Introduction:
The following factors of middle ear anatomy should be kept in mind.
1. It is an air filled cavity
2. Structures of the middle ear cavity are so small that microscope is needed during the surgical procedures involving this area
3. Facial nerve is an important structure found within the middle ear cavity. This nerve provides motor innervation to the muscles of facial expression
Considering the above factors, grate care must be taken during the perioperative period. The following are some of the special considerations that are needed:
1. Provision of bloodless field
2. Patient head positioning
3. Airway management
4. Facial nerve monitoring
5. Effect of nitrous oxide on the middle ear cavity
6. Smooth calm recovery from anesthesia
7. Prevention of post op nausea and vomiting
Importance of bloodless field in middle ear surgery:
Bloodless field is a must for successful middle ear surgery. Even small amounts of blood will obscure the surgeon’s view. Anesthetist should use a combination of physical and pharmacologic techniques to minimize bleeding. Specific attention should be paid to the head position of the patient as this is an important step to eliminate venous obstruction and congestion. Extreme hypertension can cause increased middle ear bleeding.
In patients with carotid atherosclerosis, carotid flow could be compromised / plaque of emboli could get dislodged causing problems. Hence it is worth auscultating for carotid bruit before surgery. Hyperextension of neck / torsion of neck can cause injury to cervical spine / brachial plexus.
General anesthesia:
If general anesthesia is preferred then air way can be maintained using a laryngeal mask airway or endotracheal intubation. Endotracheal intubation could be more appropriate if neck hyperextension or rotation of head is required. If intraoperative facial nerve monitoring is essential using a nerve stimulator muscle relaxants should be avoided. If neuromuscular blocking is needed for smooth intubation then an agent like mivacurium or similar fast acting muscle relaxant that ensures muscle function return before the need for nerve monitoring arises.
To prevent sudden movement of patient during the surgical process the depth of anesthesia should be monitored. Use of nitrous oxide is considered to be a little controversial as this gas could replace oxygen inside the middle ear cavity. This can cause expansion of gas volume within the middle ear cavity causing displacement of ossicular prosthesis if used.
Post op vomiting should be prevented as it could cause displacement of the middle ear prosthesis. It is prudent for the anesthetist to administer antiemetic prophylaxis.
Since majority of middle ear surgical procedures are performed as a day care surgery, rapid recovery, good analgesia and avoidance of nausea and vomiting are essential.
Preop assessment:
In adults short duration middle ear surgery (myringoplasty) can be performed under local / general anesthesia. Stapedectomy is ideally performed under local anesthesia as it provides the advantage of reduced bleeding. Hearing can also be tested on the table after placement of stapes piston.
Patients who are able to understand the procedure, and those who in all probablity would cooperate during the surgical procedure can safely undergo the procedure under local anesthesia.
Leaving the hearing aid on the non surgical ear before induction and before extubation will minimize the anxiety of the patient. Use of oral anxiolytics like Benzodiazepine can be administered as premedication.
History of cardiovascular disease, hypovolemia and anemia will limit the degree of hypotension possible. In pediatric patients during pre op assessment, it is important to check for the presence of coexisting syndromes and recent URI.
Choice of anesthesia:
Middle ear surgeries can be performed either under local / general anesthesia. Sensations from the ear is via four nerves. They are:
1. Auriculotemporal nerve supplying the outer auditory meatus
2. Greater auricular nerve supplying the medial and lower aspect of the auricle and part of the external auditory meatus
3. Auricular branch of vagus supplying the concha and the external auditory meatus
4. Tympanic nerves supply the tympanic cavity
Uncomplicated middle ear surgeries can be performed under local anesthesia. If the middle ear surgery is to be performed under local anesthesia patient can be premedicated using fortwin and atropine given intramuscularly half an hour before the surgical procedure. Infiltration can be performed using 2% xylocaine with 1 in 10,000 adrenaline. Major advantage of local anesthesia is the ability to test hearing during surgery. In addition there is less bleeding. Supplementary oxygen is provided with nasal canula.
In general anesthesia TIVA (total intravenous anesthesia) is preferred to that of inhalational anesthesia as nitrous oxide administration is avoided in TIVA.
TIVA:
TIVA is defined as a technique of general anaesthesia which involves use of intravenous drugs to anaesthetize the patient without the use of inhalational agents. In fact chloral hydrate1 was the first anaesthetic agent to be introduced intravenously way back in 1870. Introduction of Propofol in 1986 gave a new lease of life to TIVA. It was Sigismund Elsholtz who first attempted intravenous anaesthesia in 1665 . Real advance in intravenous anaesthesia took place during 1921 when Daniel and Gabriel Bardet published their experiences using somnifaine. Fredet and Perlis combined somnifaine with subcutaneous injection of morphine to supplement the effects of somnifaine. To begin with a Vann’s 10 ml syringe was used for this purpose. To facilitate continuous intravenous infusion Abel’s syringe was used. The current popularity of TIVA has been attributed to the pharmacokinetic and pharmacodynamics properties of Propofol and opioids. These drugs are ultra-short acting and hence suitable for continuous infusion. With the advent of advanced computer based drug administration system intravenous drug administration has become safer and predictable. The currently available intravenous drug delivery system allows the anaesthetists of vary the depth of anaesthesia by just controlling the infusion rate of the drug. This is in fact similar to that of conventional inhalational systems currently available.
