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Transfer Physiology

What is the impact of transferring a patient outside a hospital? We will explore the effects of motion and the environment in this article.
Air ambulance crew simulating a transfer. They are securing a mannequin in the helicopter before departure.

Transfer can lead to detrimental effects on the critically ill, who are poorly able to compensate as they are already deficient in physiological reserve. Understanding the physiological impact that movement has on the patient enables appropriate preparation and reduces the risk of patient instability and deterioration.

Newton’s 3rd Law

For every action there is an equal and opposite reaction
  • Newton’s 3rd law explains why, when we are accelerated by force in one direction, there will be a force exerted on us in the opposite direction called the inertial force.
  • These forces act along three axes:
head-to-toe (Gz)
front-to-back (Gx)
side-to-side (Gy)
  • When we think about the effects of motion on the body and thus physiological changes, we have to take into account the patient’s position.
The standard position for a patient on a road transfer via ambulance is on a stretcher with the head towards the front of the vehicle and feet towards the rear.


  1. As the patient accelerates in the ambulance, the external force causing acceleration acts towards the patient’s head. This positive G force is known as +Gz.
  2. The inertial force acts in the opposite direction towards the patient’s feet (-Gz).
  3. This causes displacement of non-tethered organs and body fluids towards the patient’s feet.
  • The amount of displacement depends on:
  1. Rate of acceleration
  2. Duration of acceleration
  3. Direction of acceleration (anterior-posterior or cephalo-caudal axis). In ambulance transport, the axis of acceleration is in the cephalo-caudal direction (i.e. head to feet).
In patients with raised intracranial pressure (ICP), extra caution is required to ensure cerebral perfusion pressure (CPP) is maintained when mean arterial pressure (MAP) is reduced. In the equation below CVP is central venous pressure. We’ll look more closely at neurological patients in Week 4.


  1. Deceleration has the opposite effect to acceleration. The external force causing deceleration acts in the direction of patient’s feet.
  2. The inertial force acts in the opposite direction with displacement of intravascular volume towards the patient’s head.
  3. The braking force of an ambulance is much greater than its acceleration and so has a greater impact on a patient’s physiology.
As blood and CSF are directed towards the head during deceleration, this can increase ICP. In patients with raised ICP and cerebral oedema this can compromise their cerebral perfusion.

And don’t forget that, with all patients, inertial forces displace the stomach and other viscera in the cephalad direction. This will lead to:

  • Increased risk of regurgitation and aspiration
  • Increased trans-diaphragmatic pressure and reduced lung compliance.

Axial loading can also cause displacement of unstable spinal fractures. This document provides more information on spinal immobilisation.

Remember that inertial forces not only affect the patient but also staff and equipment. All equipment has the potential to become a projectile unless it is secured properly and as staff, we risk injury or death if we do not wear seat-belts.

The Transfer Environment

  • Removing a critically-ill patient from the hospital environment exposes them to a number of other risks that should be considered closely. The transfer environment not only causes physiological disturbance but it may disrupt our ability to care for the patient.


  • This is a problem in many vehicles, especially when travelling at high speed. It can worsen pain and increase the risk of pressure sores and neurological injury on longer transfers.
  • Vibration may also cause artefact on patient monitoring.


  • Not only can noise cause damage to a patient’s hearing, it impacts upon the ability to communicate during the transfer and for medical staff to hear monitors and alarms.


  • Exposure to the non-hospital environment may lead to hypothermia or hyperthermia, which have a range of negative consequences. This is particularly important in critically-ill patients whose ability to regulate their own temperature is impaired.
  • Also, in extremes of heat and cold, equipment may not function properly.

Atmospheric Pressure

  • This is an important concept for aeromedical transfers, which will be explored in Week 3.


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A Journey Through Transfer Medicine

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