Sat27November0445PM 28

Which one of the following is the most important immediate physiological change in response to acute hypoxia at altitude?

(Please select 1 option)

Increased renal bicarbonate excretion

Increased cardiac output Correct

Increased muscle capillary density

Raised intracranial pressure

Polycythaemia

Explanation

General changes upon acute exposure to altitude include:

A sudden increase in resting and submaximal heart rate

Increased resting and submaximal ventilation

Increased blood pressure

Increased catecholamine secretion, and

Decreased VO2 max.

These changes result in:

Increased oxygen transport to the tissues

Increased alveolar PO2 with a concomitant decrease in carbon dioxide (CO2) and hydrogen ions (H+)

Increased vascular resistance

Increased lactate production, and

Decreased work capacity.

The stroke volume progressively decreases over a period of a week (reduced plasma volume and left ventricular dysfunction secondary to hypoxia).

The sensors of a reduced PaO2 are the carotid bodies.

Acute exposure to lowered PaO2 and subsequent tissue hypoxia results in a number of haematological changes.

Haemoglobin (Hb) concentration and haematocrit (Hct) have been shown to increase within 24 hours of exposure to altitude. The stimulation of red blood cell (RBC) production occurs as the juxtaglomerular cells within the kidneys stimulate the release of erythropoietin (EPO). However, with a reduction in plasma volume and the lag between EPO secretion and new RBC production, the true initial increases in Hb and Hct actually occur after approximately three to four days of exposure.

In susceptible individuals, acute hypoxia causes cerebral vasodilatation and a rise in intracranial pressure. This can be associated with altitude mountain sickness (AMS) particularly if the ascent is rapid.

Increases in renal bicarbonate excretion in response to a respiratory alkalosis from hyperventilation are less acute than the cardiovascular changes (hours-days).

Increase in cardiac and skeletal muscle capillary density is a chronic change.

Answer Statistics

1

8%

2

86%

3

1%

4

3%

5

5%

Times answered: 256