If there’s one thing that most often misleads even experienced paramedics used to working with adults when it comes to a pediatric patient, it’s precisely the cardiovascular system. A child can maintain a normal blood pressure for a long time even in an advanced stage of shock — and it’s exactly this compensatory ability that can be dangerous if a paramedic unknowingly relies on it. This third installment of the PED series therefore covers two closely linked systems: circulatory and thermoregulatory, which together determine how quickly and in what way a child decompensates.

The cardiovascular system

Rate-dependent cardiac output

A child’s stroke volume is relatively fixed — the myocardium has a limited ability to increase contractility and stretch according to the Frank-Starling mechanism compared to an adult heart. As a result, cardiac output in children is significantly dependent on heart rate. While an adult organism can respond to a drop in volume by increasing contractility, a child responds primarily with tachycardia — this is why tachycardia is such a sensitive (though nonspecific) early indicator of shock in children.

Compensated and decompensated shock

Two clinical stages are distinguished in children:

• Compensated shock: tachycardia, prolonged capillary refill (> 2 s), cold acral parts, weaker peripheral pulse compared to central — with a preserved normal systolic pressure thanks to increased systemic vascular resistance.
• Decompensated shock: hypotension and/or bradycardia (late signs), altered consciousness, weakened to nonpalpable peripheral pulses — a state immediately preceding circulatory arrest.

Reference circulating blood volume

Even a relatively small absolute blood loss can therefore represent a large percentage of total volume in an infant — one reason bleeding must be assessed more strictly in young children than the absolute amount might suggest.

Thermoregulation

A child — particularly a newborn and infant — loses heat significantly faster than an adult, for several concurrent reasons:

• Larger body surface area relative to weight: the surface-to-volume ratio decreases with age, so a small child loses heat through radiation and evaporation faster per kilogram of body weight.
• Thinner layer of subcutaneous fat: poorer insulation against cold compared to an adult.
• Immature thermoregulation: a newborn can’t shiver effectively and relies on non-shivering thermogenesis through brown adipose tissue, which is energy-intensive and has limited capacity.
• Large head: in an infant it represents a disproportionately large share of body surface area — and therefore also a significant source of heat loss if not covered.

Practical implications for the paramedic

Connection to the next part of the series

The first three parts of the PED series laid the anatomical-physiological foundation. In PED-04, we’ll move to practical examination — the Pediatric Assessment Triangle (PAT) as a rapid initial-assessment tool that builds precisely on the principles described in the previous installments.