Effects of acetazolamide on cerebrovascular function and breathing stability at 5050 m

Abstract

Key points• Acetazolamide improves breathing stability during sleep in newcomers to high altitude, but the mechanism remains unclear.• We examined the effects of a single I.V. dose of acetazolamide on brain vascular function and breathing at sea level and following 7 days at high altitude (5050 m).• We demonstrated that acute I.V. acetazolamide at high altitude enhances the brain blood flow response to changes in CO 2 and improves breathing stability.• We speculate that the enhanced brain blood flow responses following acetazolamide ingestion may account for the well-documented acetazolamide-induced improvement in abnormal breathing at high altitude.Abstract One of the many actions of the carbonic anhydrase inhibitor, acetazolamide (ACZ), is to accelerate acclimatisation and reduce periodic breathing during sleep. The mechanism(s) by which ACZ may improve breathing stability, especially at high altitude, remain unclear. We tested the hypothesis that acute I.V. ACZ would enhance cerebrovascular reactivity to CO 2 at altitude, and thereby lower ventilatory drive and improve breathing stability during wakefulness. We measured arterial blood gases, minute ventilation (V E ) and middle cerebral artery blood flow velocity (MCAv) before and 30 min following ACZ administration (I.V. 10 mg kg −1 ) in 12 healthy participants at sea level and following partial acclimatisation to altitude (5050 m). Measures were made at rest and during changes in end-tidal P CO 2 and P O 2 (isocapnic hypoxia). At sea level, ACZ increased resting MCAv and its reactivity to both hypocapnia and hypercapnia (P < 0.05), and lowered restingV E , arterial O 2 saturation (S a,O 2 ) and arterial P O 2 (P a,O 2 ) (P < 0.05); arterial P CO 2 (P a,CO 2 ) was unaltered (P > 0.05). At altitude, ACZ also increased resting MCAv and its reactivity to both hypocapnia and hypercapnia (resting MCAv and hypocapnia reactivity to a greater extent than at sea level). Moreover, ACZ at altitude elevated P a,CO 2 and again lowered resting P a,O 2 and S a,O 2 (P < 0.05). Although theV E sensitivity to hypercapnia or isocapnic hypoxia was unaltered following ACZ at both sea level and altitude (P > 0.05), breathing stability at altitude was improved (e.g. lower incidence of ventilatory oscillations and variability of tidal volume; P < 0.05). Our data indicate that I.V. ACZ elevates cerebrovascular reactivity and improves breathing stability at altitude, independent of changes in peripheral or central chemoreflex sensitivities. We speculate that P a,CO 2 -mediated elevations in cerebral perfusion and an enhanced cerebrovascular reactivity may partly account for the improved breathing stability following ACZ at high altitude.

AltitudeOmics: enhanced cerebrovascular reactivity and ventilatory response to CO2 with high-altitude acclimatization and reexposure

“…Our findings extend those from Fan et al (16,17) by demonstrating that the MCAv-CO 2 slope is elevated upon arrival at 5,260 m and further elevated following 16 days of acclimatization (Fig. 1A).…”

“…The details of the modified rebreathing method have been previously described in Fan al. (16,17). The rebreathing bag was filled with gas to achieve inspired PCO 2 and PO2 of 0 mmHg and 300 mmHg, respectively, at each altitude.…”

“…It is difficult to interpret the findings from these studies due to the timing of measurements at high altitude (1,16,17,24,25), the confounding effects of previous high altitude exposure (1), artificial normobaric hypoxia (28, 46), and the method used to assess reactivity (24,30,49). Data obtained by Fan et al (16,17) on subjects at different stages of altitude acclimatization suggest that cerebrovascular CO 2 reactivity is elevated with prolonged exposure to high altitude when using a modified rebreathing technique. In contrast, Lucas et al (30) reported, using a steady-state technique (poikilocapnic hypoxia), reduced cerebrovascular CO 2 reactivity in the same subjects assessed at the end of a 14-day stay at 5,050 m. More recently, Rupp et al (49) reported a reduced cerebrovascular CO 2 reactivity during steady-state hypoxic hypercapnia following 5 days at 4,350 m. Thus the effect of altitude acclimatization on cerebrovascular CO 2 reactivity remains unclear.…”

“…To date, only a handful of studies have investigated the effect of acclimatization to high altitude on cerebrovascular CO 2 reactivity (1,16,17,24,30,49). It is difficult to interpret the findings from these studies due to the timing of measurements at high altitude (1,16,17,24,25), the confounding effects of previous high altitude exposure (1), artificial normobaric hypoxia (28,46), and the method used to assess reactivity (24,30,49). Data obtained by Fan et al (16,17) on subjects at different stages of altitude acclimatization suggest that cerebrovascular CO 2 reactivity is elevated with prolonged exposure to high altitude when using a modified rebreathing technique.…”

Sightings edited by John W. Severinghaus

“…A few seconds after leaving the capillaries, blood reaches equilibrium with PaCO 2 higher than PetCO 2 . Fan et al (2012) tested whether ACZ (10 mg/kg IV) increased the sensitivity of middle cerebral artery blood flow velocity (MCAv) to changing PaCO 2 in 12 normal subjects at 5050 m altitude. They thought this could lower entilator drive and improve breathing stability during wakefulness, unlikely in my opinion since ACZ increases ventilation.…”