Ch 50 Monitoring Under Anaesthesia

CHAPTER 50

MONITORING DURING ANAESTHESIA

Outline: Cardiovascular system: Pulse Blood pressure Central venous pressure The electrocardiograph Blood loss Respiratory system: Tidal volume Respiratory rate Colour of blood Arterial gases Oximetry Capnography Urinary system Temperature Neuromuscular blockade Other parameters monitored in special units

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Monitoring equipment should never replace sound clinical judgment and common sense. The anaesthetist must always remain with the anaesthetised patient and must be aware of every problem, surgical and otherwise, to which the patient is exposed. Good communication with the surgeon is therefore essential. The colour of the blood, mucous membrane of the mouth, skin temperature and the patient's depth of anaesthesia must be constantly monitored. CARDIOVASCULAR SYSTEM Pulse Always make sure there is access to the pulse when the patient is draped. The radial or the brachial pulse is usually palpated. Note the rate and the regularity of the pulse, every 5 minutes in the average patient and even more often in the very ill patient. The volume of the pulse is important. It is a good habit to place a hand that is free on the patient's pulse whenever possible. The normal pulse rate in adults varies between 50 and 90 beats per minute. Blood pressure (BP) Measure blood pressure is normally measured with the use of a sphygmomanometer. Monitor and chart it every five minutes in patients who are stable and more often in ill patients. Both systolic and diastolic blood pressures must be recorded. The blood pressure cuff must be wide enough to cover two-thirds the length of the upper arm. Palpation method The pressure in the cuff is increased to a point above which the radial artery is impalpable, then decreased at a rate of 2-3 mmHg per second. The point at which the radial pulse re-appears is the systolic blood pressure. Auscultation method Both systolic and diastolic blood pressure are obtained by this method. The systolic blood pressure is about 10 mmHg. above that obtained by the palpation method. Oscillotonometer. The blood pressure may be recorded by noting the oscillations of a needle in the oscillotonometer. Automatic electronic devices are commonly used today. The normal blood pressure varies. For an adult it is 100/70 to 140/90, while for babies and small children it is 80 or 90 systolic and increases gradually with age.

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Central venous pressure (CVP) The CVP is the pressure in the venous side of the circulation measured by a catheter inserted into the large veins opening into the heart. Normal CVP is from 5 to 10 cm of water. About 80% of the blood volume is contained in the venous circulation, so when the blood volume falls, the venous pressure falls. Venous pressure falls before arterial pressure. The measurement of the venous pressure gives us an early indication of blood loss. If the patient has been transfused and the blood volume has been increased, the venous pressure rises. The venous pressure also rises if the heart is failing. When the heart action fails there is a banking up of blood on the venous side of the circulation. The central venous pressure should always be interpreted together with the other parameters monitored - arterial blood pressure, pulse, peripheral circulation, etc. Guide to CVP measurements CVP Low

BP Low or normal

Diagnosis Fluid Lack

Fluids Increase

Drugs

High

Normal

Fluid overload

Reduce

Diuretics e.g. frusemide

High

Low

Cardiac Failure

Restrict

Diuretics Digoxin(for rapid AF) IV inotropes eg dobutamine

Indications for CVP monitoring • Whenever massive infusions or transfusions are given • In severe shock • In patients with heart failure receiving fluid therapy • In acute cardiac failure of obscure origin.

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The Electrocardiograph (ECG) The ECG gives an accurate assessment of: • Rate • Rhythm of the heart • The degree of oxygenation of the heart muscle. Hypoxia may show as a change in the ST segment. Ideally, all patients anaesthetised should be monitored electrocardiographically. The ECG is especially useful in: • Patients with heart disease such as arrhythmias, ischaemia, hypertension, heart failure • Patients who have a previous history of anaesthetic problems • Patients who are shocked, hypotensive or bleeding • Patients who are poor risks due to underlying medical conditions, e.g. thyrotoxicosis, diabetes, renal or liver disease. Blood loss Blood loss can be measured by several methods: The gravimetric method: The swabs, sponges and packs are weighed dry and then with the absorbed blood. The difference in weight gives the weight of the blood absorbed. One gram of blood is then taken to be equal to 1ml of blood. To this weight is added the volume of blood in the suction bottles. A further 25% should be added to the total to account for the blood in the drapes, etc. Colorimetric method: The swabs and sponges are mixed with a large known volume of liquid and the haemoglobin level of the resultant solution is estimated. By using a formula the blood loss may be worked out. RESPIRATORY SYSTEM Tidal volume To estimate the tidal volume you need a spirometer. A Wright’s respirometer is commonly used as it is small, portable and does not require any power supply. The gas flow drives a spinning vane in one direction only and the speed of rotation is converted to volume which is then displayed on a dial. The Respiratory Rate This should be noted, especially in the case of spontaneous respiration. Colour of blood This must be constantly observed. It shows how well oxygenated the patient is and therefore gives an idea of the respiratory and cardiovascular systems but is not nearly as accurate as pulse oximetry.

