Ch 16 Conventional Anaesthetic Techniques

CHAPTER 16

CONVENTIONAL ANAESTHESIC TECHNIQUES

Outline: Making the choice of technique General anaesthetic checklist Spontaneous respiration Controlled ventilation Special techniques • techniques used for patients who are likely to regurgitate or vomit under anaesthesia • inhalational induction • pre-oxygenation

227

CHOICE OF TECHNIQUE The anaesthetic technique used will depend on several considerations. There is firstly the choice between a regional and a general anaesthetic. This will depend on: • The patient – time of last meal, general medical state, general mental state. • The surgery – the site of operation, the anticipated duration of surgery, the nature of surgery, (bleeding, intestinal obstruction etc), position in which the surgery is to be performed, the use of diathermy, the use of adrenaline by the surgeon. • The anaesthetic – availability of anaesthetic gases, experience of the anaesthetist (especially in relation to regional techniques), cost. Four important checks must be made. To help the memory we can call them the 4 Ms. • The medical state of the patient. • The time of the last meal. • Medication. This means that pre-operative medication, if ordered, must be given. • The machine to be used must be checked, as described in the previous chapter. If a general anaesthetic is decided on, then the most suitable anaesthetic technique using the gas machine must be chosen - either spontaneous respiration or controlled ventilation. Whichever one is chosen, all of the items in the following list will be needed. Use it as a checklist.

228

GENERAL ANAESTHETIC CHECKLIST Equipment which should always be available on the machine or trolley • • • • • • • • • • • • • • •

Laryngoscope Endotracheal tube (the cuff and connector must be checked) Syringe for inflating the cuff Artery forceps Catheter mount Mask Airway Suction apparatus (pharyngeal sucker) Stethoscope Strapping IV needle/cannula Blood pressure cuff ETT introducer Laryngeal mask, if available Oximetry, BP monitor, ECG and capnography if available

Anaesthetic drugs • • • • • • • •

Thiopentone and water for injection or propofol Ketamine Suxamethonium chloride Pancuronium, vecuronium or any other long acting relaxant An inhalational agent e.g. halothane or ether Atropine sulphate Neostigmine Opioid for analgesia

Drugs for resuscitation • • • • • • • •

Sodium bicarbonate Metaraminol (Aramine) Ephedrine Aminophylline Hydrocortisone Frusemide Naloxone Adrenaline

229

• •

230

Lignocaine Calcium gluconate or chloride

SPONTANEOUS RESPIRATION Preliminary measures • • • • •

Check the four Ms. (See above) Make sure all the items of equipment are available and ready. Draw up the drugs to be used and label the syringes. It is essential to check at the beginning of every operating list, that all emergency drugs required are available. Check the patient's blood pressure and leave the cuff in place. Insert a cannula or start an intravenous infusion.

Induction In the fit adult patient 2.5% thiopentone is used. This is prepared by mixing 0.5 g of thiopentone in 20 ml of water (1.0 g in 40 ml or 2.5 g in 100 ml). Each ml of thiopentone contains 25 mg. Dose of thiopentone is 4-5 mg /kg for a patient of average size. 200 mg (i.e. 8 ml) of this solution can be injected first. Test the eyelash reflex. If it is absent then it may be assumed that sufficient thiopentone has been given (though the patient often may require a further 3-4 ml). If the eyelash reflex is still present after 200 mg, then give further increments of 50 mg at a time and test the reflex. Once the eyelash reflex has disappeared, no further doses are generally required. Test the eyelash reflex by brushing the eyelashes with a finger. The reflex is said to be absent when the patient does not blink when stimulated in this way. The loss of the eyelash reflex is taken to indicate the loss of consciousness. Loss of consciousness is often followed by a deep breath and a period of respiratory depression. Once normal breathing has resumed, but not before, another 2-3 ml of thiopentone may be given if necessary. In the elderly or very ill or "poor risk" patient, induction may be carried out using IV ketamine 1mg/kg or etomidate or thiopentone in very small doses. Thiopentone must be used cautiously, bearing in mind the patient’s prolonged circulation time. Inject 100mg (4ml) of thiopentone slowly. After a pause, test the eyelash reflex and if present inject another 25 mg of thiopentone. Repeat this pattern until the eyelash reflex is no longer discernible. Do not use more than 150mg of thiopentone in a premedicated elderly patient or a very ill patient.

