Anesth Analg 2004;98:40-45
© 2004 International Anesthesia Research Society
CARDIOVASCULAR ANESTHESIA
Orthostatic Hypotension Occurs Frequently in the First Hour After Anesthesia
Dean A. Cowie, FANZCA*,
J. Kevin Shoemaker, PhD
, and
Adrian W. Gelb, FRCP*,
*Department of Anesthesia and Perioperative Medicine, London Health Sciences Centre, and The University of Western Ontario; and
School of Kinesiology, The University of Western Ontario, London, Ontario, Canada
Address correspondence and reprint requests to Adrian W. Gelb, Department of Anesthesia and Perioperative Medicine, LHSC-UC, 339 Windermere Rd., London, Ontario, N6A 5A5, Canada. Address e-mail to agelb{at}uwo.ca
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Abstract
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Symptoms of orthostatic intolerance are common after general anesthesia and are associated with an increased risk of postoperative morbidity. The contribution of orthostatic hypotension (OH) has not been well defined. We conducted a head-up tilt test on patients after general anesthesia for minor surgery to assess the incidence of and risk factors for OH after general anesthesia. One-hundred-four patients were enrolled and were prospectively divided into four groups: older female, older male, young female, and young male. The incidence of OH was 76.0%, 72.0%, 45.5%, and 62.5% respectively and was associated with increasing age (P < 0.05) and posttest dizziness (P < 0.05). Body mass index, preoperative blood pressure, ASA class, anesthetic duration, IV fluid administration, and use of analgesics and antiemetics in the postanesthetic care unit were not different in subjects who demonstrated OH compared with those with a normotensive response. Subjects with OH after general anesthesia did not increase their heart rate and diastolic blood pressure with a head-up tilt which may have been caused by persistent effects of anesthetics on reflex cardiovascular control and/or bedrest-induced dysregulation of reflex cardiovascular control. We conclude that OH is common after general anesthesia for minor surgery and may be the major cause of postoperative orthostatic intolerance.
IMPLICATIONS: Orthostatic hypotension, a failure to maintain blood pressure on assuming an upright posture, is common after general anesthesia for minor surgery and may be the major cause of postoperative orthostatic intolerance.
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Introduction
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Orthostatic intolerance (OI) is an inability to maintain an upright posture because of several symptoms that include light-headedness, altered vision, headaches, fatigue, and frank syncope. It can occur as an isolated condition or may be associated with other medical conditions such as diabetes mellitus and can also be induced in previously normal individuals by a period of bedrest or prolonged spaceflight (1). Subtypes of OI have been described. These are divided according to the blood pressure and heart rate response to the change to an upright posture, and include the classic neurocardiogenic response (syncope caused by sympathetic withdrawal and bradycardia), the dysautonomic response (an inability to mount a tachycardic response to a change in posture), and the postural tachycardia syndrome (a decrease in blood pressure despite a tachycardia) (2). A common mechanism of OI is the failure to maintain normal blood pressure on assuming the upright posture, a response termed orthostatic hypotension (OH). OH is defined by the American Autonomic Society as a decrease in systolic blood pressure of 20 mm Hg or a decrease in diastolic blood pressure of >10 mm Hg (3). Thus, OH is a response specifically related to reflex blood pressure regulation and is differentiated from the more general term of OI which can include nonhypotensive symptoms that are unrelated to cardiovascular control.
Symptoms of OI are common after general anesthesia and are associated with an increased risk of postoperative morbidity and readmission (4–7). However, the mechanism(s) leading to postoperative OI has not been extensively investigated, nor has the contribution of OH been defined. Therefore, the aim of the current study was to assess the incidence of OH after general anesthesia and to determine the role of risk factors such as age and gender and thereby gain some insight into the mechanisms of OI after general anesthesia. We hypothesized that there would be a significant relationship between postoperative OH and dizziness or nausea and that this is related to age and gender.
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Methods
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This study was approved by the Ethics Review Board for Health Sciences Research involving human subjects at The University of Western Ontario. Consenting patients aged 16 yr and older were enrolled if they had an ASA physical status score of I or II, and were undergoing an elective surgical procedure of expected duration of <3 h, where a general anesthetic technique was to be used. Patients with a history of diabetes mellitus, recurrent postural hypotension, or OI were excluded. The latter included patients who indicated that they had a history of a postural decrease in blood pressure, intermittently low blood pressure, or frequent postural dizziness. To study the impact of age and gender on the incidence of OH, the subjects were divided prospectively into four groups: females 40 yr of age or older (OF), males 40 yr or older (OM), females younger than 40 yr (YF), and males younger than 40 yr (YM).
