Published audit data from the Intensive Care Society and the Scottish Patient Safety Programme consistently identify critical care transfer as a period of disproportionate risk. Adverse events occurred in 5–70% of transfers, depending on how rigorously they are defined and reported. Physiological deterioration — haemodynamic instability, hypoxia, arrhythmia — accounts for the majority, but equipment-related events contribute significantly in centres where preparation is inadequate.

The underlying reasons are structural rather than related to individual clinical competence. In the ICU, a nurse is assigned to one or two patients, with immediate access to the full clinical team, pharmacy, blood bank, and diagnostic services. During transfer, the same patient is managed by two clinicians in a confined space with limited equipment, finite consumables, and no immediate backup. The physiological reserve that keeps a critically ill patient stable in optimal conditions may be insufficient to tolerate even minor perturbations — a brief syringe pump change-over, a bump in the road causing coughing against the ventilator, or a change in position during a radiograph can each trigger a haemodynamic cascade that would be trivial to manage in ICU but dangerous in transit.

Human factors also play an important role. Transfers are often undertaken under time pressure, particularly for inter-hospital transfers where receiving units have allocated theatre or ICU slots. The pressure to depart on time can compress the preparation phase — the single most important determinant of transfer safety. Checklists exist specifically to counteract this pressure and ensure that every safety step is completed before the doors close.