Advantages of TIVA :
1. Recovery is smooth and predictable
2. No post-operative vomiting
3. There is no pollution
4. Allows high dose of oxygen to be inspired
5. There is virtually no bowel distention
6. Reduced requirement for muscle relaxants
7. Intraocular pressure is reduced
8. Malignant hyperthermia is virtually unknown
9. Controlled hypotension is possible
10. Produces adequate amnesia
11. Produces adequate analgesia
12. Less neuro humoral response
Drug pharmacokinetics: This is actually the use of mathematics to describe how body handles a certain drug. This is in a nutshell a calculation of the mathematical relationship between the administered dose of drug and the resulting observed changes in its plasma concentration. This is very important in deciding which drug / drug combinations can be safely used to administer TIVA.
Goals of TIVA:
1. Smooth induction
2. Reliable and titratable maintenance of anaesthesia
3. Rapid emergence out of the effects of infused drug as soon as the infusion is terminated
Drug combinations used in TIVA:
1. Propofol with remifentanil
2. Propofol with sufentanil
3. Midazolam with sufentanil (used in patients susceptible to hyperthermia) Only flip side to TIVA is the expense involved. The newer drugs are highly expensive and coupled with the cost of computerized delivery system adds to the cost.
Target controlled infusion: This is the basis of TIVA. This system calculates the drug concentration, the delay in the transfer of blood brain barrier. Drugs like Propofol affects its own pharmacokinetics, probably by decreasing cardiac output and also by decreasing hepatic blood flow 4. Target controlled infusion is performed using a syringe pump.
Important functions of syringe pump include 7: 1. Bolus – This gives the ability to rapidly increase plasma concentration of the drug administered 2. Flow rate – The pump should be able to function accurately even at low flow rates. Alarms – Facilitates identification of improper positioning of syringe in the pump 4. Tight syringe fitting – This prevents syringe from moving when the pump is in action 5. Battery indicator
Precautions that should be taken while using syringe pump:
1. High concentration of drugs that run at slow speed should be avoided
2. The syringe pump should be connected close to the patient
3. Vasoactive drugs should not be combined with the primary drug
4. Pump should not be placed above the level of the patient
Otolaryngological surgeries where TIVP is preferred:
1. Functional endoscopic sinus surgery
.2. Thyroid surgeries where recurrent laryngeal nerve monitoring is needed
3. Advanced endoscopic surgical procedures involving skull base where hypotensive anaesthesia is needed to control bleeding
Propofol:
This drug is GABA receptor agonist. This drug produces deep state of unconsciousness within 30 seconds of administration of loading dose (1.5-2.5 mg/kg body weight). It is also known to cause respiratory depression in 90% of patients. Effects of this drug wanes within 5 minutes of administration due to redistribution of the drug. This drug causes lower incidence of post-operative vomiting. Liver plays a vital role in elimination of Propofol.
Opioids: Commonly used opioids in TVIP include: Morphine Fentanyl – 100 times more potent than morphine Remifentanyl – Short half life Sufentanyl – Ultra short half life Currently only Remifentanyl / Sufentanyl is being used.
Effect of nitrous oxide in middle ear surgery:
Use of nitrous oxide in middle ear surgery is contoversial. Nitrous oxide is more soluble than nitrogen in blood. In high concentrations it enters the middle ear cavity more rapidly than nitrogen leaves it. This causes a rise in the middle ear pressure if the eustachean tube is obstructed. During tympanoplasty, the middle ear is open to the atmosphere and hence there is no build up of pressure. Once the tympanic membrane graft is placed, nitrous oxide might cause displacement of graft.
Endotracheal intubation during GA is associated with complications such as sore throat, cough, dental injury. In comparison Laryngeal mask airway is free of such complications.
Position of the patient:
Head up tilt of 15 degrees to 20 degrees helps in avoidance of venous obstruction, normocapnia and controlled hypotension. A slightly elevated position of the head reduces arterial and venous pressures in areas above the heart. There is also a risk of air embolism in this position. Controlled hypotension is defined as reduction of systolic pressure to 80 – 90 mm Hg. In the presence of hypotension, elevating the head could compromise perfusion of the head and neck region.
Pharmacoligical agents used to produce hypotension:
They include inhalation anesthetics like isoflurane and sevoflurane.
Vasodilators like nitroprusside and nitroglycerin, beta adrenoceptor antagonists like labetalol and esmolol. Recently magnesium sulfate is being used for this purpose.
Avoidance of post operative nausea and vomiting:
Middle ear surgery is associated with a high incidence of post op nausea and vomiting. The etiology of post op nausea and vomiting depends on various factos like anesthetic technique, use of nitrous oxide and duration of anesthesia. Prophylactic administration of antiemetic medication also decreases the incidece of post op nausea and vomiting. The drugs used include a combination of ondansetron 0.1 mg/kg and dexamethasone 0.15 mg/kg.