1. Posterior Fossa Surgery
Anesthetic Challenges of the Sitting Position
Rakhi Pal MD*,
Rafi Avitsian MD**
Fellow*, Section Head** Neuroanesthesia
CCF

2. Jason is 27 years old, previously healthy, Presented to the PACE for evaluation for posterior fossa surgery in the sitting position.

3. History: Clinical exam: Airway:
Occasional attacks of bronchial asthma.
Clinical exam:
free.
Airway:
Malapmati II
Dentition: full, no loose.
Thyromental distance more than 6 cm.

4. Lab Work CBC: CMP: Type & Screen: H&H 14& 41 Normal
A Pos, no antibodies.

5. Risks General risks of anesthesia. General risks of surgery.
Risks pertinent to post fossa surgery in the sitting position.

6. Respiratory Problems Inability to extubate.
Airway problems after breathing tube removal.

7. Neurological problems
Neurological complications:
Single nerve deficit.
4 limbs paralysis.
Stroke
Coma.

8. Cardiovascular Problems:
Hypotension.
Arrhythmia
Death.

9. The patient wants everything done to minimize the risks

10. Anything else need be done at this point?

11. Work up for PFO?

12. Options: Alternate surgery position.
Do the surgery in the sitting position and take all the precautions.
Send the patient to close the PFO.

13. Patent Foramen Ovale and Neurosurgery in the sitting Position: A Systematic review BJA 102 (5) (2009)
The databases Medline, Embase, and Cochrane Controlled Trial Register were systematically searched.
4806 patients were considered for neurosurgery in sitting position and 5416 patients underwent percutaneous PFO closure.

14. Patent Foramen Ovale and Neurosurgery in the Sitting Position: A Systematic Review BJA 102 (5) (2009)
The overall rate of VAE during neurosurgery in sitting position was 39% for posterior fossa surgery and 12% for cervical surgery.
TEE detected paradoxical air embolism between 0% and 14%.
The overall success rate for PFO closure is 99%, with an average risk of major complications is 1%.

15. TEE
A detection rate of 10% to 30% which is comparable with the 20% to 30% incidence reported in autopsy findings.
Not sensitive enough (i.e. nondetection of PFO does not guarantee its absence).

16. Sitting Position Better surgical access for the surgeon.
Improved postoperative cranial nerve function has been reported in acoustic neuroma resection in the sitting position.
Less blood loss.

17. Physiologic Changes in the Sitting Position

18. Sitting Position Paradoxical air embolism
Head elevation to the 90-degree decreases dural sinus pressure of up to 10 mm Hg.
Jugular bulb venous pressure is not a reliable indicator of dural sinus pressure.
Increased incidence of VAE (25-45%)
Paradoxical air embolism

19. Physiologic Changes
Hemodynamic instability: decreased venous return, CO.
For each 1.25-cm movement of the head above the level of the heart, local arterial pressure is reduced by approximately 1 mm Hg.
Increases in pulmonary and systemic vascular resistance and decreases in cardiac output, venous retur and CPP.
Dysrhythmias: from manipulation or retraction of cranial nerves or the brainstem regardless of patient position.

20. Contraindications Absolute: Patent ventriculo-atrial shunt
Right atrial pressure in excess of left atrial pressure
Patent foramen ovale
Cerebral ischemia when upright and awake
Relative:
Extremes of age .
Uncontrolled hypertension
COPD

21. VAE
Slow continuous air entrainment air → peripheral pulmonary circulation → sympathetic reflex vasoconstriction, and chemical mediators→ pulmonary hypertension and hypoxemia.
A rapidly entrained air bolus → an air lock within the right side of the heart → heart failure, myocardial and cerebral ischemia, dysrhythmias, cardiovascular collapse.

22. The “symptomatic dose” of venous air is not well documented in humans, more than 50 mL →decreases in blood pressure, dysrhythmia, and ECG changes.
The lethal dose of intravascular air in humans has been estimated to be greater than 300 mL.
Investigations in dogs reported tolerance of as much as 1000 mL of air infused over 50 to 100 minutes, but fatality with a 100-mL bolus.