IV access was secured preoperatively, and fluids were given according to the usual practice of the attending anesthesiologist who determined the choice of general anesthetic. No regional technique was permitted. The amount of administered IV fluid and the amount of blood loss was estimated. Postoperative analgesic and antiemetics were used at the discretion of the anesthetic and recovery staff to treat pain or nausea. Drugs were not given prophylactically. In each case their use was recorded. Subjects in whom intraoperative blood loss was >5 mL/kg, or who had uncontrolled pain, nausea, or vomiting 30 min after emergence were not subjected to a tilt study.
To examine the time course of postural responses after surgery, each patient was assessed at 2 study periods corresponding to 15 min (test A) and 45 min (test B) after emergence from anesthesia. Emergence was defined as the first time at which the subject could open their eyes and move a limb on command, and could correctly answer questions about their name and location. For each study period, the subject underwent a standardized 60° head-up tilt test. Subjects were monitored throughout the study period with a pulse oximeter, three-lead electrocardiogram (lead II), and oscillometric noninvasive blood pressure measurement (Spacelabs Medical, Toronto, Canada). After transfer to a dedicated manual tilt-table, the subject was asked to rate the presence of nausea, dizziness, and drowsiness on a four-point scale (none, mild, moderate, or severe). After baseline supine blood pressure, heart rate, and respiratory rate were determined, the table was tilted to 60° head-up over a period of 30 s. Blood pressure and heart rate were then recorded every minute during the tilt test. The tilt test was terminated at 10 min or sooner if nausea or dizziness (other than mild), weakness, tunnel vision, sweatiness, or hypotension were reported or observed. At the end of each tilt test, the patient was returned to a supine position and asked to give a semiquantitative assessment of their worst nausea, dizziness, and drowsiness for the period during which they were upright (none, mild, moderate, severe). In both pre- and post-tilt questioning, symptoms were considered significant if rated more than mild.
The primary end-point was the presence of OH as defined by a decrease in systolic blood pressure of >20 mm Hg and/or a decrease in diastolic blood pressure >10 mm Hg. Secondary end-points included a reported increase in nausea or dizziness on assuming the upright position.
The sample size was calculated to detect a 15 mm Hg difference in systolic blood pressure among groups. Group differences were analyzed for significance using either the 
2 test or a repeated measures analysis of variance test. In the latter case, Bonferroni’s correction was used when analyzing a significant interaction. A P value of < 0.05 was taken to be statistically significant. Statistical calculations were performed using the Statview statistical analysis package (SAS Institute Inc., Cary, NC). All steady-state values were given as mean ± SD, or number (percentage) as appropriate.
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Results
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One-hundred-four subjects were enrolled in the trial, with a mean age of 39 (range 16–74) yr. The characteristics of the patients are described in Table 1. Eight subjects were subsequently excluded and not subjected to a tilt-table test because of excessive postoperative nausea (n = 4), pain (n = 2), intraoperative blood loss (n = 1), or intraoperative cardiac arrest (n = 1). The older groups had a larger body mass index than the YF group (Table 1). The OM group had higher systolic and diastolic blood pressures on admission. There was no difference among groups with regard to anesthetic duration, IV fluid administration, or proportion of patients who required analgesics in the postanesthesia care unit. Individual group comparisons of antiemetic use revealed administration in a larger proportion of subjects in the YF group than the OM and YM groups (26% versus 4% and 4%; P < 0.05). There were no significant between group differences in the proportion of patients who were ASA II. However, when the older groups (OF and OM) were combined, the proportion of ASA II subjects was significantly larger than in the younger groups (YF and YM) combined (37% cf. 18%; P < 0.05).
Of the 96 tested participants, 9 did not complete a full 10 min of head-up tilt in test A (2 normal responders and 7 hypotensive individuals; 1 OF, 1 OM, 5 YF, 2 YM). For test B, 4 normal responders and 14 hypotensive individuals did not complete the 10 min of tilt (2 OF, 4 OM, 5 YF, 7 YM). The number of subjects remaining at each measurement time is given in Table 2. The overall incidence of OH in both tests was 65%, with an incidence of 49% for test A and 41% for test B. There was a trend to a less frequent incidence of OH in the YF group (Table 1). Fifty-seven patients (60%) for test A and 54 patients (58%) for test B had neither nausea nor dizziness. Demographic and perioperative variables for those with OH are compared with those with a normal blood pressure response in Table 3. Subjects with OH were older than those without OH (P < 0.05). In the group with OH, more patients experienced dizziness.