231

To summarise the technique of induction for elective surgery: Fit patient: Thiopentone; sleep dose of 200mg, increments of 50mg. Loss of eyelash reflex; period of respiratory depression. A further 50-100mg of thiopentone if required. Poor risk patient: Ketamine IV 1mg/kg or etomidate 150 micrograms/kg or thiopentone. Sleep dose of 100 mg then PAUSE, then 25mg increment, then PAUSE. Repeat increment once more (total dose to 150mg maximum). No further thiopentone is required. Maintenance A clear airway is maintained by extending the head and lifting up the jaw. Place the mask on the patient's face. Use an oxygen flow rate of 3L/min and a nitrous oxide flow rate of 5L/min. Increase the concentration of halothane gradually starting with 0.5% halothane, to a maximum of 2% halothane. After 3 - 4 minutes check the tone in the jaw muscles. Once the jaw is relaxed, insert a Guedel airway of the appropriate size. If the patient does not cough or react to the airway lower the concentration of halothane approximately 1%. If the patient resists the airway (e.g. by coughing or holding the breath) it is best to remove it and re-insert it after the anaesthetic has been deepened. The following parameters must be monitored at least every 5 minutes and more frequently if necessary: blood pressure, pulse, colour and respiration. If oximetry is available monitor the SaO2 (saturation of arterial oxygen) as well. Uses of the technique This technique can be used for limb surgery, hydrocele, "superficial" surgery of the chest or abdominal wall, curettage, cystoscopies and can be used for all herniotomies if a GA is required. It is not advocated for very prolonged procedures. Dangers of the technique • •

•

232

The airway may become obstructed, especially by the tongue falling back on the posterior pharyngeal wall. Spasm of the vocal cords may occur under the following conditions: − Premature surgical stimulation while the patient is still only lightly anaesthetised. − Direct stimulation of the vocal cords by secretions or vomitus. − Stimulation of the laryngeal area, by the insertion of an airway in a patient who is only lightly anaesthetised. Hypoventilation may occur if the technique is used for prolonged periods.

•

There may be an overdose of halothane.

233

Technique using the LMA An alternative technique would be to insert an LMA at the point where the patient's jaw muscles are relaxed and the Guedel airway is tolerated. Continue maintenance as before with the patient breathing through the LMA. Advantages • Provides a more secure airway with less risk of obstruction. • The anaesthist's hands are free from holding the facemask and jaw throughout the procedure. CONTROLLED VENTILATION Preliminary measures • Check the 4 Ms. • Make sure all the items of equipment are available and in working order on the trolley. • Draw up the drugs and label the syringes. • Check the patient's blood pressure and leave the cuff in place. • Insert an indwelling needle or cannula or start an intravenous infusion. Induction As described previously for the spontaneous respiration technique. Intubation Once the eyelash reflex is lost, give the patient suxamethonium chloride in a dose of 1mg /kg. Suxamethonium chloride (Scoline) paralyses the patient for approximately 3 to 5 minutes. After the relaxant is injected, the patient is managed as follows: First minute • Ventilate the patient with 100% oxygen. • Insert a Guedel airway if needed when the jaw is relaxed. • A flow rate of 6-8 L/min is used. The inflation rate is approximately 20/minute. The chest must rise and fall with each inflation. • After an injection of suxamethonium fine muscle twitching (fasciculation) of the face and body muscles will usually be seen. When the fasciculation of the facial muscles stops, proceed to laryngoscopy and intubate. Second minute • Intubate

234

• • • • •

Third minute Auscultate the chest in the apex of both axillae to ensure the tube is correctly placed (breath sounds must be equal on both sides). Inflate the cuff of the tube with a syringe until the leak of air disappears (less than 5 ml of air should be enough). Insert a Guedel airway as a bite-block. Strap tube in place. Check the blood pressure.