23. Paradoxical Air embolism
The most likely mechanism is right-to-left shunting through an intracardiac defect.
The likelihood of right-to-left shunt increases:
If the right atrial pressure exceeds left atrial pressure.
Up to 50% of patients may experience reversal of an existing left-to-right atrial pressure gradient with the potential for PAE after 1 hour in the sitting position.
With hypovolemia.

24. Monitors for Air Embolism

25. Precordial Doppler Sensitive, noninvasive.
Earliest detector (before air enters pulmonary circulation)
Not quantitative
 Difficult to place in obese patients and patients with chest wall deformity.
False-negative if air does not pass beneath ultrasonic beam 10% of cases.
Useless during electrocautery.
IV mannitol may mimic intravascular air

26. Pulmonary Artery Catheter
Quantitative.
Easy placement in experienced hands
Can detect right-atrial pressure greater than pulmonary capillary wedge pressure
Small lumen.
Placement for optimal air aspiration may not allow pulmonary capillary wedge pressure measurement

27. Etn2 Specific for air Detects air earlier than etco2
May indicate air clearance from pulmonary circulation prematurely
 Accuracy affected by hypotension

28. TEE Most sensitive . Can detect air in left heart and aorta.
Invasive, cumbersome and expensive
Must be observed continuously
Not quantitative
May interfere with Doppler ultrasonography

29. ETCO2 Noninvasive Sensitive Quantitative Widely available
Accuracy affected by:
Tachypnea.
Low cardiac output.
COPD

30. Other Non Specific Monitor for Air Entrainment
TCD monitoring of Carotid and Middle Cerebral Arteries
Brain Electrical Activity
Acute decrements in the bispectral index

31. Right Atrial Catheter (RAC)
Used to Aspirate air entering the right side of the heart.
Optimal catheter placement requires that the orifice(s) be placed in or near the air-blood interface, with the tip at or 2 cm below the SA node.

32. How to confirm Site of RAC?

33. ECG Guidance
ECG lead II is attached to the right arm lead to the conductive connector.
Manipulate the catheter to be in the mid-right atrium to detect a biphasic P wave (P= QRS hieght) .
Withdraw an additional centimeter, at which point the P wave should be slightly smaller than the QRS complex, and then secure the catheter.

34. X ray.
ECG guides
Pull back from RV while monitoring pressures

35. Prevention of VAE No maneuver is 100% effective.
Incidence and severity can be decreased by the use of :
Controlled PPV.
Adequate hydration
Proper wrapping of the lower extremities
Positioning so that head elevation is the lowest possible.
Meticulous surgical technique with careful dissection and liberal use of bone wax.
Avoidance of N2O in patients with known intracardiac defects.

36. Intraoperative Period
The intraoperative goals in the treatment of VAE:
To stop further air entry.
To remove air already present.
To correct hypotension, hypoxemia, and hypercapnia.

37. Rx of VAE Inform surgeon immediately if:
doppler ULS changes occur or
Etco2 decreases more than 2 mm Hg.
Discontinue N2O, increase O2 flows.
Modify the anesthetic.
Have the surgeon flood the surgical field with fluids.
Provide jugular vein compression.
Aspirate the right atrial catheter.

38. Rx of VAE Lower the head to heart level.
Left lateral decubitus to limit airflow through the pulmonary outflow tract – of limited benefit in the presence of a continuous stream of air.
CV support:
- IV fluid.
Vasopressors.
Antiarrythmics.

39. External cardiac massage has been
shown to be effective in disrupting a large air lock
in the event of cardiovascular collapse.
PEEP or Valsalva maneuver may increase the likelihood of PAE after VAE has occurred and should therefore be avoided.
Hyperventilation is not helpful.

40. Postoperative Gare Supplemental O2. ECG, CXR. Serial ABG.
Air bubbles in the retinal vessels, seen through funduscopic examination, is diagnostic sign for cerebral air embolism.
Brain CT scan for cerebral air embolism.
If arterial air emboli are suspected, provide hyperbaric oxygen compression if available.

41. Acknowledgement
We would like to thank Dr Marco Maurtua for providing some of the material of this presentation