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Pulse oximetry Arterial oxygen saturation (SaO2) is the measure of oxygenation of the blood. The relationship between oxygen saturation of haemoglobin (Hb) and the partial pressure of oxygen in the blood is not linear (See Respiratory physiology in Chapter 3). As saturation drops below 80% partial pressure drops dramatically. Any value below 90% is clinically dangerous. A small sensor is placed on the end of the finger, toe or ear lobe. The sensor consists of 2 light-emitting diodes one red and one infrared illuminating in turn. They emit light of 650 nanometers for oxyhaemoglobin (HbO2 ) and 900 nanometers for reduced or de-oxyhaemoglobin (Hb). The blood absorbs this light depending on the content of HbO2 & Hb. The photo diode detects the residual light energy. This is interpreted as a Hb/HbO2 ratio and the resultant SaO2 is calculated and presented on the machine. The pulse oximeter depends therefore on the pulsatile perfusion of the tissues and so doubles up as a pulse meter. Pulse oximetry must be interpreted with caution in the following conditions: When abnormal Haemoglobins are present • Carboxyhaemoglobin (smokers may have 10-20 % carboxy Hb). The pulse oximeter does not distinguish between oxyhaemoglobin and carboxyhaemoglobin and in heavy smokers the reading will be falsely high. Following poisoning with carbon monoxide a pulse oximeter is very inaccurate. • Methaemoglobin. This is normally only 1% but it rare cases can reach levels of over 20%. Methaemoglobinaemia can be caused by certain drugs e.g. nitrites, sulphonamides and prilocaine and some poisons. It can also occur spontaneously. It is of importance to the anaesthetist as it reduces the oxygen carrying capacity of the blood and shifts the oxygen dissociation curve to the left. Treatment is with methylene blue 1% given IV at a dose of 1–2mg / kg over 5 mins. When limb blood flow is low • Low blood pressure • Hypothermia • Vasoconstriction from any cause • Peripheral vascular disease. Miscellaneous causes • High intensity light • Heat lamps • IV dyes such as methylene blue • Electrocautery • Venous congestion.

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Capnography A capnograph measures carbon dioxide in inspired and expired gases using infra–red absorption. The information can be displayed in numerical form or as a waveform or capnograph tracing. Hypercapnia (also called hypercarbia) means an abnormally high carbon dioxide. Hypocapnia (also called hypocarbia) means an abnormally low carbon dioxide. Carbon dioxide is produced in the tissues as a result of cellular metabolism. It is carried from the tissues in the venous blood to the right side of the heart and from there into the lungs via the pulmonary artery. The pulmonary arterioles perfuse the alveoli of the lung. The carbon dioxide diffuses from the arteriole into the alveolus and via the expiratory gases through the upper airway into the atmosphere. If this process of elimination is interfered with, carbon dioxide accumulates in the blood stream (See Chapter 3 Respiratory failure). The closer to the alveoli we sample the gas, the more accurately will the sample reflect the changes in the alveoli and in the blood stream. Methods of sampling of end-tidal CO2 Mainstream The gas is analysed as it passes through the circuit. Sidestream Here the gas is led through a small capillary tube to a capnograph and then analysed. Hence a slight delay in analysis. Principle of carbon dioxide measurement This depends on infra-red absorption. An infra-red light beam is projected through a gas source and the intensity of transmitted light is measured. The difference will indicate the amount of carbon dioxide absorbed.

