Patient Positioning and Associated Risks Flashcards
Physiological changes produced by the Trendelenburg position
Initial placement of the patient in head-down supine position will increase cardiac output approximately 9% in less than 1 minute via an autotransfusion from the lower extremities. This effect is not sustained and within approximately 10 minutes the cardiac output begins to return to baseline.
Functional residual capacity is decreased due to gravitational pull of the diaphragm
cephalad. In a spontaneously breathing patient, the work of breathing increases. In patients under general anesthesia, these pulmonary changes result in higher airway
pressures.
Intracranial and intraocular pressures (IOCs) also increase in Trendelenburg position. Trendelenburg is contraindicated in patients with increased intracranial pressures.
Prolonged head-down positioning can also lead to swelling of the face, conjunctiva, larynx, and tongue, with an increased potential for postoperative upper airway obstruction.
The Trendelenburg position increases intraabdominal pressure and displaces the stomach placing the patient at a higher risk for aspiration
Back pain is common in the supine position because …
… increases the risk of back pain further due to loss of tone in the paraspinous muscles
the normal lumbar lordotic curvature is often lost
General anesthesia with muscle relaxation and neuraxial block
Physiologic changes caused by lithotomy position
When the legs are elevated, venous return increases, causing a transient increase in cardiac output and, to a lesser extent, cerebral venous and intracranial pressure in otherwise healthy patients.
In addition, the lithotomy position increases intraabdominal pressure and causes the abdominal viscera to displace the diaphragm cephalad, reducing lung compliance and potentially resulting in a decreased tidal volume.
In obese patients, or when large abdominal mass is present (e.g., tumor, gravid uterus), abdominal pressure may increase enough to obstruct venous return to the heart.
The curvature of the lumbar spine is lost in lithotomy and can put the patient at risk of back pain.
Complication of lithotomy position
Lower extremity compartment syndrome is a rare but potentially devastating complication of the lithotomy position. Compartment syndrome is caused by increased tissue pressure within a fascial compartment due to tissue ischemia, edema, and rhabdomyolysis. Inadequate arterial inflow (from lower extremity elevation) and decreased venous outflow (due to direct compression or excessive hip flexion) elevates the risk of compartment
syndrome for patients in lithotomy.
Reperfusion after ischemic injury further increases edema, exacerbating the problem
Local arterial pressure decreases … for each centimeter the leg is raised above the right atrium
0.78 mm Hg
In lithotomy position
… was the only distinguishing characteristic of the surgeries during which patients developed lower extremity compartment syndromes
If surgical time extends beyond …, periodically lowering the legs is recommended
Additional risk includes factors known to compromise tissue oxygenation, such as …
Elevated body mass index is also a risk factor for compartment syndrome
Long procedure time
2 to 3 hours
blood loss peripheral vascular disease, hypotension, and reduced cardiac output
Ventilation changes in the lateral decubitus
In a patient who is mechanically ventilated, the combination of the lateral weight of the mediastinum and the disproportionate cephalad pressure of abdominal contents on the dependent lung favors overventilation of the nondependent lung. At the same time, the effect of gravity causes the pulmonary blood flow to the underventilated, dependent lung to increase. Consequently, ventilation-perfusion matching worsens, potentially affecting gas exchange and ventilation.
Venous air embolism is a constant concern in the sitting position due to the position of the surgical field above the level of the heart and the tented open dural venous sinuses. The reported incidence of VAE varies greatly in the literature due to a lack of
standardization of measurement and grading scale for VAE. VAE can cause …
arrhythmias, oxygen (O2) desaturation, acute pulmonary hypertension, circulatory compromise, and cardiac arrest. If there is a patent foramen ovale (PFO),
then the patient is at risk for a paradoxical arterial embolism causing stroke or myocardial infarction
Treatment for VAE includes …
first stopping further air entrainment. The surgon is asked to stop operating, to
flood the field with normal saline, and possibly apply bone wax.
The inspired percent of O2 is changed to 100%. This will aid in treatment during hypoxemia or hypotension and may help reduce the volume of the air embolism via denitrogenation.
Hemodynamic compromise is treated with intravenous fluids and vasoactive agents.
Consideration is given to placing the patient in left side down and Trendelenburg in order to move an air lock in the right ventricular outflow track (although this can be difficult or impossible in some surgeries).
A central venous catheter is often placed preoperatively in order to aspirate entrained air
Patient comorbidities that contribute to peripheral nerve injuries include: …
hypertension, diabetes, peripheral vascular disease, older age, and heavy alcohol and tobacco use an extremes of weight (both low body mass index and obesity, are also risk factors)
What are the main causes of postoperative visual loss?
Ischemic optic neuropathy (ION), and retinal arterial occlusion (RAO)
Surgerys associated with higher incidence of POVL
Spine fusion and cardiac surgery
Describe the four mechanisms associated with retina arterial oclusion
(1) external compression of the eye
(2) decreased arterial supply (embolism to retinal arterial circulation or decreased systemic blood flow)
(3) impaired venous drainage
(4) thrombosis from a coagulation disorder
The cause of perioperative CRAO is usually …
external compression of the eye
The largest case series of perioperative RAO is in cardiac surgery. This recent study retrospectively examined RAO (CRAO and BRAO) in cardiac surgery using the Nationwide Inpatient Sample. More than 5.8 million cardiac operative procedures were estimated from 1998 to 2013, with 4564 RAO cases, an incidence of 7.8/10,000.
Pacient conditions associated with increased RAO were …
Perioperative factors were …
giant cell arteritis, transient cerebral ischemia, carotid artery stenosis, embolic stroke, hypercoagulability, myxoma, diabetes mellitus with ophthalmic complications, and aortic insufficiency
bleeding, aortic and mitral valve surgery, and septal surgery
