How does the physiology of an infant aged 6 months differ from that of an adult?

This is a standard question which is orientated towards basic science and which aims to test your understanding of the important physiological differences in paediatric patients.

Introduction

Anatomical and physiological differences between adults and children are most marked at birth and converge with increasing age. At 6 months of age the physiological differences are best described by reference to the individual systems.

Surface area to mass ratio

• The smaller the child the larger is the ratio of surface area to mass. This is probably the most important single factor and it explains a number of the physiological differences.

• Alarge surface area is associated with increased heat loss, and all infants are at greater risk than adults or larger children of hypothermia.

• The need to maintain body temperature via heat production results in a higher metabolic rate and higher tissue oxygen demand. Desaturation occurs quicker.

• The higher BMR is associated with higher resting heart and respiratory rates.

Cardiovascular

• In the cardiovascular system the capacity of the heart to increase cardiac output by increasing stroke volume is limited. Cardiac output increases predominantly by increasing heart rate.

• The limbs are smaller in relation to the body. This means that there is less blood volume to mobilise from the periphery in response to hypovolaemia. Infants have less reserve.