Monitoring and maintenance of Anaesthesia

 

 

MONITORING ANAESTHESIA

INTRODUCTION

·  Pre, intra and post operative monitoring are most important for the final out come of anaesthesia and surgery.

The monitoring procedures are aimed to assess the functions of cardiovalscular, pulmonary and CNS and body temperature, fluid and electrolyte balances. PRE OPERATIVE PATIENT MONITORING

·  Preoperative assessment of the patient is done for the safe administration and maintenance of anaesthesia.

HISTORY

Identification

·  Identification includes the details of species, breed, sex, age and other identification marks.

Main complaint

·  To find out whether the disease condition will interfere with the normal anaesthetic practice and to tailor suitable anaesthetic regimen.

History of the present illness

·  Details of the duration of illness, clinical signs and severity of illness are collected.

Previous medical history

·  This includes the collection of details regarding the previous illness, medication, vaccination, deworming, anaesthetics administered, poisoning, application of ectoparasiticide etc.

PHYSICAL AND CLINICAL EXAMINATION

·  Physically examination includes general body condition, palpation, percussion, auscultation, measurement of heart, pulse and respiratory rates, examination of lymph nodes, rectal temperature, appearance of the mucous membrane, reflex status, integument, location of the lesion and weight of the animal.

·  Weight calculation

o Horse = (Heart girth cm- 63.7)/0.38 = body weight in Kg.

o Cattle= G x G x L(in inches)/300 = BW in pounds.

Systemic examination

·  Systemic examination includes the assessment of cardiovascular, pulmonary, hepatic, renal gastrointestinal, central nervous system, endocrine and musculoskeletal functions.

Presurgical laboratory examination

·  It includes the determination of a complete blood count and total plasma protein.

Further tests

·  Includes ECG, X-rays and other special examinations.

INDIRECT MONITORING

·  Indirect monitoring of CNS function is assessed by the reflex status. The reflex status is modified by the stages of anaesthesia, drugs used and cerebral blood flow.

·  The following reflexes are assessed

o Pedal reflex

o Palpebral reflex

o Corneal refle

o Lacrimation

o Yawning

o Swallowing reflex

o Laryngeal reflex

o Anal reflex

o Pupillary reflex

o Eyeball position

o Hearing sense

PEDAL REFLEX

·  This reflex is elicited by applying firm pressure on the interdigital skin in dogs and cats.

·  This reflex is abolished in stage III anaesthesia.

·  Pedal reflex is reliable in barbiturate anaesthesia to assess the depth of anaesthesia, where as with halogenated inhalants it disappears even in the light plane of anaesthesia.

PALPEBRAL REFLEX

• Tapping the skin at the medial canthus or running the finger along the eyelashes stimulates this reflex.

·  It is abolished in the light plane of anaesthesia in dogs.

·   Palpebral reflex is not abolished during ketamine anaesthesia

 

CORNEAL REFLEX

·  This reflex is stimulated by gentle palpation of the cornea on the lateral aspect.

·  The response is observed by the closure of eyelids.

·  In horses absence of corneal reflex indicates deep plane of anaesthesia, in dogs its not reliable.

·   Corneal reflex is not abolished during ketamine anaesthesia.

LACRIMATION AND YAWNING

Lacrimation

·  In horses and cattle lacrimation is reduced during deep plane of anaesthesia, leading to drying of cornea. It may result in keratitis and ulceration..

Yawning

·  Dogs under light plane of anaesthesia yawn when the mouth is opened.

SWALLOWING AND LARYNGEAL REFLEX

Swallowing reflex

·  This reflex disappears at the light plane of anaesthesia. This reflex is protected in ketamine anaesthesia.

Laryngeal reflex

·  This reflex is abolished in the light plane anaesthesia except with ketamine induction.

ANAL REFLEX

·  This reflex is abolished in the middle of III stage of anaesthesia in dogs and cats.

·  In horses it is abolished soon after induction with ketamine.

·  This reflex is elicited by sudden gentle manipulation of the anus and the response will be sphincter contraction.

PUPILLARY REFLEX

·  In general the pupil in un premedicated animals will dialate during early excitement phase and then constricts progressively upto surgical anaesthesia.

·  Again the pupil will dialate as the animal enters into the IV stage of anaesthesia.