Fig 50.1 Capnograph tracing

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The normal capnograph is shown in the diagram. Any significant change indicates respiratory abnormality, a mechanical defect in the circuit or a complication. Description of the capnograph Phase b (2nd phase ascending part) represents exhalation. Carbon dioxide builds up during exhalation as the gases containing it are expired from the alveoli into the atmosphere via the sensor. Phase c (3rd phase plateau) This remains nearly constant until the next inspiration sweeps the carbon dioxide away. Phase d (4th phase descending part) The fresh gas flow entering the lungs during inspiration contains no carbon dioxide. The carbon dioxide level falls rapidly to the base line, indicating the start of inspiration. Phase a (1st phase) The capnograph remains steady at zero, indicating inspiration. How to interpret the capnograph Phase b abnormal A slanted upstroke means • Uneven emptying of the lungs. Carbon dioxide rises at different times (a gradual rise). • The sampling mechanism is too slow. Phase c The Plateau. Normal peak values are 36 - 44 mmHg. The highest carbon dioxide is found at the end of expiration and is called the End tidal carbon dioxide (EtCO2) High plateau (greater than 44 mmHg) may be caused by • Hypoventilation: − The effect of anaesthesia and opioids on the spontaneously ventilating patient − Respiratory failure (see Chapter 57) • Increased production of carbon dioxide, secondary to malignant hyperpyrexia, fever. • Transient increase in carbon dioxide: − Release of limb tourniquets after surgery − IV sodium bicarbonate (NaHCO3) − Insufflation of carbon dioxide – laparoscopy Low plateau (less than 36 mmHg) • Hyperventilation (normal minute volume 100 - 120 ml/kg)– caused by hypoxia, fever, (hysteria in awake patient). • Low carbon dioxide delivery to the lungs: − Hypotension − Pulmonary embolus

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− Decreased cardiac output from any cause. Irregular plateau indicates a physiological cause - a dip showing an attempt to breathe spontaneously e.g. muscle relaxant wearing off. Slanted plateau indicates COAD or asthma. Shorter plateau indicates a leak in the system. Phase d Slanted downstroke Normally the downward slant is very brisk (almost vertical). If it is slow it may mean there is carbon dioxide in the inhaled gases, or the inspiratory valve is not functioning properly. Phase a An abnormal base line means the inspired air contains carbon dioxide. Uses of Capnography • Confirming intubation of the trachea. A normal capnograph indicates tracheal intubation. There is no carbon dioxide detected with oesophageal intubation. • Apnoea. The capnograph alarms when the next wave of carbon dioxide fails to arrive. This reveals apnoea from whatever cause (i.e. airway obstruction, drugs such as excessive opioid usage or inadvertent muscle relaxant etc.). • Warning of a disconnection in the circuit. • Measuring the adequacy of mechanical ventilation. • Diagnosis of rebreathing. • Measuring the adequacy of cardiac output (i.e. PE or cardiac event such as acute myocardial infarction). • Early detection of malignant hyperpyrexia.

URINARY SYSTEM The urine output is measured every hour if the patient is catheterised. A good urine output is 60 ml/hr in an adult or 1 ml/kg/hr in a child. Minimum urine output should be 30 ml/hr or 0.5ml/kg/hr. TEMPERATURE This should be checked routinely in very young patients. The temperature should be monitored in several other groups of patients, e.g. febrile patients, those having major or prolonged surgery. • Rectal temperature. This only poorly reflects central body temperature. • Oesophageal temperature. A probe is inserted into the naso-pharynx or into the upper oesophagus between the heart and the descending aorta.

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This reflects the temperature in the central circulation.

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NEURO-MUSCULAR BLOCKADE A peripheral nerve stimulator is used for monitoring neuromuscular function, the aim being to provide optimum recovery from paralysis after anaesthesia. A nerve stimulator applies a current to a peripheral nerve and measures the response in the muscle that the nerve supplies. The features of the nerve stimulator are: • The pulse waveform is a unipolar square wave of 0.2 - 0.3 ms duration. • The output from the nerve stimulator should provide supra-maximal stimulation (more than 50 mA). That means a current is applied which guarantees every muscle fibre to be stimulated with each pulse. • Stimulus patterns Single twitch (1.0 Hz) Tetanus (50 Hz) Train of four Double burst. These are discussed a little later. A digital print-out is useful. Site of stimulus The ulnar nerve is commonly used. The muscle observed is the adductor pollicis which is innervated solely by the ulnar nerve. Setting up • The electrodes should be positioned as close as possible to the nerve. • There are two types of electrodes - surface and needle. Surface electrodes are generally used. Needle electrodes are useful in the obese, those with thick skin and burns patients. • Preparation of skin involves: − Removing excess hair using a razor. − Light abrasion of the skin. − Cleansing with alcohol. • Placement of electrodes: − Place the distal electrode, which is connected to the negative (black) terminal, 1-2 cm proximal to the proximal skin crease, just lateral to the tendon of the flexor carpi ulnaris. − Place the proximal electrode as close to the distal electrode as possible along the line of the ulnar nerve (3 - 4 cm apart). Baseline adjustment Before administering the muscle relaxant, adjust the nerve stimulator to provide supramaximal stimulation, which means the level of stimulation at which all the muscle fibres are depolarised. Increasing the current above this level does not increase the twitch response. Once this setting has been found for the patient, it should be kept for the entire procedure.

