ALTITUDE AND COMMON MEDICAL CONDITIONS - Medicine for Mountaineering and Other Wilderness Activities - page 361

Carotid Surgery
Carotid body responsiveness to hypoxia is necessary for optimal acclimatization to alti-
tude,andalessenedresponsetohypoxiahasbeenlinkedtosusceptibilitytoaltitudeillness.
The carotid body is commonly damaged or ablated during carotid artery surgery (usually
for arteriosclerotic narrowing or occlusion). Investigative studies have found that the vent-
ilatory response to the hypoxia of 5000 feet (1500 m) observed prior to surgery was abol-
ishedbytheprocedure.Similarly,individualswithbilateralcarotidbodyresectionfortreat-
mentofasthmaoremphysemalackadequateventilatoryresponsestoacutealtitudehypox-
ia. However, although altitude acclimatization might be impaired in such persons, whether
therelative hypoventilation leadstoaltitude illnessremainstobedetermined. Nonetheless,
these individuals can be easily identified, and should be cautioned about altitude exposure.
Sleep Disordered Breathing and Apnea
Persons with abnormal breathing rhythms during sleep at sea level, such as snoring, ob-
structive apnea, or sleep disordered breathing (SDB), particularly the elderly, may have
greater nocturnal hypoxemia at altitude and a consequent increase in abnormal cardiac
rhythms,anincreasedpulmonaryarterypressure,andagreatersusceptibility toaltitudeill-
ness. A period of apnea at altitude would certainly produce greater hypoxemia than at sea
level. However, altitude-induced changes in ventilatory control and breathing conceivably
could improve certain apnea syndromes. Unfortunately, virtually nothing is known about
theinteractionofSDBwithaltitude,eventhoughSDBhasbeeninvokedinthepathogenes-
isofacutealtitudeillnesses,includinghigh-altitudepulmonaryedema.(Incontrast,normal
periodic breathing at altitude has not been related to development of acute mountain sick-
ness [AMS], and, in fact, is associated with a brisk hypoxic ventilatory response, which is
generally considered beneficial at altitude.)
At altitude individuals using a continuous positive airway pressure (CPAP) machine
without pressure-compensating features as treatment for SDB need to make adjustments
because altitude decreases the delivered pressure. The error is greater with higher altitudes
and higher initial pressure settings.
Lung Disorders
Impairmentofgasexchangebyhypoventilationleavesthoseafflictedwithalowerarter-
ial oxygen pressure than their healthy counterparts. Physiologically, individuals with lung
disease breathe as if they are already at a higher altitude and may suffer even greater ef-
fectswithascent.Inaddition,greaterhypoxemiamayexacerbate theirunderlyinglungdis-
ease—increasing pulmonary artery pressure by exaggerating hypoxic pulmonary vasocon-
striction, for instance.