Anesthesia for surgery in the sitting position

I will include in this chapter  posterior fossa surgery, which in our hospital is performed in the sitting position in most of the cases  , and  anesthesia for cervical spine surgery done in this position. I will also include considerations for posterior fossa surgery which are relevant  for any surgical position.

 Preoperative assessment

Increased ICP due to Hydrocephalus-many patients undergo ventriculo-peritoneal shunt  before their definitive surgery.

Assess compromise of cranial nerves and consciousness.

Hydration- due to vomiting, mannitol administration and contrast media,many patients are hypovolemic, which demands correction before sitting the patient.

Planned surgery

Posterior fossa tumors. Due to the presence of neural and vascular structures,the surgical dissection can  stimulate or lesion these structures.As a consequence, cardiovascular reflexes  like hypertension, bradychardia  can suddenly appear  during stimulation of the brain stem ,the trigeminal nerve or the vagus nerve.

Bleeding –from  venous sinuses,from the tumor or arterial.

The sitting position is preferred by most of our surgeons due to excellent exposure, reduced bleeding and bulging albeit the risk of venous air embolism(VAE)

In patients who can be compromised by the hemodynamic instability posed by the sitting position like those with severe cardiac disease, we opt for lateral or park bench position.Patients  known to have PFO or ASD are not operated in this position due to the risk of paradoxical air embolism.

Premedication

Not routine, in anxious patients we give small doses of midazolam just before entering the OR.Patients scheduled for trigeminal nerve decompression or cervical laminectomy are premedicated  as usual.

Goals

Keep adequate CPP and hemodynamic stability
MAP is kept at  least at 70 mmHg at  circle of Willis level.

Adequate positioning, normoventilation.or mild hyperventilation to facilitate exposure.

Minimize VAE  risk

Anesthesia Technique

Monitoring:  EKG ,Pulse oximetry ,non invasive blood pressure, esophageal thermometer,ETCO₂, FIO₂,ET anesthetic agent,Train of four

The capnography is by large the most useful  monitor in VAE diagnosis.

Set  the alarm of low  ETCO₂   2-3 mmHg  below  the baseline in order to have an early warning in case of VAE. Tipically ETCO₂ will decrease in a few respiratory cycles.

Invasive  blood pressure : arterial line, generally Lt.radial ,20G after induction.The  arterial line is placed before induction under local anesthesia and light IV sedation in cardiac and unstable patients.

The transducer is placed at ear level (circle of Willis  level  ) to facilitate the assessment of CPP.

 Volemia is assessed by  the analysis of the arterial wave contour and by measuring   systolic variation (delta down)

Urinary catheter , esophageal  thermometer

1  or 2 wide bore IV's (depending on the vascularity  and site of the tumor ) 

Central venous line : Place a multiorificed ,EKG directed central line.Using an anterior approach of the right internal jugular,and lead l as the sensing lead  the catheter is advanced until  the P wave becomes biphasic, near the superior vena cava-right atrium junction.After fixation, check always patency.

The transducer is placed at the level of the right atrium (fifth intercostal space in the mid –axillary  line) .

In most of the cases of VAE, the amount  of air bubbles aspirated is minimal, as compared to the air that gets to the pulmonary circulation so generally  the central line is another diagnostic device and  is less important as a real retriever of air  from the circulation .

Precordial Doppler-placed after the patient is sitting .Audible heart tones and the typical noise after a rapid injection of saline via central line indicates correct placing of the transducer.The Doppler is affected  by bipolar diathermy  and is a significant noise polluter .Requires continuous listening.

IV induction  After preoxygenation, via  existing IV line

Fentanyl  1-2 mcg/kg

Propofol   2mg/kg or Thiopental or Etomidate until loss of eyelid reflex.

Facilitation of intubation  Rocuronium 0.6 mg/kg.Non depolarizing relaxants are always used during induction,-even when motor responses are monitored- in order to ease the positioning of the patient. Monitoring can be started about  45 min. after induction.

The ET tube is safely fixated and eyes are taped. In sitting position we fixate the tube with a  thin suction catheter.First,pass the catheter through one nostril, and pick it from the mouth with a Magill forceps.Make a knot  over the upper lip ,then knot it around the ET tube.Then fixate it as usual with tape This fixation does not interfere with the surgical field.
 
Maintenance The patient is ventilated with a mixture of O₂/air  to provide normoventilation or mild hyperventilation and normal oxygenation.A potent inhalatory agent , sevoflurane or isoflurane is added at less than  1 MAC.
Analgesia: Fentanyl increments .

