Hypovolaemic shock

Shock is commonly defined as, 'the life-threatening failure of adequate oxygen delivery to the tissues and may be caused by decreased blood perfusion of tissues, inadequate blood oxygen saturation, or increased oxygen demand from the tissues that results in decreased end-organ oxygenation and dysfunction' (Alexiou and Rau, 2022).

If left untreated, shock results in sustained multiple organ dysfunction and end-organ damage with possible death. Tissue hypoperfusion may be present without systemic hypotension, but at the bedside shock is commonly diagnosed when both arterial hypotension and organ dysfunction are present (Alexiou and Rau, 2022).

Hypovolaemic shock (hypo=low, vol=volume and anaemic=blood) is characterised by a loss of intravascular volume of 15% or more, leading to inadequate perfusion of the tissues (Peate, 2020).

Shock is generally classified according to its cause. There are four main pathological mechanisms that can result in a state of shock:

1. Hypovolaemia: loss of intravascular volume from internal or external fluid loss

2. Cardiogenic: pump failure

3. Obstruction: barriers to cardiac filling or circulatory flow

4. Distributive shock: as a result of poor or abnormal vasoregulation and loss of vascular tone (Vincent and De Backer, 2013; Alexiou and Rau, 2022)

Hypovolaemic shock occurs when the volume of the circulatory system is too depleted to allow adequate circulation to the tissues (Tidy, 2022). Patients with hypovolaemic shock have severe hypovolaemia with decreased peripheral perfusion. If left untreated, ischaemic injury of vital organs can occur, leading to multi-system organ failure. The first factor to be considered is whether the hypovolaemic shock has resulted from haemorrhage or fluid losses as this will dictate treatment.

Symptoms of hypovolaemic shock can be related to volume depletion, electrolyte imbalances or acid base disorders that accompany hypovolaemic shock. Patients with volume depletion may experience thirst, muscle cramps and/or orthostatic hypotension (decrease in systolic blood pressure of 20 mmHg or decrease in diastolic blood pressure of 10 mmHg within 3 minutes of standing compared to a sitting or supine blood pressure).

In severe hypovolaemic shock, patients can experience abdominal or chest pain caused by mesenteric and coronary ischaemia. Brain malperfusion can cause agitation, lethargy or confusion.

Physical assessments as a result of volume depletion may find:

• dry mucous membranes
• decreased skin elasticity
• low jugular venous distention
• tachycardia
• hypotension
• decreased urinary output (Annane et al, 2013)

Patients may also appear cold, clammy and cyanotic.

The clinical features of hypovolaemic shock are:

• weak pulse, tachycardia, tachypnoea
• wold, clammy extremities, poor capillary refill
• hypotension with narrow pulse pressure in the decompensated stage

Common investigations for hypovolaemic shock are shown in Table 1.

A healthy adult can withstand the loss of 0.5 litre of fluid from a circulation of about 5 litres without ill effect (Tidy, 2022). However, larger volumes and rapid loss cause progressively greater problems. Risk of shock is related to the degree of hypovolaemia and the speed of correction. In children and young adults, tachycardia is one of the earliest signs of hypovolaemia as the circulatory system is better able to cope with the rigours of loss. The risk of morbidity and mortality is much greater as age increases because older people often do not tolerate having low blood volume (Tidy, 2022). Pathophysiology in the cardiovascular, respiratory and renal systems increases risk.