112 Prone Positioning
112 Prone Positioning
112 Prone Positioning
Prone Positioning
Dr D G Hovord
Specialist Trainee Registrar - Anaesthetics
University Hospitals of Coventry and Warwick
d_hovord@hotmail.com
Self-assessment
Key Points
Introduction
1. Cardiovascular system
The most important feature of turning a patient prone under anaesthesia is a
drop in cardiac output. Cardiac index in one study dropped by an average of
24%. This was mainly as a result of decreasing stroke volume with little
change in heart rate. Of the three factors involved in cardiac output (preload,
afterload and contractility), it seems likely that decreased preload was most to
blame compression of the inferior vena cava (IVC) reducing venous return
to the heart. When the IVC is itself obstructed, blood uses a collateral return
route the vertebral wall venous plexuses. As prone positioning is often used
for spinal surgery, this can cause increased bleeding in the surgical field. It is
important to decrease the pressure on the abdomen directly this can be
achieved with specially designed operating tables, or by placing wedges
under the chest and pelvis care must be taken not to stress the back too
much.
2. Respiratory system
Prone positioning has a broadly positive effect on the respiratory system
providing that abdominal compression is avoided as described above.
Functional residual capacity and arterial oxygen tension both increase. This
is partly the reason why prone positioning has been used in intensive care
settings for patients with poor lung function often secondary to acute lung
injury. The exact reason for this improved function is unclear, although it is
most likely that changes in ventilation and perfusion result in better V/Q
matching, and thus improved arterial oxygen tension.
There are a number of body structures that can be injured these are
generally as a result of direct pressure to structures on which body weight
would not normally rest.
Pressure injuries
Since pressure equals force divided by area, care must be taken to ensure
that small or vulnerable areas such as the eyes or nose do not bear a
disproportionate load. Pressure can cause damage by direct pressure, or by
occlusion to an arterial supply or venous drainage. Areas at risk depend on
exactly which prone positioning technique is being used this tutorial will
focus on two commonly used. Firstly a simple prone position with a Montreal
mattress (a standard soft operating table with a hole cut out to avoid pressure
on the abdomen) and the tuck position, where the hips and knees are flexed
to allow better access to cervical/thoracic spine.
Ocular damage
Ocular damage is caused by two mechanisms. First is direct pressure to the
eye - incorrect positioning leading to the weight of the head being supported
by the globe will intuitively result in damage secondary to ischaemia. The
second is a result of poor perfusion. In the same way that cerebral perfusion
pressure equals mean arterial pressure (MAP) minus the intracranial pressure
(ICP), ocular perfusion pressure can be defined as MAP minus the intraocular
pressure (IOP) Occlusion to the venous drainage, or any generalised rise in
venous pressure will raise the IOP, as will use of a head-down position. MAP
may be reduced either by deliberate hypotension or abdominal compression.
If ocular perfusion pressure is too low to adequately perfuse the eyes then
ischaemic damage will result
Swelling of the tongue and mouth has been reported, causing potential airway
obstruction and delayed extubation. The mechanism is assumed to be
obstruction of venous drainage from the head and neck secondary to
excessive flexion of the neck. The subsequent increase in hydrostatic
pressure causes the equivalent of dependent oedema.
The chest wall is normally able to withstand the pressure put on it under
anaesthesia. However, it can be weakened by a congenital abnormality
(scoliosis or pectus excavatum), or by previous chest surgery. Cardiac output
can be severely compromised by right ventricular compression against the
sternum. If the sternum itself is abnormal, this is more likely to occur.
The lower limbs are at increased risk of compartment syndrome in the tuck
position, where both knees and hips are flexed. Arterial blood flow and MAP
decrease, and limb pressure increases. This can result in rhabdomyolysis
and compartment syndrome, which when complicated by renal failure can be
fatal.