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Table 3. Demographics and Perioperative Factors: Subjects with Orthostatic Hypotension Versus Those with a Normal Response
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Supine heart rate, systolic, and diastolic blood pressure were not different between normal and hypotensive groups before test A or B (Table 4). The blood pressure and heart rate responses relative to baseline measurements of both groups for test A are given in Figures 1–3. In contrast to those individuals with normal responses, including maintained systolic blood pressure and increased diastolic blood pressure during tilt, the systolic blood pressures were lower in those with OH from 2–10 min of head-up tilt, and the diastolic blood pressures were lower from 1–10 min. The heart rate tended to be faster for subjects who had a normotensive response, and this reached significance by the end of the test (period A). The pattern of blood pressure and heart rate responses for normotensive and hypotensive groups was similar in the second test.
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Figure 1. Systolic blood pressure response in test A: normal subjects and those with orthostatic hypotension. *P < 0.05.
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Figure 2. Diastolic blood pressure response in test A: normal subjects and those with orthostatic hypotension. *P < 0.05.
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Figure 3. Heart rate response in test A: normal subjects and those with orthostatic hypotension. *P < 0.05.
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Discussion
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The relationship between general anesthesia and postoperative OH has not been extensively studied. The major findings of the current study are that postoperative OH is (a) evident in up to 65% of patients, (b) late in onset for some patients, and (c) associated with dizziness or nausea.
Many of the symptoms of OI are common after general anesthesia. In one study of 3722 ambulatory surgical patients, 20% reported dizziness after discharge from the day-stay unit (4) and in another its incidence was up to 25% depending on anesthetic technique (5). Our findings are consistent with this incidence. Although it is generally appreciated that frank hypotension is an important perioperative event (6), the impact of apparently benign symptoms such as light-headedness may be underestimated, as demonstrated by two prospective studies (7,8). Dizziness occurred in 1.3% of ambulatory surgical patients and was associated with a 31% increase in the length of postoperative stay (7). Furthermore, dizziness ranked behind only hypotension, nausea, and drowsiness as a cause of unanticipated postoperative admissions with a relative risk of 2.7 (8). In the majority of these cases, the dizziness did not correspond to hypotension as measured in the supine subject. However, none of these studies included postural blood pressure measurements. Our study has demonstrated a link between early postoperative dizziness and a postural decrease in blood pressure so that at least some cases of postoperative dizziness are attributed to OH.
The aim of this study was to determine the incidence of postoperative OH and to define the factors that may be modified to reduce the morbidity associated with perioperative dizziness. The only perioperative factor associated with OH in this study was age, with older patients more likely to be in the OH group. The incidence of OH dissipates somewhat over time so that 56% of older patients (groups OM and OF) had hypotension in test A but only 44% in test B and in the younger groups, 40% were hypotensive for test A and 33% for test B. There was a trend toward a larger proportion of older patients receiving analgesics in the postanesthesia care unit. This was not statistically significant and analgesic administration was not associated with OH (Table 3). Older patients were also more likely to be ASA II, but ASA status was not a significant predictor of OH. This is despite the fact that some of the ASA II patients were in this group because of treated hypertension and were thus likely to be on antihypertensive medications. Thus, older patients are at increased risk of early postoperative hypotension and dizziness and consideration should be given to protocol changes that allow for longer postoperative stays for those patients.
The mechanism of OH was not directly addressed in the current study. Intravascular volume may affect orthostatic blood pressure control; however, the subjects in the current study had on average been given a volume of IV fluid that has previously been reported to adequately rehydrate fasted ambulatory patients (9). There was also no difference in the amount of fluid given to patients with and without OH. Thus, although we cannot totally exclude intravascular volume depletion, it seems unlikely.
A recent study (10) demonstrated a correlation between preoperative OH and postoperative nausea and vomiting in women undergoing elective gynecological surgery. Because we did not perform preoperative orthostatic testing, we do not know whether those of our patients who developed postoperative OI also had it preoperatively. Other possibilities, which were not examined in this study, include positional vertigo and a subjective feeling of light-headedness caused by a reduction in cerebral blood flow secondary to hypocapnia (11) or unexplained vasoconstrictor factors (12).