·   Premedicants alter the papillary reflex.

·  E.g. Atropine induces pupillary dialatation and narcotics induce constriction in dogs.

EYEBALL POSITION

·  In small animals the eyeball rotates medially and ventrally in the early stages and then centrally placed in third plane and at plane I when inhalants like halothane or isoflurance is used.

·  In horses under halothane anaesthesia nystagmus is common during light plane of anaesthesia and it is centrally placed at the surgical plane of anaesthesia.

·  In ruminants the eyeball rotates ventrally in  plane I and then gradually rotates dorsally in plane II and finally fix to the central position.

OTHER REFLEXES

Muscle relaxation

·  In small animals the jaw tone is used as the criteria of muscle relaxation and anaesthetic depth.

Hearing sense

·  It is the last sense to disappear during induction and the first sense to reappear during recovery.

ELECTROENCEPHALOGRAPH

·  The normal EEG pattern is low voltage high frequency activity in the activated state of brain.

·  During cerebral hypoxia, hypoglycemia, hypothermia, hyponatremia and at excessive depth of anaesthesia it becomes high voltage and low frequency.

HEART RATE

·  Heart rate can be monitored by using stethoscope.

·   Heart rates below 50 to 60 bpm in dogs and cats, 25 bpm in horses and ruminants is considered to be low heart rate.

·  Heart rate above 250bpm in dogs, 300 bpm in cats, 75 bpm in horses and ruminants are considered as high heart rate.

·  The alteration in heart rate must be simultaneously compared with cardiac output and blood pressure.

BRADYCARDIA

·  Bradycardia may arise due to

o Excessive depth of anaesthesia

o Excessive vagal tone

o Terminal hypoxia

o Endogenous and exogenous toxaemias

o Conduction disturbances in myocardium

o Hyperkalaemia

o Hypothyroidism

Treatment

o Administration of atropine or glycopyrrolate.

o Dopamine 2.5 to 20ug/kg/min.

o Isoproterenol 5 to 10 ug/kg/min.

TACHYCARDIA

·  Tachycardia may arise due to

o Light level of anaesthesia

o Hypovolaemia

o Hypoxia

o Hyperthyroidism

·  Normally pulse rate may either be equal or slightly deficit of heart rate.

·  If the  ventricular contraction is persistent with heart rate exceeding 180 to 200 the following treatments must immediately be adopted.

o Check the oxygen supply and maximize the inspired oxygen level.

o Institute intermittent positive pressure ventilation

o Start fluid administration

o Administer anyone of the following.

·         Lignocaine 1 to 5 mg/kg I.V.

·         Procainamide 1 to 5 mg/kg I.V

PERIPHERAL PERFUSION

·  It is assessed by the colour of the mucous membrane and the capillary refill time.

·  The normal capillary refill time is less than 2 seconds.

·  Pale mucous membrane and prolonged refill time are due to reduction in perfusion.

·  The reasons for reduced peripheral perfusion

o Stress induced  sympathetic tone

o Hypovolemia

o Low cardiac output

o Fear and pain

o Exogenous alpha – Receptor agonist.

BLOOD PRESSURE Blood pressure is one of the important parameter to be monitored during anaesthesia because adequate blood pressure is needed to perfuse the brain and heart. A minimum 50 to 60 mm of Hg mean arterial blood pressure (MAP) is to be maintained for coronary and cerebral perfusion. Arterial blood pressure can be measured by Direct and Indirect techniques in animals. Indirect technique-By BP meter. Direct technique Direct method of blood pressure measurement is done by catheterization of a suitable artery and connecting it to an aneroid manometer to assess the mean arterial pressure. The mean arterial pressure can be calculated from systolic and diastolic pressure using the formula: MAP in mm of Hg = Diastolic + ((Systolic – diastolic)/3) The difference between the systolic and diastolic pressure is called as pulse pressure.

 

TREATMENT OF HYPOTENSION

·  Discontinue the anaesthetics and adjuncts, which induces hypotension and use the agents like diazepam and ketamine.

·  Lactated Ringer’s 10 to 40 ml/kg is administered over a period of 10 to 30 minutes.