If intubation has not been achieved after the second minute, ventilate the patient with oxygen for another minute before attempting intubation again. If the patient is breathing by this time (i.e. the last dose of relaxant has worn off) half of the original dose of suxamethonium may be given before a second attempt at intubation is made. Atropine 0.3mg IV is sometimes needed to block the bradycardia caused by the second dose of suxamethonium. It is important not to let the patient become hypoxic while intubation is being attempted. If still unsuccessful, consider waking the patient up and getting a more experienced person to help. A maximum of three attempts at intubation is allowable. It is dangerous to continue trying to intubate. This leads to hypoxia and airway oedema, making possible ventilation more difficult. The priority at this point is maintaining adequate ventilation. Wake the patient up and review the options for anaesthesia. (See Chapter 21 for difficult and failed intubation) Maintenance of anaesthesia Begin with a gas mixture of nitrous oxide 5 L/min and oxygen 3 L/min. These flow rates are reduced after the first 3 to 4 minutes depending on the type of circuit being used. (See Chapter 15 for flow rates required with each circuit). When the patient shows signs of returning muscle power e.g. swallowing or breathing, in the fit adult 6 mg of pancuronium (Pavulon) may be given (See Chapter 12 Muscle relaxants). It is important to note the time the muscle relaxant is given on the anaesthetic chart. Further increments may be needed later. The anaesthetic will require to be supplemented and this will depend on the availability of drugs. Halothane 0.5%, enflurane 1% or isoflurane 0.5% may be added. If N2O is not available then higher concentrations of inhalational agents may be required. For painful operations, boluses of IV opioid are given as well (i.e. morphine 1mg, pethidine 10mg, or fentanyl 20 micrograms for adults) so that the patient will be comfortable when they wake up. (See Chapter 55 Acute pain relief.)

235

Reversal This is attempted at the end of the operation. Wait a minimum of 20 minutes after the last dose of relaxant before attempting reversal. Atropine 1.2 mg or glycopyrrolate 0.5mg and neostigmine 2.5 mg are used in the adult. Rules for extubation • The patient must breathe deeply and regularly before extubation is attempted. Neuromuscular function should have returned to normal. • Administer 100% oxygen for at least 3 minutes before extubation to allow the nitrous oxide to be eliminated and to provide a safe reservoir for the patient. • Suction the pharynx before and after deflating the cuff and also after extubation. Active reflexes will protect the airway after extubation. • Do not extubate patients who are hypotensive or cyanosed until the underlying problem has been understood and corrected. • It is very important that an active cough reflex be present after extubation in certain types of patients: − Patients who have had surgery of the upper respiratory tract and in whom post-operative bleeding may occur. − Patients with a full stomach, e.g. intestinal obstruction or a recent meal. In these patients, it is necessary to leave the tube in place until the patient is fully awake, i.e. responds to verbal stimuli. • Give oxygen using the anaesthetic mask. • If there is any risk of aspiration the patient should be extubated awake in the left lateral position. A high proportion of aspiration syndromes occur on extubation. SPECIAL TECHNIQUES RAPID SEQUENCE INDUCTION (RSI) Technique used for a patient who is likely to regurgitate or vomit under anaesthesia Preliminary measures: Check the 4 Ms • The medical state of the patient and the availability of blood • The time of the last meal • The pre-medication • The machine

236

Check the equipment required as described previously (page 229). Draw up the drugs and label the syringes.

237

Take special precautions for patients with a "full stomach". • Empty the stomach using a naso-gastric tube. • Check the suction equipment, making sure it is turned on and the suction head is left tucked under the mattress of the operating table. It must be easily accessible and ready for use when needed. • Check the cuff of the endotracheal tube. The inflating syringe is left attached to the cuff. • Explain the principle of cricoid pressure to the assistant (and to the patient). Cricoid pressure is applied over the cricoid cartilage to compress the oesophagus that lies behind it. This prevents gastric contents rising up the oesophagus and contaminating the respiratory tract. The cricoid cartilage lies immediately below the thyroid cartilage (Adam's apple) which is easily palpable. The cricoid pressure is applied as the patient is being induced (i.e. before the loss of protective airway reflexes). The cricoid pressure is maintained until the patient is intubated and the cuff of the endotracheal tube is inflated and tested for the presence of a leak.