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Monitoring the neuromuscular function Twitch response The stimulus is delivered at a rate of 1 per second. After the relaxant is given the twitch response starts to fade. The twitch may completely disappear, which means more than 90% of receptors are blocked. However, it is better not to abolish the twitch completely under anaesthesia but to give just adequate amounts of relaxants to maintain a faint visible muscular contraction. If the twitch has been abolished by an overdose of relaxant, wait until it reappears before giving a subsequent dose or reversing the patient. Tetanic stimuli When the simple twitch returns to normal one can say that 20% of the receptors are free. But this also means that 80% of the receptors are blocked and this is dangerous, so we cannot rely on the twitch response to tell us if recovery is adequate. The tetanic stimulus was designed for this purpose: 50 or 100 stimuli per second (Hz) are delivered for 5 seconds. This rapid stimulation depletes the neuromuscular junction of acetylcholine when the number of free (unblocked) receptors is decreased. The tetanic response gets less and less, that is, it fades. The higher the rate of stimulation the more obvious the 'fade'. Although tetanic stimulation is a means of assessing recovery it is painful and therefore of limited value in the unanaesthetised patient. Train of four (TOF) stimuli The nerve is stimulated with 4 supramaximal currents 0.5 of a second apart. The response can be assessed in the form of the TOF count (TOFC) or the TOF ratio (TOFR). The TOFC indicates the number of palpable or visible twitches after the TOF stimulation. When a patient is paralysed with a muscle relaxant the twitches are lost in the order 4-3-2-1. During recovery from the relaxant the 1st twitch reappears first and the others follow. When only 1 twitch is present after TOF stimulation it means there is 90% block. This is a good level of relaxation to maintain for surgery. It is important not to completely abolish the thumb twitch (TOFC = 0). This would indicate too much relaxant. Recovery from the block occurs when the TOFC is 4 and all the twitches are of equal height. Double burst stimulation (DBS) This pattern of stimulation is available on the newer nerve stimulators. It consists of 2 short bursts of tetanus (50Hz for 60ms) separated by 0.75 of a second. It is easier to see fade on DBS than using TOF. DBS is more useful in the post-operative and recovery phase than TOF.

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The Post-tetanic count (PTC) is sometimes used to assess deeper levels of block when all response to TOF stimulation is absent (no twitching at all). It consists of 50Hz for 5 seconds, (like the tetanic stimulus mentioned earlier), a 3 second pause and then a single twitch. The PTC is the number of visible twitches. One (1) means deep block; 2-8 moderately deep. Clinical application Induction TOFC 1 Good relaxation sufficient for laryngoscopy. PTC 0-1 Paralysis of diaphragm if required in special conditions. Maintenance

TOFC -1

(90% depression of single twitch) Stimulate with TOF every 5 – 15 minutes and seek to maintain a TOFC 1. If there is no response to TOF a PTC is performed.

Reversal

Aim to have the TOF at 3 - 4 by the time of reversal. If before reversal the TOFC is 0 the patient should not be reversed but simply ventilated and kept anaesthetised. If the TOFC is 1 - 2, neostigmine plus atropine (or glycopyrrolate) is used and the patient must be carefully observed for up to 30 minutes for complete recovery.

Recovery

The simplest test for adequate recovery is the sustained head lift for 5 seconds. Using the nerve stimulator: • A TOFC of 4 and all 4 twitches are the same height. • A sustained response to tetanus. If you cannot detect a fade, use a DBS or 100Hz tetanus for 3-5 seconds. Any detectable fade will imply residual muscle paralysis.

Possible errors in monitoring blockade • Technical error • The presence of nerve and muscle disease

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OTHER PARAMETERS MONITORED IN SPECIALISED UNITS Cardiac output, pulmonary artery pressure, arterial blood gases, electroencephalogram (EEG) and blood volume can all be measured using electronic equipment. Monitoring of these parameters is not routine but there are specific conditions where they are useful. This type of monitoring would normally be carried out in an Intensive Care Unit. For more detailed information see an advanced textbook of anaesthesia or intensive care medicine.

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