Effect of hypobaria on ventilatory and CO2 responses to short-term hypoxic exposure in cats

Use of high frequency oscillatory ventilation (HFOV) in neurocritical care patients

INTRODUCTION

Adult respiratory distress syndrome (ARDS) can be a common problem associated with the treatment of acute brain injury. High frequency oscillatory ventilation (HFOV) is a developing therapy for the treatment of ARDS in adult patients that can be life saving. However, often patients with acute, severe brain injury demonstrate intracranial hypertension (hICP) due to a variety of injuries (e.g., traumatic brain injury, mass lesion, acute hydrocephalus). There is concern over the use of HFOV due to its effects on intracranial pressure in patients with hICP.

METHODS

Retrospective case series study.

RESULTS

We describe the effects of HFOV on hemodynamics, respiratory function, and intracranial pressure in five patients with acute brain injury being treated for ARDS.

CONCLUSIONS

HFOV did not cause unmanageable or sustained increases in ICP in our series of patients. It appears HFOV may be a relatively safe and effective means of oxygenating patients with severe ARDS and concomitant hICP secondary to acute brain injury.

Cytochrome P450 enzyme-mediated drug metabolism at exposure to acute hypoxia (corresponding to an altitude of 4,500�m)

OBJECTIVE

To investigate the effect of acute hypoxia and concomitant changes in portal blood flow on the disposition of drugs mainly metabolized by the cytochrome P(450) enzymes (CYP) 3A4 (verapamil) and CYP1A2 (theophylline).

METHODS

Twenty healthy male participants were studied on two 14-h study days in a normobaric hypoxic chamber and were allocated randomly to one of two groups receiving short infusions of either theophylline (6 mg kg (-1) body weight) or verapamil (5 mg) intravenously. According to a randomized, cross-over design, participants were once exposed to normoxia and once to hypoxia (12% oxygen corresponding to the ambient( P)O(2) at an altitude of 4,500 m above sea level). The concentrations of theophylline, 1,3-dimethyluric acid, verapamil, and norverapamil were determined in serial blood samples by means of liquid chromatography-mass spectrometry (LC/MS/MS). Portal blood flow was assessed by transabdominal duplex ultrasonography.

RESULTS

Acute hypoxia did not alter the pharmacokinetics of theophylline [half-life+/-SD: 9.29+/-1.77 versus 9.39+/-1.40 (hypoxia)], 1,3-dimethyluric acid (12.9+/-4.72 versus 15.1+/-8.59), verapamil (2.00+/-0.98 versus 1.79+/-0.58), or norverapamil (7.98+/-2.94 versus 9.91+/-6.40). Individual changes of elimination half-life and changes in capillary oxygen saturation,( P)O(2), or portal vein flow were not correlated. Portal vein flow was unaffected by hypoxia.

CONCLUSIONS

Acute hypoxia corresponding to hypoxia at altitudes of 4,500 m does not impair the metabolism mediated by CYP1A2 or CYP3A4. At rapid ascent to and short-term stay at altitudes up to 4,500 m, the doses of drugs metabolized by these CYPs do therefore not require dose modification, and major changes in the disposition of already administered drugs are not to be expected.

Ventilation, EELV and diaphragmatic activity in rats during the early phase of normobaric hypoxia

We tested whether the enhancement of end-expiratory activity of the diaphragm (DE) induced by acute hypoxia persists during long-lasting hypoxia and participates in the enlargement of end-expiratory lung volume (EELV). We thus measured these two parameters together with ventilation (VE) in 30 rats, either awake or anesthetized, exposed to (1) poikilocapnic hypoxia sustained for 2 or 3 h; or (2) chronic normobaric hypoxia for 7 days interrupted by short episodes of normoxia. Twelve control animals were also studied. (1) Sustained hypoxia induced a stable increase in DE, VE and EELV. (2) In awake rats, chronic hypoxia induced a transient increase in VE after 1 day of hypoxia, and an increase persisting during acute normoxia throughout the exposure. DE followed the same, although less pronounced, course as VE. In anesthetized animals, only EELV was increased in both chronic hypoxia and acute normoxia, but its enlargement in normoxia was not associated with a concomitant increase in DE. The transition from hypoxia to normoxia always induced a decrease in DE and EELV. Therefore, (1) during hypoxia sustained for 2 or 3 h, the ventilatory and diaphragmatic responses were stable; (2) during chronic hypoxia lasting 1 week, a ventilatory acclimatization was expressed by a transient increase in hypoxic VE and a hyperventilation continuing during acute normoxia; (3) EELV enlargement in chronic hypoxia was partly related to changes in DE and partly due to another mechanism possibly involving morphological adaptations.

Ventilation, EELV and diaphragmatic activity in rats during chronic normobaric hypoxia

We determined the effects of chronic hypoxia on end-expiratory lung volume (EELV), end-expiratory diaphragmatic activity (DE) and ventilation (VE) in 27 intact (awake and anesthetized) and six carotid body-denervated (CBD; anesthetized) rats. Twenty-nine control animals were also studied. Recordings were made during hypoxia and normoxia before and after 2 or 3 weeks of hypoxia (+3 days of recovery from chronic hypoxia). In awake rats, 2 weeks of chronic hypoxia increased only normoxic VE, while 3 weeks of chronic hypoxia did not change VE or DE. In anesthetized intact rats, after both exposures, hypoxic and normoxic VE tended to decrease, DE did not change and hypoxic and normoxic EELV were enlarged. In CBD animals, 2 weeks of chronic hypoxia did not affect hypoxic VE but decreased normoxic ventilation and enlarged EELV similar to the intact animals. After 3 days of recovery in normoxia, all parameters except EELV were restored to prehypoxic values. Also, transition from hypoxia to normoxia induced parallel changes in EELV and DE while chronic hypoxia increased only EELV. Therefore, chronic normobaric hypoxia induced, (1) an increase in normoxic ventilation reflecting a process of acclimatization; (2) an enlargement of EELV that did not depend on changes in DE and carotid chemoreceptors.