On 28 January, 1848, 15-year-old Hannah Greener was the first anaesthetic mortality. She died while under chloroform anaesthesia for the removal of an ingrown toenail. This incidence did nothing to improve objective monitoring during the remainder of the 19th century. Assessment of the patient was qualitative and subjective.
The precordial stethoscope was first put into use by Guedel in the 1930s. Blood pressure was, however, monitored before this in 1903. Harvey Crushing used a Riva-Rocci sphygmomanometer, a technique that was only first reported in 1896. In 1905, Korotkoff described the sounds that have taken his name as an aid to blood pressure measurement. In 1931, von Recklinghausen introduced the oscillotonometer for the assessment of blood pressure. This method is still used today. Direct arterial blood pressure was first performed in 1733 by Stephen Hales. Its routine use in clinical practice had to await the development of safe percutaneous arterial catheterisation. Petersen and Dripps achieved this in 1947. Deep venous catheterisation for cardiac catheterisation was first performed by Werner Forssman in 1929, on himself. With this in place, Fick followed with the measurement of cardiac output.
The goal of monitoring the cardiovascular and respiratory system is to ensure the delivery of oxygen to each cell. Most of the parameters we monitor assess global perfusion and not regional perfusion. The error in this is that we have no idea of how the cardiac output is distributed.
Monitoring comes from a Latin term "monere" meaning "to warn". The aim of monitoring is to supply the anaesthetist with enough information as to give a warning before an anaesthetic accident happens. An anaesthetic incidence is any event that occurs during an anaesthetic that could be potentially harmful or have a negative outcome to the patient. An incident may happen acutely or result from a slower decompensation of a vital organ system. An anaesthetic accident results when an anaesthetic incident has occurred and it has not been rectified in time. As a result, the patient suffers. The time between an incident and an accident determines the safety factor and severity of a particular incident.
The term vital signs refers to those parameters that indicate the response of a patient's homeostatic mechanisms. This includes heart rate, respiratory rate and capillary refill time. The patient's vital signs should indicate how well the patient is maintaining basic circulatory and respiratory function during anaesthesia.
The term reflex refers to an involuntary response to a stimulus. Reflex responses give the anaesthetist valuable information on the depth of anaesthesia but do not convey information on the patient's homeostatic mechanisms.
The anaesthetist originally used the senses of sight, hearing and touch to monitor a patient. With modern medical science has come a host of machines to aid in the monitoring of patients, but these machines have not replaced the clinical judgement of a skilled anaesthesiologist. What these machine have done is enable us to more closely monitor vital functions and give us advanced warning of anaesthetic incidences and accidents. This has made anaesthesiology safer.
Monitoring During the Induction Period
Parameters to be monitored during anaesthesia
Respiration rate, depth, and character (assess both reservoir bag movement and chest movements)
Mucous membrane colour and capillary refill time (CRT)
Jaw tone, eye position, and palpebral reflex activity
The key to effective and safe anaesthesia during the maintenance period is adequate monitoring. The word monitor meaning, "to warn." The anesthetist who monitors the animal under anaesthesia closely will usually receive ample warning of problems as they arise.
Vital signs may be monitored by the anaesthetist's senses (touch, hearing, and sight) or through the use of electronic devices such as an ECG machine or pulse oximeter. Electronic surveillance, although convenient, should not be relied on to give a complete picture of patient status, as instruments are subject to power failure, interference from artefacts, and loss of contact with the patient. Instrumentation cannot replace the presence of a skilled and conscientious anaesthetist.
Vital signs that should be monitored during anaesthesia include heart rate and rhythm, blood pressure, central venous pressure, capillary refill time, mucous membrane colour, blood loss, respiratory rate and depth, blood gases, and thermoregulation.
Physiological Aspect to Cardiopulmonary Monitoring
The delivery of oxygen to each cell requires that sufficient haemoglobin is available, blood is fully saturated in the lungs with oxygen and that sufficient blood passes each cell. Essentially, two major systems are involved, and these are generally monitored separately, but one cannot ignore the fact that they are integrated. Oxygen delivery is a function of three major components: haemoglobin, saturation and cardiac output. Saturation is influenced by the partial pressure of oxygen. The amount of oxygen dissolved in plasma is small and this is generally ignored.
Visual inspection of the mucous membranes for cyanosis as a method for determining hypoxia is a very unreliable indicator. This was determined in 1947 in a trial involving 7,200 subjects. In this trial, 37% of nonhypoxic people were deemed hypoxic while 12% of hypoxic people were deemed nonhypoxic. In order for cyanosis to be detected, at least 5 g/dl of haemoglobin are required. The degree of hypoxia is not quantified by cyanosis, but its presence can be regarded as clinically significant.
Transcutaneous oximetry uses the principle of light absorption to determine the percentage of saturation. In order for oximetry to function, the following criteria must be met:
The light must transilluminate arterial blood.
No significant quantities of other haemoglobins should be present.
The absorption of light due to tissues should be negligible.