Pre-assessment
First discuss with the surgeon the position required and the anticipated length
of the procedure. Then fully pre-assess the patient, including examination
and consent for anaesthesia. Assess the airway carefully - cervical spine
surgery is one indication for prone positioning, and limited head and neck
movements will be more common in this group, complicating airway
management. Focus on risk factors for peripheral neuropathy (diabetes,
alcohol consumption, B12 deficiency), and document pre-existing nerve
injuries and neuropathies. Check for signs of vertebro-basilar insufficiency.
Consider the need for invasive monitoring and consent appropriately.
Perform pre-operative investigations as appropriate.
Pre-induction
Standard monitoring should be instituted with the patient in the supine
position, and appropriate venous access gained. Avoid using the anterior
cubital fossae flexion of the arms will occlude this route after the patient is
positioned prone for surgery. Place ECG electrodes on the patients back in a
position where they will not interfere with surgery. Ensure that there is an
adequate number of staff present to turn the patient after induction they
should be drilled in the technique, using an awake volunteer for practice. The
correct operating table should be in place, and induction take place on a
separate moveable trolley.
Induction
Induce anaesthesia appropriately, and then secure the airway. A reinforced
endo-tracheal tube (ETT) is often used. The laryngeal mask airway has been
used in the prone position, but it is intuitively safer to fully secure the airway
as access intra-operatively is difficult. Secure the tube, preferably with tape
and not a tube tie. This is because when the patient is positioned prone the
tie may become tighter and occlude venous drainage from the head and neck
resulting in morbidity as discussed earlier.
Protect the eyes carefully. Initially tape shut, and then place extra protective
padding over them, and tape that in place also. Hard goggles have been
designed to help protect the eyes in the prone position if used, ensure that
they are correctly fitted, taking the pressure off the globes. Consider
temperature monitoring if continuous naso-pharyngeal monitoring is
needed, then insert prior to taping the ETT as access to the nose and mouth
may be difficult. Place arterial and central lines if required, although be aware
that CVP interpretation may be difficult in the prone position. A urinary
catheter is recommended in major procedures to aid in assessment of the
circulation.
Positioning
When the airway and all lines are secure, tell other members of the theatre
team that you are ready to position the patient prone. Place the trolley with
the patient next to the operating table. Take control of the head and airway
as with all positioning it is safest to disconnect the patient from the breathing
circuit at this point. At least five other staff (one of whom should be the
surgeon) are required to safely turn the patient two on each side and one
controlling the legs and feet. The patient should be turned slowly and gently
prone onto the operating table next to the trolley, with the anaesthetist co-
ordinating the procedure. Particular care should be taken to avoid
misplacement of intravenous lines and cannulae. Once placed, position the
head and neck carefully, preferably in a neutral position on a soft head ring
avoiding ocular pressure. Then perform a rapid but thorough assessment of
the airway, breathing and circulation. Commonly the endo-tracheal tube may
move into right main bronchus, as a result of increased neck flexion.
Perform a full top to toe assessment of the patient to ensure every pressure
point is protected by padded material. If on a Montreal mattress ensure that
the abdomen is correctly placed. Then perform another assessment of airway,
breathing and circulation prior to the commencement of surgery.
Intra-operative management
The same principles of intra-operative management apply to prone positioning
as to any other anaesthetic. The main difference is that if a problem arises
that requires a return to the supine position, there may be some delay before
this can happen safely. As with all anaesthetics, preventing problems
occurring by careful preparation and double-checking prior to commencement
of surgery is crucial.
Summary
Discuss with the operating team the exact position and expected
duration of surgery.
Carefully pre-assess the patient
Ensure that adequate numbers of theatre staff who are well trained in
prone positioning are available.
The airway must be secure prior to turning the patient prone. Check
this again after positioning.
A top to toe assessment of all pressure points should be performed,
paying particular attention to those discussed throughout this tutorial.
Maintain anaesthesia until the patient is placed supine at the end of the
procedure.
References