Posterior Fossa Surgery and Venous Air Embolism Flashcards
(11 cards)
What patient positions might be used for Posterior Fossa Surgery
Sitting Position (good for accessing midline structures plus good venous and CSF drainage, improving the surgical field)
Supine with head turned and supported by sandbags (may be suitable for acoustic neuromas or cerebellopontine angle tumours)
Prone (also offers good access to midline structures)
Lateral (good access for lateral structures)
Park Bench (a modification of the lateral position where the patient is semi-prone with their head flexed to face the floor, offering greater access to the midline and avoiding the need for the sitting or prone position)
Name some Specific Complications of the sitting position
Airway
ET Tube Displacement
Jugular Venous Obstruction (due to flexed neck which can cause laryngeal and tongue oedema resulting in post op airway compromise)
Cardiovascular
Hypotension due to reduced venous return as a result of venous pooling in dependant areas
Venous air embolism
Neurological
Spinal Cord or Brainstem Ischaemia due to head flextion and hypotension
scaitic or Femoral Nerve Damage from excessive hip flexion compounded by lower limb oedema
Pneumocephalus (air in the cranium) resulting in delayed recover, neurological deficit, confusion or headache)
Cutaneomusculoskeletal
Compartment Syndrome
Lumbosacral Pressure Damage
What abnormalities in routine intraoperative monitoring could develop as a consequence of venous air embolism?
Drop in SpO2
Decrease in etCO2
ST-segment depression on ECG monitoring
Tachyarrhythmia
Hypotension
What monitoring techniques can specifically detect the presence of venous air embolism during surgery and the features that would indicate the diagnosis for each monitoring technique
Precordial Doppler:
Sound is heard if air is present in the cardiac chamber (this is the most sensitive non-invasive device)
Transoesophageal Echo:
Air seen in right-sided cardiac chambers (In the presence of a patent foramen ovale, it can detect air in the left heart as well). The device is not overly suited for long operations where the head is flexed.
Pulmonary Artery or Right Artery Pressure (Presumably from a pulmonary artery catheter):
Pulmonary artery pressure will rise with a significant air embolus and related right ventricular outflow tract obstruction can cause a rise in right arterial pressure.
Oesophageal Stephoscope:
Mill wheel Murmur ( A large volume of air is required to cause this noise as which point cardiovascular collapse may have already occurred
End Tidal Nitrogen Level:
A sudden rise in end tidal Nitrogen due to the presence of nitrogen in the air embolus, which is more specific and sensitive for venous air embolism the EtCO2, but let’s face I,t you’ve never heard of it or seen it, so when is it ever going to be available to you.
In reality Precordial Doppler is used alongside gas exchange, invasive BP, and potentially CVP monitoring, in addition to clinical suspicion
What steps Can be undertaken to manage a significant venous air embolism in a patient in the sitting position.
Call for Help
ABCDE approach
Present further air entry:
Ask surgeon to flood the surgical site with saline and cover with wet packs
Administer IV Fluids, especially if hypovolemia is thought to be a contributory factor to air entrainment
Lower the head of the patient so that the surgical site is below the height of the right atrium (i.e. Trendelenburg position)
Aply sustained positive airway pressure until the other above measures have been achieved
Reduce Size of Air Embolism:
Administer 100% O2
Stop Nitrous Oxide if being used
Aspirate air from the right atrium via central line (if one is present)
Overcome Mechanical Obstruction:
Left Lateral or Trendelenburg positioning may help force bubbles above the right ventricular outflow tract
If the patient suffers a cardiac arrest or severe haemodynamic compromise, chest compressions may assist in dispersing the bubble
Ionotropic Support may also be required
List the structures contained within the posterior fossa
Brainstem
Cerebellum
Aqueduct of Sylvius
Vertebral and Basilar Arteries
Cranial Nerves VI, VII, VIII, IX, X, XI and XII
Venous sinuses - sigmoid, transverse, and occipital
Describe neurological features that you would specifically assess for prior to posterior fossa surgery in the sitting position
Cerebellar function, i.e., coordination, posture, and gait
Cranial Nerve Function (in particular bulbar weakness which can lead to the loss of the ability to protect the airway and hence need postoperative ventilation.
Raised ICP i.e. reduced level of consciousness, headache, vomiting (this could be due to the pathology or the development of hydrocephalus)
What are the absolute contraindications to the sitting position for craniotomy?
Ventriculo-atrial shunt: Shunt blockage could occur in the context of venous air embolism, leading to raised ICP.
Right-to-left cardiac shunt, which can allow venous air to pass to the left side of the circulation, potentially leading to stroke
What are the relative contraindications to the sitting position for craniotomy
Patent Foramen Ovale
Uncontrolled Hypertension
Severe Autonomic Neuropathy
Extremes of Age
In what ways can the risk of developing a venous air embolism be minimised during the sitting position for craniotomy
Positioning: If the sitting position is absolutely necessary, then a Trendelenburg tilt or leg elevation may help reduce the risk
Reduce venous hypotension: by avoiding dehydration and hypotension
Surgical Technique: Minimse open veins and attention to hemostasis
N.B. Venous air embolus occurs as a result of entrainment of air into the venous system due to the presence of open venous sinuses which are at negative pressure in relation to the heart and hence with this in mind one should be able to understand why the above should reduce the risk
What is the most sensitive non invasive monitoring technique for the detection of venous air embolus
Precordial Doppler
