Book Text

Anesthesia at Altitude

General Principles

Because of the reduced ambient Po₂ at high altitudes, the risk of perioperative hypoxia is likely to be magnified, particularly in newcomers to altitude. Opiates depress both the tachycardia and hyperpnea that normally occur in response to acute hypoxemia. Slow recovery of consciousness and postanesthetic headache have been reported after anesthesia consisting of thiopental with air or nitrous oxide.^[268] When ventilation was assisted or controlled during the anesthetic and when postoperative supplemental O₂ was administered, these complications did not occur, which suggests that intraoperative or postoperative hypoxemia may be a pathophysiologic factor. On the other hand, long-term residents at altitude may be more tolerant of hypoxemia; in such individuals, other considerations may be important, including the higher baseline hematocrit, pulmonary hypertension, and lower PaCO₂ and bicarbonate concentrations.^[214] ^[269] To prevent renal retention of bicarbonate, which will reduce the ventilatory drive, patients requiring controlled ventilation should have their PaCO₂ maintained at the baseline value rather than in the traditionally normal range. Similarly, maintenance of PaO₂ within the sea-level normal range will result in loss of adaptation and difficulty in readapting the patient to breathing room air.

Increased oozing of blood from surgical wounds at altitude has been reported.^[270] This effect has been attributed to higher venous pressure and blood volume, vasodilatation, and increased capillary density. Camporesi^[271] has reviewed the topic of anesthesia at both high and low ambient pressure.

Anesthesia Equipment (also see Chapter 9 )

James and White^[272] tested Fluotec Mark II and Dräger halothane vaporizers at sea level and at altitudes of 5000 ft

2692

(1524 m) and 10,000 ft (3048 m). At any given setting, the delivered percentage of halothane increased with altitude; however, its partial pressure remained constant. Therefore, when these devices are used at a given vaporizer setting, anesthetic will be delivered at a constant potency regardless of altitude.

The same authors^[272] also examined the effect of altitude on floating-bobbin or floating-ball gas flow meters. At a simulated altitude of 10,000 ft (3048 m), both nitrous oxide and O₂ flow meters under-read the actual flow rate. The percentage of error progressively increased up to 4 L/min, at which point both flow meters were approximately 20% in error. A hazard may therefore arise when a low flow of O₂ is mixed with a higher flow of nitrous oxide. Unless an O₂ analyzer is available, the delivered percentage of O₂ may be significantly lower than that calculated on the basis of the flow meter readings.

Venturi-type gas-mixing devices tend to deliver higher concentrations of O₂ at altitude than they do at sea level^[272] ; at an altitude of 10,000 ft (3048 m), a mask designed to deliver 35% O₂ at sea level actually delivered 41% O₂ .

General Anesthesia

The potency of anesthetic gases is proportional to their partial pressure. Therefore, as barometric pressure is reduced, fixed concentrations of inhaled anesthetics will have lower potency. Cleaton-Jones and coauthors^[273] reported insignificant differences in the effect of nitrous oxide at sea level and an altitude of 1700 m. On the other hand, at an altitude of 3300 m, James and colleagues ^[274] showed a significant reduction in the efficacy of 50% nitrous oxide in reducing the pain threshold in normal volunteers ( Table 70-9 ). Because of this reduced effectiveness and the errors in administered gas concentration, which can occur as a result of altered gas flow meter performance (see later), in the absence of accurate inspired gas monitoring, the risks associated with the use of nitrous oxide at altitudes higher than about 2000 m may exceed the benefit.

Because supplemental O₂ may not be available in some mountain locations, it may be imperative to choose an anesthetic technique that is least likely to suppress ventilation. Pederson and Benumof^[275] have reported using ketamine anesthesia with spontaneous ventilation in 23 patients requiring general anesthesia at an altitude of 1830 m. Significant, though brief desaturation developed in two patients, but it resolved quickly. The authors

TABLE 70-9 -- Effect of 50% nitrous oxide on pain threshold in normal volunteers at altitude*

Altitude (m) Barometric Pressure (mm Hg) Increase in Pain Threshold (%) Side Effects

0 760 71.5 3 subjects, nausea; 2 subjects, semicomatose

1460 636 40.0 None

3300 517 19.0 None

Data from James MFM, Manson EDM, Dennett JE: Nitrous oxide analgesia and altitude. Anaesthesia 37:285, 1982.

