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Physiology

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Respiration

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A 32 years old mountaineer has hematocrit of 60. He is likely to have: (AIPG 2011)



A Progressive hemodilution

B High altitude pulmonary edema

C High altitude cerebral edema

D Polycythemia with possible dehydration

Ans. D Polycythemia with possible dehydration.

I. Hematocrit levels >50% in men or >45% in women may be abnormal. Hematocrits >60% in men and >55% in women are almost invariably associated with an increased red cell mass.

II. Polycythemia can be spurious (related to a decrease in plasma volume; Gaisbock’s syndrome), primary, or secondary in origin.

III. The secondary causes are all associated with increases in EPO levels: either a physiologically adapted appropriate elevation based on tissue hypoxia (lung disease, high altitude, CO poisoning, high-affinity hemoglobin- opathy) or an abnormal overproduction (renal cysts, renal artery stenosis, tumors with ectopic EPO production).

IV. A rare familial form of polycythemia is associated with normal EPO levels but hyperresponsive EPO receptors due to mutations.

V. Hypoxia is the principal stimulus for causing an increase in red blood cell production.

VI. Ordinarily, when a person remains exposed to low oxygen for weeks at a time, the hematocrit rises slowly from a normal value of 40 to 45 to an average of about 60, with an average increase in whole blood hemoglobin concentration from normal of 15 g/dl to about 20 g/dl.

VII. In addition, the blood volume also increases, often by 20 to 30 per cent, and this increase times the increased blood hemoglobin concentration gives an increase in total body hemoglobin of 50 or more per cent.

Chronic mountain sickness or Monge disease

a. An increase in RBC production and resulting polycythemia are normal responses to high altitude. However, some high-altitude residents become severely symptomatic as a result of excessive polycythemia. This condition has been called chronic mountain sickness, or Monge disease.

b. Symptoms of Monge disease range from diminished mental and physical capacity to headaches, personality changes, unconsciousness, and coma.

c. Healthy men at 4540 m (14,900 ft) have hematocrits of approximately 60% and hemoglobin values of 19 g/dL.

d. Patients with Monge disease may have hematocrits as high as 84% and hemoglobin concentrations of 28 g/dL.

e. Normal arterial saturation at that altitude (ie, 81%) may fall as low as 60% in affected individuals.

ADAPTATION TO HIGH ALTITUDE

a. Partial pressure of oxygen falls with increasing altitude

b. Results in acute / chronic hypoxia with adaptation

Acute

a. Alveolar PO2 needs to fall to 60mmHg and below to stimulate ventilation via the carotid body *

b. As ventilation increases, CO2 is lost and the fall in PCO2 reduces respiratory drive from the central chemoreceptors, off-setting the carotid body response *

c. Carotid bodies also elicit reflex vasoconstriction with increased heart rate and cardiac out-put. Blood is diverted from the skin and splanchnic circulations to vital organs *

d. The fall in PCO2 increases the ability of Hb to bind oxygen (reverse Bohr effect) – resulting in a higher Hb saturation for a given alveolar PO2 *

e. Mountain sickness is characterised by headache, nausea, giddiness, GI disturbance, fatigue and impaired mental function.

f. Sleep apnoea occurs at altitudes > 4000m and pulmonary oedema may occur above 3000m.

Chronic

a. Increased minute respiratory volume – the respiratory alkalosis caused by hyperventilation is compensated by excretion of bicarbonate by the kidneys. Plasma pH is restored to normal within 1 week of ascent.

b. Increased red cell mass – stimulated by erythropoietin secreted by the kidneys in response to low PO2.

c. Increased production of 2,3-diphosphoglycerate by red cells resulting in a right shift of the oxygen dissociation curve.

d. Increased cardiac out-put.

e. Increased vascularization of tissues.

f. Relative polycythemia is an apparent rise of the erythrocyte level in the blood; however, the underlying cause is reduced blood plasma.

g. Relative polycythemia is often caused by loss of body fluids, such as through burns, dehydration and stress. Rarely, relative polycythemia can be caused by apparent polycythemia also known as Gaisböck's syndrome.

h. Apparent polycythemia primarily affects middle-aged obese men and is associated with smoking, increased alcohol intake and hypertension.

Respiration Flashcard List

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