Hypoxia produces tachycardia in the older infant and child. Anxiety and a fever will also contribute to tachycardia making this a non-specific sign. Severe or prolonged hypoxia leads to bradycardia. This is a pre-terminal sign.
Hypoxia (via catecholamine release) produces vasoconstriction and skin pallor. Cyanosis is a late and pre-terminal sign of hypoxia. By the time central cyanosis is visible in acute respiratory disease, the patient is close to respiratory arrest. In the anaemic child cyanosis may never be visible despite profound hypoxia. A few children will be cyanosed because of cyanotic heart disease. Their cyanosis will be largely unchanged by oxygen therapy.
The hypoxic or hypercapnic child will be agitated and/or drowsy. Gradually drowsiness increases and eventually consciousness is lost. These extremely useful and important signs are often more difficult to detect in small infants. The parents may say that the infant is just "not himself". The doctor must assess the child's state of alertness by gaining eye contact, and noting the response to voice and, if necessary, to painful stimuli. A generalised muscular hypotonia also accompanies hypoxic cerebral depression.
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