which might influence the integrity of the corneal epithelium or alter its susceptibility to disease.