Fig 16.1 The technique of cricoid pressure

238

ANAESTHETIC TECHNIQUE – RAPID SEQUENCE INDUCTION • • • • • • • • •

• •

•

Check the blood pressure. Insert an indwelling needle or cannula into the vein. Start an intravenous infusion if necessary. Place the patient on the table in the horizontal position. Give 100% oxygen for 4 minutes. Give a pre-selected dose of thiopentone (or ketamine), followed by a selected dose of suxamethonium. (This is the one instance where the dose of the induction agent is not titrated by testing the eyelash reflex). The assistant applies cricoid pressure and maintains this until the cuff of the endotracheal tube is inflated. (The patient is not ventilated with a mask before intubation). Insert the laryngoscope when the jaw is relaxed and intubate the patient. Inflate the cuff of the endotracheal tube. Hand ventilate the patient while the position of the tube is being checked by auscultation of the chest and upper abdomen. This is to ensure that the oesophagus or the right main bronchus has not been intubated. It is best to listen out in the axillae to ensure bilateral air entry. Strap the tube in place. Note: it is possible to hear transmitted sounds in the axilla from an oesophageal intubation. Capnography is extremely useful. If there are any doubts remove the tube and re-intubate. Connect the patient to the anaesthetic machine. Insert a Guedel airway as a bite block. Maintain the anaesthetic by air- nitrous oxide-oxygen-volatile-relaxant. − Give I.V opioid as required. − Monitor carefully for signs of inadequate anaesthesia. − Calculate and give IV fluids as necessary. Reversal − Atropine / glycopyrrolate and neostigmine are used in the dosage described above. − The rules for extubation are followed but extubate these patients when they are fully awake.

239

INHALATIONAL INDUCTION This is an alternative to intravenous induction in the following situations: • Very young children where an experienced anaesthetist prefers this mode of induction. • Patients with poor veins. • Patients who request inhalational anaesthesia for fear of an injection. • Patients who may have intubation problems and where the use of a relaxant may be dangerous, e.g. patients with known or possible airway obstruction after induction of anaesthesia. Do not paralyse any patient who may be difficult to ventilate. Partial airway obstruction can become complete airway obstruction on induction with loss of airway tone. An awake technique must be used. Enflurane, halothane and ether are suitable agents for inhalational induction of anaesthesia. This technique will often be used with only oxygen or air but if N2O is available it can be used in addition to the halothane or ether and may aid a smooth induction. The use of halothane as an induction agent is described under the pharmacology of halothane. If ether is being used as an induction agent it is important to remember that the concentration of ether must be increased very gradually. Start the induction with the lowest concentration of 1% and make the patient take 8 breaths before you increase the concentration by a further 1%. Induction is therefore a slow process. It is important that the patient be deep enough before intubation is attempted, or laryngeal spasm will result. The patient must be able to tolerate a Guedel airway and breathe 2-4% halothane or 10-15% ether after the insertion of the airway for at least another minute before intubation is attempted. If when the airway is introduced, or later the laryngoscope, the patient swallows, holds breath, reacts in any way, withdraw the airway or laryngoscope immediately and deepen the anaesthesia. An inhalational induction or intubation can never be a hurried process; be as comfortable as possible. For example sit near the head end of the table while inducing anaesthesia.

240

PRE-OXYGENATION Many anaesthetists believe that all patients should be pre-oxygenated but oxygen should always be administered before the induction of anaesthesia in the following situations: • • • •

•

•

Very ill or moribund patients. Patients with a full stomach (e.g.unfasted or with intestinal obstruction) in which a rapid sequence induction with cricoid pressure is used. Patients with possible intubation problems, where a slightly longer time interval may be required for intubation. Patients who are not desperately ill but who have some medical problem which cannot guarantee a normal degree of oxygenation by conventional anaesthetic technique, e.g. severe anaemia, chronic obstructive airway disease, obesity. Patients in whom even a small fall in the oxygen tension would have very adverse effects. For instance, those with − Sickle cell anaemia − Ischaemic heart disease − Liver disease Obese patients.

If a patient breathes 100% oxygen by mask, the nitrogen is eliminated from the lungs in 2 minutes, from the blood stream in 5 minutes and from the brain in 20 minutes. The average healthy adult has some oxygen reserves in the body which will be utilised during a period of apnoea. The FRC (see Chapter 3) is the most important source of this and under normal circumstances i.e. breathing room air, allows approximately 90 seconds before hypoxia develops. Four minutes of pre-oxygenation with 100% oxygen increases the FRC stores of oxygen significantly delaying the onset of hypoxia after apnoea by 3-4 minutes. If pre-oxygenation is considered necessary, administer 100% oxygen by mask for 4-5 minutes. Note: it is important to ensure a good seal of the mask to prevent dilution of oxygen with air.

241