3. Prone Position Flashcards
Manual Handling Issues
Turning the patient is often problematic, particularly in those with raised BMI.
Several handlers will be needed to ensure that the patient is logrolled into position
without any twisting of the lumbar and cervical spines.
The head must move as one with the shoulders with obvious care not to dislodge whichever airway device is in place.
Airway Access
It is accepted wisdom that restricted access to the airway once the patient is inverted
mandates endotracheal intubation, probably with an armoured tube, and this would
be the ‘safe’ exam answer.
In clinical practice, however, by no means all anaesthetists
are quite as dogmatic. The prone position confers some respiratory advantages in a
patient who is breathing spontaneously and a standard laryngeal mask airway is
relatively easy to resite should it move, unlike a tracheal tube (or a reinforced
laryngeal mask airway). It would probably be wise not to volunteer this option as it
remains contentious, but be prepared to discuss it should an examiner raise the issue.
Mechanical Pressure Effects
Ophthalmic problems
Peripheral nerve injuries.
Ophthalmic problems
. These range from relatively benign complications such as
chemosis and subconjunctival haemorrhage through retinal detachment to complete
visual loss (which is quoted as occurring in 0.02–0.2% following spinal surgery).
Decreased ophthalmic perfusion pressure secondary to
hypotension, hypovolaemia and an increase in direct orbital pressure
can contribute to ischaemic optic neuropathy
or central retinal artery occlusion, both of which may result in blindness.
The central retinal artery can also thrombose secondary to a reduction in flow, but in
some cases this may be unilateral. Intraocular pressure can rise in patients with
narrow-angle glaucoma who are positioned prone.
Peripheral nerve injuries.
Nerves of the brachial plexus are at particular risk of
traction injury because they are effectively fixed at the cervical vertebrae, and they
may also be compressed at the level of the first rib, clavicle and head of the humerus
as they pass down into the upper limb across these potentially mobile structures.
The ulnar nerve is vulnerable both to ischaemia and to direct compression, as is the
lateral cutaneous nerve of the thigh, which may be in prolonged contact with pelvic
supports.
Pressure sores and compartment syndromes.
The prone position is no more likely
to cause pressure sores than the supine, except that there are probably more bony
prominences to put the overlying tissues at risk. These include the anterior ankle
joints, knees, anterior superior iliac spines, thorax, chin and forehead. Ischaemia
develops after around 2 hours of unrelieved pressure and tissue necrosis after around
6 hours.
Duration of surgery is therefore the major risk factor along with increased
skin fragility associated with advanced age or corticosteroid therapy. Obesity is also a
contributing
Surgical bleeding
. The venous drainage from the spine is via the vertebral valveless
venous plexus of Batson. The absence of valves means that any increase in intraabdominal
pressure secondary to external compression is likely to cause a significant
increase in operative bleeding and potential compromise of any planned spinal
surgery.
Physiological Effects
CVS
consistent finding across various studies has been a
decrease in cardiac index. This fall in cardiac output is attributed primarily to a
reduction in stroke volume secondary to reduced venous return, which is due largely
to venous pooling but to which inferior caval obstruction can make a contribution if
positioning is poor and the abdomen is compressed
The compensatory sympathetic response to
this effective hypovolaemia is reflected by a tachycardia and a rise in systemic
vascular resistance. Patients who are anaesthetized may also tolerate poorly rapid
changes in position.
Respiratory system.
Functional residual capacity decreases by
around 45% from the conscious, upright position to the anaesthetized supine position,
but only by 12% from upright to prone. The change from supine to prone therefore,
with all other factors unchanged, is associated with an improvement in PaO2 which is
due to better matching of ventilation-perfusion
However, more sophisticated investigation has demonstrated that blood
flow is much less altered with changes of position (and in weightlessness) than
previously believed, and a different model based on the structural features of the
airway and associated vasculature has been proposed
A factor that is simpler to understand is the fact that the dorsal lung areas receive
preferential perfusion independently of position, and this may be related to an
intrinsically lower pulmonary vascular resistance in those regions. In the injured lung,
the proximal architecture of the bronchioles and vessels may be distorted by oedema
and inflammation, and this mismatch can be amplified by the distal branching with
substantial V/Q mismatch in the alveoli.
