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8 out of 9

Hypoxia is if PaO2 is less than ------ mmHg? (LQ)

A 40

B 45

C 50

D 60

Ans. D



1). Hypoxia is defined as a paO2 <60mm Hg.

2). It is a parameter depends upon Ventilatory efficiency:

a. Normal paCO2 is 40 to 45 mmHg Q

b. PaCO2 is directly related to alveolar ventilation.

c. A paCO2 <40 mmHg indicates hyperventilation

d. PaCO2 > 45 mmHg indicates hypoventilation.

Causes of hypoxemia -

The four basic, mechanisms of hypoxemia are -

1). Decrease in an inspired PO2 (FiO2)

2). Hypoventilation

a. Decreased respiratory drive

b. Neuromuscular disease (Myasthenia Gravis)

c. Drug overdose causes respiratory depression resulting in hypoxemia.

3). Shunt

a. Alveolar collapse (atelectasis)

b. Intra cardiac shunt

c. Intra alveolar filling (pneumonia)

d. Vascular shunt within lungs

4). V/Q Mismatch

a. Airway disease (asthma, COPD)

b. Alveolar disease

c. Interstitial lung disease

d. Pulmonary vascular disease (pulmonary embolism)

Extra Edge: Alveolar Gas Equation

1). Alveolar-arterial O2 concentration gradient: This is the difference in th O2 partial pressure between the alveolar and arterial sides.

2). This gradient is normal Q in type II respiratory failure.

3). The gradient is increased in type I respiratory failure.

4). If the gas exchange at alveolar capillary levels occurs normally, the alveolar arterial difference in PaO2 is not elevated.

5). In infiltrative lung disease the alveolar capillary bed is damaged and there is a decrease in diffusion capacity.

6). This results in a increase in alveolar arterial differences in PaO2.

Sign / Symptom of Hypoxia

1). Dyspnea

2). Agitation
3). Confusion

4). Central cyanosis.

If longstanding hypoxia'

1). Polycythemia

2). Pulmonary hypertension

3). Cor pulmonale.


1). Headache;

2). peripheral vasodilatation;

3). tachycardia;

4). bounding pulse,

5). tremor/flap;

6). papilledema;

7). Drowsiness;

8). Coma.

Extra Edge:

Management depends on the cause:

1). Type I respiratory failure:

a. Treat underlying cause.

b. Give high flow oxygen Q

2). Type II respiratory failure the respiratory centre may be relatively insensitive to CO2 and respiration could be driven by hypoxia.

a. Oxygen therapy should be given with care. Give low flow Q oxygen

b. Treat underlying cause.

c. Assisted ventilation