*Pain threshold was measured in 20 subjects at each altitude by applying pressure to the anterior aspects of the tibia with a spring balance. Simulated altitude exposures were in a hyperbaric chamber. At increasing altitudes, a fixed concentration of N₂ O is progressively less effective.

**TABLE 70-9** -- Effect of 50% nitrous oxide on pain threshold in normal volunteers at altitude*
Altitude (m)	Barometric Pressure (mm Hg)	Increase in Pain Threshold (%)	Side Effects
0	760	71.5	3 subjects, nausea; 2 subjects, semicomatose
1460	636	40.0	None
3300	517	19.0	None
Data from James MFM, Manson EDM, Dennett JE: Nitrous oxide analgesia and altitude. Anaesthesia 37:285, 1982.

concluded that this form of anesthesia is a practical method in a rural hospital setting at altitude without access to supplemental O₂ . A consecutive series of 11 patients (American Association of Anesthesiologists [ASA] physical status I to II) requiring general anesthesia for short procedures at Kunde Hospital ( Fig. 70-17 ) in Nepal (3840 m) all received ketamine (total dose, 1 to 6 mg/kg) after intravenous premedication with atropine, 0.02 mg/kg, and midazolam, 0.05 mg/kg. ^[276] Nine patients required reduction of dislocations or fractures, one (9 years old) required suturing of a facial laceration, and one needed drainage of an abscess. The induction dose of ketamine was 1 mg/kg intravenously administered over a period of 1 to 2 minutes, followed by additional incremental doses titrated to effect. Baseline SpO₂ on room air was 86% to 91%. Oxygen was administered in 3 of 11 cases by means of an O₂ concentrator if SpO₂ fell below 80% for 1 minute and was not corrected by a forward jaw thrust or vocal and tactile stimulation. Both low-altitude residents required supplemental O₂ for 20 minutes during recovery, whereas only one of nine high-altitude residents required O₂ . Nunn reported successful induction of general anesthesia with halothane in a hypobaric chamber at a barometric pressure of 375 mm Hg (equivalent altitude, 5490 m).^[277] A standard halothane vaporizer (Fluotec Mark 2) with an inspired O₂ concentration of 60% was used. Recovery was rapid and uneventful.

Regional Anesthesia (see Chapter 43 and Chapter 44 )

Spinal anesthesia has been reported to result in an unacceptably high incidence of post-dural puncture headache.^[268] Possible causes include chronically increased CSF pressure, dehydration, and altered sensitivity of the high-altitude resident's CNS to intracranial pressure changes. ^[268] Headaches were not reported in 20 young native mountain dwellers in whom Severinghaus and coworkers obtained CSF samples with 25-gauge needles.^[215]

Perioperative Supplemental Oxygen

At high altitude, alveolar PO₂ is normally increased by hyperventilation induced by the hypoxic ventilatory drive. Administration of anesthetics or narcotic analgesics, both of which blunt the hypoxic ventilatory drive, may therefore precipitate hypoxia. The resulting hypoxic symptoms of irritability, confusion, and restlessness may be misinterpreted as being due to pain, and misguided

2693

Figure 70-17 Surgical procedure under general anesthesia with ketamine at Kunde Hospital, Nepal (altitude, 3840 m), the site of the published series of general anesthetics described in the text.^[276] The patient is spontaneously breathing room air. The photo depicts Dr. David Murdoch assisted by Kami Temba Sherpa, now the physician in charge of Kunde Hospital. (Courtesy of Dr. Lynley Cook.)

administration of additional narcotics would only compound the problem. Nunn considered analgesic-induced respiratory depression the probable cause of death in a Sherpa who underwent débridement of frostbitten fingers at an altitude of 4300 m. ^[277] Therefore, if opiate analgesics are administered, supplemental O₂ may be required. At sea level, treatment of insomnia in healthy people with sedative/hypnotic drugs has few complications in the short term.