If  SSEP are monitored,we use total IV anesthesia with remifentanyl infusion, between 0.1 and 0.25 mcg/kg/min and propofol.Deep TIVA  generaly requires vasopressor support.
Muscle relaxation Generally atracurium, 0.5 mg/kg/h (not in case of EMG monitoring or transcranial motor  evoked potentials)

Positioning

Before sitting the patients  bandage both legs to reduce venous pooling, and administrate a preload of NS.

When sitting, try to maintain a physiological position, avoiding extreme neck flexion which can compromise the spinal cord perfusion. Check that  the chin to chest distance is at least 4cm. Pad all pressure points.Pay attention to flexion of hips and knees.Do not leave an oral airway in place because of the risk of tongue edema.If motor evoked potentials are monitored, place a gauze roll between the teeth.

Fluids- Normal saline ( mildly hyperosmolar).  Metabolic acidosis is common with this fluid regime

Volume loading  is partially effective in VAE  prevention because it is difficult to rise CVP in patients with normal heart function.

Consequently the venous negative pressure present in the  surgical field cannot be reduced (except by jugular pressure).

 Hypovolemia is to be avoided.

Mannitol is not routinely given because of its hemodynamics effect .

Prophylactic antibiotics  Cefazoline 1g IV .In penicillin allergic patients, clindamycin 600 mg.

Seizure prophylaxis  Phenytoin or  Valproate (loading or maintenance)

In posterior fossa surgery ,some surgeons request them because  of the epileptogenic effect of  pneumocephalus.

Laboratory. In stable patients, Arterial blood gases and Ht are measured at  least   once every 2  hours .In bleeding patients, a full coagulation profile and a CBC is sent  at the beginning of the episode and  then  hourly. In massive bleeding , inform the blood bank which enters in a "massive transfusion mode" providing us  with all the blood components needed.

Intraoperative problems

Hypotension

Due to the change of position,hypotension  is common. Do not start remifentanyl  until the patient is sitting.Treatment: fluids, vasopressors.

If  muscle relaxants are avoided due to monitoring of motor evoked potentials, deeper anesthesia is needed to avoid movements,and a  continuous infusion of phenylephrine is given for hypotension.

Venous air embolism

About 50% incidence  in posterior fossa  . Can occur in any stage of surgery , more frequent during craniotomy.

In cervical laminectomy its incidence is negligible. Generally occurs without serious hemodynamic consequences ,although massive VAE can occur, causing  collapse.

In such cases, the operating table should be placed in the Trendelemburg position to stop VAE and allow resuscitating maneuvers  

Prevention- most probably depends on surgical technique.Avoid hypovolemia. Peep is not used.(may facilitate paradoxical air embolism)

Diagnosis-Typical sounds in precordial  Doppler ; a reduction of ETCO₂ due to an increase in physiological dead space.

Treatment-Inform the surgeon , will flood surgical field with saline

Ventilate with O₂ !00%. Denitrogenation  accelerates the absorption  of the bubbles.(N₂ is less soluble and will stay longer in the circulation)

Jugular pressure- Stops entrance of air and shows the surgeon the open veins responsible for the episode. We  also use this maneuver as a prevention of VAE. When the surgeon performs burr holes  and during the craniotomy, frequent intermittent  application of jugular pressure help the  identification of open veins which otherwise can become a source of VAE.

Aspiration of blood and air via central line.

Wait for a return of the ETCO₂ to baseline values to allow the surgeon to resume surgery.

Cardiovascular reflexes. Inform the surgeon. If repetitive bradycardia occurs,  give atropine to allow the surgeon  complete the dissection or resection of a tumor.

Hypertension. Short bouts of hypertension are generally self limited and are not treated.

Awakening the patient

If possible we  prefer early awakening in the OR

 Goals :patient awake, responds to simple orders, minimal cough,hemodinamically stable,no risk of hypoxia  or hypoventilation

In the following situations do not awaken the patient in the OR, and  transfer him to  the ICU via CT scan:

Patients which are no fully awake before surgery, long procedures, difficult hemostasis, hypothermia, massive transfusion, major hemodynamic instability ,brainstem edema,repiratory failure.

Stop atracurium infusion 30 min before end of procedure

Fentanyl  1.0 -1.5 mcg/kg   and droperidol 1 mg  for PONV prevention   after returning the patient to supine position.

These may cause   hypotension , be ready with ephedrine

After pin removal (on supine position to avoid VAE) and dressing , close inhalatory agent and remifentanyl. Reverse  residual curarization with atropine/neostigmine

The patient will awake in about 10-15 minutes.Emergency hypertension may occur and is treated with Labetalol, beginning with 0.25 mg/kg .

Cough -IV lidocaine  1.5 mg/kg

Extubate when patient is awake, normal Vt ,obeys orders ,no signs of residual curarization.

Transfer the patient to Recovery Room