These devices cannot differentiate between arterial and venous blood. Oximetry relies on the position of application and ensuring that the area was kept warm to ensure that arterial blood predominated. Modern oximetry makes the difference between arterial and venous haemoglobin by assuming that the pulsatile portion of the absorption spectrum is entirely due to arterial blood. This is known as pulse oximetry and ensures that the first condition is met. The normal patient has less than 5% of his haemoglobin in another form in the arterial blood. This meets condition two. The last condition is met by careful selection of light wavelength and the application to nonpigmented areas of the skin.
Pulse oximetry improves the detection of hypoxaemia 20 fold and that of hypoventilation 3 fold. Desaturation is often accompanied by an increase in heart rate and electrocardiographic evidence of hypoxia. Carboxyhaemoglobin absorption spectrum falls into the same spectrum as oxyhaemoglobin. This results in a falsely elevated saturation reading. Methaemoglobin results in an underestimation of saturation. Low haemoglobin concentration tends to under estimate saturation. Methylene blue and other dyes result in altered readings. Saturation below 75% leads to gross errors in readings, but at this point gross hypoxia is present.
Heart rate is one of the basic parameters to monitor during anaesthesia. Auscultation or direct palpation of pulse may be used to monitor heart rate. The pulse can be palpated over the femoral artery (not in horses and cows), lingual arteries, facial artery (horse), transverse facial (horse), internal iliac arteries (horses and cows), metacarpal arteries (horses), metatarsal arteries (horses), auricular artery (cows) and directly over the heart (usually small animals). Auscultation can be done with an oesophageal stethoscope. The acceptable heart rate under anaesthesia is dependent on the normal resting heart rate of the patient. Normally, an allowance of 10 to 20% below normal is allowed. Large animals (horses, cattle) have slower resting heart rates than small dogs, and even slower than mice. Generally, it is accepted that a heart rate below 60 beats per minute constitutes a bradycardia and should be managed appropriately. Neonates, geriatrics and patients with cardiac disease have very little ability to adjust stroke volume and are therefore more dependent on heart rate for cardiac output than other groups of patients. Heart rate does not tell us anything about cardiac output, blood pressure or circulation.
Careful auscultation or palpation of heart rate can diagnose certain arrhythmias. An electrocardiogram is a useful device to monitor the rhythm of the heart. The ECG does not, however, diagnose pulseless electrical activity (electromechanical dissociation). For this reason, it is important to be able to palpate a pulse.
Pulse pressure is the difference between systolic and diastolic blood pressure. A strong pulse pressure indicates to us that there is large difference between these pressures. It does not, however, give us an indication of actual blood pressure. A blood pressure of 40/80 mm Hg has the same pulse pressure as a blood pressure of 100/140 mm Hg. With weak cardiac contractions, the pulse pressure becomes small.
Invasive blood pressure gives us continual information with regards to cardiac contraction (no contraction no blood pressure). Invasive blood pressure is preferred to noninvasive blood pressure for the following reasons:
Real time information is available. This is especially useful when positive inotropes and vasoactive drugs are used to maintain blood pressure. Noninvasive blood pressure usually only takes a reading every few minute.
Invasive blood pressure monitors are more accurate than noninvasive blood pressure machines. Noninvasive blood pressure machines do give useful trend information, especially when a number of readings are averaged.
Blood pressure, however, does not give us an indication of cardiac output. Cardiac output and total peripheral vascular resistance are the two dependent factors in determining blood pressure. A low cardiac output with a high peripheral vascular resistance may give us the same blood pressure as high cardiac output and a low peripheral vascular resistance.
Capillary refill time gives us an indication of peripheral blood pressure and circulation. A normal capillary refill time is 1 to 2 seconds. It is important to realise that a dead patient also has a normal capillary refill time. Instead of arterial blood filling the capillary bed, venous blood fills the bed. A prolonged capillary refill time is usually an indication of hypotension or shock. Mucous membrane colour is also observed when testing the capillary refill time. Normally the mucous membranes are pink. Having said that, pink mucous membranes do not indicate that all is well. After euthanasia, an animal's mucous membranes will remain pink for several minutes. Blue mucous membranes (cyanosis) and very bright pink mucous membranes with very fast capillary refill times (hyperdynamic shock) indicate impending doom if not corrected. Certain breeds of dogs (Chows, Dalmatians, German shepherds) may have normally pigmented mucous membranes. The tongue, buccal, conjunctiva, prepuce and vulva may be used to assess mucous membrane colour.
Urine output is a useful tool for monitoring blood pressure and perfusion. Normal urine production is 1 to 2 ml/kg/min. If renal perfusion (blood pressure, blood flow) is not adequate, urine production ceases. Although this does not give us real time information, over an hour it gives us very useful information.
Respiratory rate is easily counted by observing the movement of the reservoir bag of the anaesthetic machine. Respiratory rate is only part of alveolar ventilation and it is important to pay attention to tidal volume. Respiratory rate and tidal volume give us minute ventilation and an idea of alveolar ventilation if dead space is kept to a minimum. It should also be born in mind that spontaneous ventilation (respiratory rate measured from chest movement) does imply that ventilation is occurring or is adequate.