An additional consideration is the impact of bedrest on normal reflex cardiovascular control. Previous studies have demonstrated that periods of bedrest are associated with measurable decrements in the sympathetic response to a tilt-table test (13). Prolonged bedrest is associated with a decrease in plasma aldosterone levels and a diuretic effect (14). However, this effect is unlikely to be important with the short periods of rest seen in this study. By contrast, as little as four hours of bedrest results in a decrease in stroke volume and an increased incidence of OI in response to a tilt test, indicating an alteration in cardiovascular homeostasis in the absence of changes to blood volume (15). Hence, even the brief period of bedrest that patients experience during day-stay surgery may contribute to postoperative OH. The mechanism(s) leading to this deconditioning effect on reflex cardiovascular control is an area of active research. Animal studies suggest that the deconditioning-induced autonomic dysregulation is linked to altered central processing of reflex afferent input and sympathetic discharge (16). In the current study, the heart rate and blood pressure responses were also indicative of abnormal autonomic function in the hypotensive group. Although similar in both groups while supine and early in tilt, there was little if any increase in heart rate more than supine by the end of the tilt period in the hypotensive patients. This postural bradycardia may have been caused by an increase in parasympathetic outflow to the heart rate that attends neurocardiogenic syncope (17,18). However, the small maximal increase in heart rate in both groups, and especially the hypotensive patients, was much different than a normal orthostatic response in healthy individuals or those with chronic OI. Specifically, an approximately 20-bpm (approximately 30%) increase in heart rate is observed with a 60° tilt protocol (13), even in chronically orthostatically intolerant individuals (17) and those after prolonged bedrest or after 14 days of bedrest (13). This is much larger than the 5% increase observed in the normal subjects of the current study. Therefore, it is unlikely that the current heart rate or blood pressure responses were entirely caused by an effect of the bedrest.
Therefore, it must be considered that residual effects of anesthetics contributed to the postoperative blood pressure decrements by interfering with normal cardiovascular homeostasis. By anesthetics, we mean all drugs given as part of the balanced anesthesia, including sedatives, IV and inhaled anesthetics and analgesics given during surgery. This study was not designed to explore the impact of different anesthetics on the incidence of OH. The anesthetic given was at the discretion of the attending practitioner, and hence it is not possible to comment on the impact of individual anesthetics. However, the majority of subjects underwent the outpatient procedure with spontaneous ventilation and sevoflurane through a laryngeal mask airway. Sevoflurane attenuates autonomic nervous system function (19). In one study (20), nonsurgical volunteers had baroreflexes remain depressed for at least 60 minutes after sevoflurane and isoflurane anesthesia whereas Tanaka and Nishikawa (21), studying surgical patients similar to ours, found that pressor but not the depressor responses had returned by 20 minutes after a sevoflurane anesthetic. Depressed baroreflex gain could account for the diminished heart rate response in both the normotensive and hypotensive groups. However, if the autonomic dysregulation noted in the hypotensive group of the current study was strongly related to the ongoing effects of the anesthetics, then the incidence of OH should be less in the second tilt test performed 30 minutes or more after the initial tilt test. This was not observed. Moreover, the anesthetic effect should be equally present at the beginning and end of a particular 10-minute test rather than increasing in effect with tilt duration. Therefore, it may be that the anesthetics affected the ability to sustain an autonomic defense during tilt. We have recently (22)advanced this same hypothesis for bedrested individuals who demonstrate a normal sympathetic and heart rate response to a 20-second Valsalva’s maneuver but develop progressive hypotension after 5 or more minutes of head-up tilt. It will be of interest to examine the differential and overlapping effects of bedrest per se versus anesthetic in determining the short- and long-term reflex autonomic responses.
In conclusion, OH was prevalent in this postoperative population and was associated with dizziness. The pathophysiology of postoperative OI involves a failure to increase the heart rate and diastolic blood pressure on assuming a head-up position which suggests an inadequate autonomic adjustment to baroreflex unloading probably caused by a combination of a period of bedrest and the residual effects of anesthesia.
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Acknowledgments
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This study was supported by the Natural Sciences and Engineering Research Council of Canada (JKS) and Department of Anesthesia Intramural Funds (DAC).
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Footnotes
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Present address for DAC is Austin & Repatriation Medical Centre, Studley Rd., Heidelberg VIC 3084 Australia.
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Accepted for publication August 14, 2003.
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Abstract
Introduction
