The distribution of IHCA usually depends upon the hospital’s proportion of intensive care unit (ICU) and high-dependency care unit (HDU) beds. These beds are usually equipped with continuous or automated patient vital signs monitoring. Hospitals with a high proportion of ICU and HDU beds tend to have a greater proportion of cardiac arrests occurring in monitored patients. In hospitals with few such beds, most arrests occur in unmonitored patients and are usually also unwitnessed. Irrespective of arrest location, the primary cardiac arrest rhythm in hospital is usually either asystole or pulseless electrical activity (PEA).

Only about 15–20% of patients suffering an IHCA survive to hospital discharge. Survival is better if the arrest occurs in a monitored area, is witnessed, or the victim has a shockable rhythm (i.e. ventricular fibrillation or pulseless ventricular tachycardia). Most adult survivors of IHCA are defibrillated immediately. Many IHCA survivors have a full return of neurological status to pre-arrest levels, but approximately 40% may have moderate or severe neurological disability. Although cardiac arrest is rare in hospitalised pregnant women and children, the underlying causes are similar and outcomes are similarly poor.

Hospitalised patients with an admission diagnosis of heart disease may suffer a cardiac arrest caused by a sudden change in cardiac rhythm due to an irritable myocardium. However, most IHCAs occur during non-cardiac illness and are heralded by unrecognised, or untreated, respiratory and circulatory deterioration due to the patent’s underlying admission diagnosis, associated complications or coexisting diseases (e.g. hypertension, diabetes mellitus, renal disease).

IHCA is rare in pregnant women, but when it occurs it can often also be linked to a failure of prompt recognition of physiological deterioration by hospital staff. Specific risks that may predispose to deterioration include pre-existing medical conditions, maternal sepsis, eclampsia and obstetric haemorrhage. In children, cardiac arrest due to primary cardiac disease is extremely rare. More often profound hypoxaemia and hypotension are the cause, with asystole or PEA being the most common initial arrest rhythms.

The role of ‘false’ arrests

A ‘false’ arrest can be defined as an IHCA call for which no cardiopulmonary resuscitation (CPR) is needed. This usually occurs when one of the following exists: (a) a cardiac arrest call is cancelled by the hospital telephone exchange soon after having been made; (b) the patient is found to have been dead for some time and no CPR is undertaken by the resuscitation team; or (c) the patient is found not to have suffered a cardiac arrest. The false arrest is often caused by one of the conditions shown in Box 3.1. Particular attention should be paid to patients who have a ‘false cardiac arrest’, as the event often signifies unrecognised illness. The subsequent hospital mortality for patients suffering a false cardiac arrest is approximately 20–30%.

Other factors influencing the occurrence of IHCA

Adequate patient monitoring and assessment are crucial to preventing adverse outcomes, such as IHCA. Ideally, the sickest patients should be admitted to areas that can provide the greatest supervision and the highest level of nursing care and organ support. Higher nurse–patient staffing ratios are associated with a reduction in IHCA rates. IHCA survival rates are lower during nights and weekends. Hospital processes, such as admission to hospital at night or at weekends, or discharge from an ICU to a general ward at night, also increase the risk of cardiac arrest and death.