Capnography is a useful tool to monitor respiratory and cardiovascular function. The device measures inspired and expired carbon dioxide. In order for the carbon dioxide to appear in the expired gas, adequate alveolar ventilation and pulmonary circulation are required. With a drop in alveolar ventilation, expired carbon dioxide rises and with a drop in perfusion, expired carbon dioxide drops. This device also determines respiratory rate, and some machines will determine tidal volume and minute ventilation.
Pulse oximetry measures peripheral oxygen saturation. For a normal reading of 97% to be obtained, normal pulmonary function (oxygen diffusion, alveolar ventilation, pulmonary blood flow) and peripheral perfusion are required. Any changes in these parameters will alter peripheral saturation.
Blood gasses are used to evaluate respiratory function and acid base balance. The interpretation of blood gas analysis is complex, and these tests are expensive to run. However, they form the gold standard for this type of evaluation.
Throughout the anaesthetic period, the patient's temperature will fluctuate. The normal homeostatic mechanisms responsible for thermoregulation are disrupted and the patient will cool to environmental temperature. The patient will initially cool by a degree on induction due to redistribution of central and peripheral blood. This temperature drop cannot be avoided, but further drops in temperature can be managed.
Several factors contribute to the drop in temperature:
Patients are routinely shaved, removing their thermo-insulator layer.
Patients are prepared with cool water and alcoholic preparation that further drop body temperature.
Anaesthetised patients cannot shiver to generate body heat.
Metabolic rate of anaesthetised patients drops.
Body cavities are opened during surgery, increasing exposed surface area and allowing for evaporative cooling.
Several preanaesthetic and anaesthetic agents induce vasodilatation, resulting in increased heat loss through the skin.
Operating theatres and metal tables are cold.
The unfortunate results of hypothermia include a prolonged recovery for anaesthesia, cardiac complications (bradycardia, low cardiac output) and increased oxygen demands in the postoperative period.
In order to prevent hypothermia, the following should be done:
Warm water blankets and other heating devices should be used during induction, maintenance and recovery from anaesthesia.
Environmental operating room temperatures should be raised.
Insulation from cold surface should be provided.
During preparation, the patient should be kept as dry as possible.
Warm intravenous fluid should be administered.
Hyperthermia is rare and usually occurs in patients with tetanic muscle activity or in patients which develop malignant hyperthermia. Malignant hyperthermia is usually rapidly fatal. Temperature should be monitored every thirty minutes under anaesthesia.
Monitoring anaesthetic depth is trickier than monitoring a physiological parameter, as it has no defined end point. Anaesthetic depth is usually determined after a number of factors have been considered. This makes the determination of anaesthetic depth more of an art than a science.
All conscious animals demonstrate predictable reflex responses to certain stimuli. These reflexes are usually protective in nature and disappear with increasing depth of anaesthesia. The palpebral reflex can be tested by lightly tapping medial or lateral canthus of the eye and observing whether the animal blinks or not. This reflex may also be tested by gently stroking the hairs of the upper eyelid. This reflex is maintained until Stage III Plane III of anaesthesia. Touching the cornea with a sterile object (drop of sterile water) and noting whether or not the eyeball is withdrawn into the orbital fossa or the patient blinks can test the corneal reflex. This reflex is usually present until the end of Stage III Plane III of anaesthesia and indicates that the patient is too deep. Pinching a digit across one of the bones to elicit deep pain, noting whether or not the limb is withdrawn tests the pedal reflex. The pedal reflex is a reliable indicator of anaesthetic depth and usually disappears in Stage III Plane III of anaesthesia.
The coughing, swallowing and laryngeal reflex are present in light planes of anaesthesia (Stages I and Stages II). These reflexes may complicate endotracheal intubation. The swallowing reflex is important for the protection of the upper airway. The return of this reflex indicate that endotracheal extubation may be performed. Tickling the inside of the ear and observing for a flicking of the ear tests the ear reflex. This reflex may be unreliable but is usually lost in Stage III Plane III of anaesthesia.
Muscle tone may be used as a guide to anaesthetic depth. The deeper the anaesthetic plane, the greater the degree of muscle relaxation. This reflex is assessed through the muscles of mastication (jaw tone), flexion and extension of the fore and hind limbs and abdominal muscle tone. Muscle tone may be influenced by a number of drugs.
The rotation of the eyeball in relation to the stages of anaesthetic plane has already been discussed. The pupil diameter also changes with anaesthetic depth. Increased production of tears and saliva indicates a lightening of anaesthetic plane. Changes in heart and respiratory rate may also be used to indicate changes in anaesthetic plane. Usually these increase with a lightening anaesthetic depth. Painful stimuli under anaesthesia may also influence heart rate, respiratory rate, pupil diameter and a relative lightning of anaesthetic plane.
A distinction between surgical pain and anaesthetic depth should be made. A high degree of surgical pain will give the impression of a light surgical plane of anaesthesia. One is inclined to turn up the anaesthetic delivered to the patient. As the plane deepens, less of a response to the pain is seen. When the surgical stimulus is removed, the patient suddenly lapses into a dangerously deep anaesthetic plane. These patients require more analgesia not anaesthesia.
References are available upon request.