·  Multielectrolyte sodium containing crystalloid replacement solutions can be administered routinely at the rate of 10 ml/kg/hr plus 2 to 3 times the volume of estimated blood loss. During major procedures like thoracotomy, fracture repair and laparotomy it can be increased upto 20 ml/kg.

·   If the PCV is less than 20%,  blood is indicated and if the total serum protein is less than 3 to 3.5 g/dl further volume replacementis done only by plasma or dextran.

CENTRAL VENOUS PRESSURE

·  Central venous pressure (CVP) is the pressure inside anterior vena cava or right atrium.

·  The central venous catheters are positioned in jugular vein.

·  The nomal CVP is 0 to 10 cm of H2O in small animals, and 5 to 10 cm of H2O in cattle, sheep and goats.

·  Increase in CVP could be noticed in reduced cardiac output, vasoconstriction and hypervolemia.

·  CVP decreases during vasodilatation, hypovolemia and obstruction to venous return.

Fluid therapy is indicated when increase in CVP is noticed with heart failure. RESPIRATORY RATE

·  Carbon dioxide is the primary chemical stimulant of respiratory centers to maintain normal respiratory pattern.

·  Hypocapnia is in anaesthetized patients.

·  In anaesthetized patients each respiration must be long and large to satisfy the ventilatory requirement and oxygen demand.

COLOR OF MUCOUS MEMBRANE

·  Cyanosis indicates severe hypoxemia.

·  If cyanosisis noticed during anaesthesia immediately the oxygen supply must be checked for the correct delivery.

·  The oxygen is supplied at the rate of 10 ml/kg/min in circle system and 20 ml/kg/min in non rebreathing system.

·  The other reasons for cyanosis are shock, hypothermia, cardiac arrest and intra thoracic lesions.

VENTILOMETRY

·  Measurement of ventilation volume is ventilometry.

·  Visual observation suggests the volume roughly.

·  Ventilometers are fitted on the expiratory side of the breathing circuit will indicate the tidal and minute volume.

·  The normal ventilation is 150 to 250 ml/kg/min. Minute volume below 100 ml.kg/min is considered as hypoventilation and above 300 ml/kg/min as hyperventilation.

BLOOD GAS Arterial blood is collected in 2 ml heparinized syringe with 22 to 25 gauge needle and the needle is corked or the needle guard is replaced immediately. The syringe is kept in ice and sends for analysis using blood gas analyzer.   Partial pressure of carbon dioxide (PaCO2) The normal PaCO2 is 35 to 45 mm Hg. PaCO2 less than 35 mm Hg indicates hyperventilation. PaCO2 above 45 mm Hg indicates hypoventilation. PaCO2 above 60 mm Hg indicates severe respiratory acidosis PaCO2 less than 20mm Hg indicates severe respiratory alkalosis and decreased cerebral blood flow Partial pressure of oxygen (PaO2) The normal PaO2 is 90 to 100 mm Hg. Pa02 less than 60 mm Hg indicates hypoxia and hypoventilation

METABOLIC ACIDOSIS

·  The pH will indicate the metabolic acidosis and is attributed to the lactic acidosis secondary to inadequate tissue perfusion due to vasoconstriction, hypotension, hyperthermia or infusion of acidotic fluids.

·  Bicarbonate is administered only for the patients having bicarbonate deficit, not for all acidotic conditions.

TEMPERATURE

·  Recording body temperature is important in anaesthetized patients as it indicates the systemic function.

·  The temperature can be recorded at deep rectum, cervical oesphagus, pharynx and under the tongue.

·  During anaesthesia drop in temperature could be noticed due to the reduction in metabolic rate.

·  Anaesthesia depletes  catecholamine in the thermoregulatory center and render the animal to pick up the environmental temperature.

URINE OUTPUT

·  It is an indirect assessment of visceral perfusion. Urinary catheters are placed aseptically and the urine is collected.

·  The normal expected urine output in anaesthetized animals is 1 to 2 ml/kg/hr.

·  If the urine output is reduced lactated Ringer’s is administered at the rate of 20 to 40 ml/kg rapidly to induce diuresis.

·  The other agents administered to induce diuresis are

o Frusemide 5 mg/kg.

o Glucose, Mannitol 0.5 g/kg over 20 to 30 minutes.

o Dopamine 1 to 5 µg/kg/min.