Anesthetic monitoring (Proceedings)

Monitoring anesthetized patients is imperative for all procedures. All anesthetic drugs act as cardiovascular and respiratory depressants on varying levels, and they can compromise a patient's homeostasis at unpredictable times in unpredictable ways. Crises are rapid in onset and devastating in nature. The term "young and healthy" never implies the animal is exempt from a crisis while under anesthesia. There is always the potential of underlying disease that routine and even thorough screening can miss. Therefore, the goal of the anesthetist is to maximize the beneficial aspects of anesthesia while minimizing the effects on the organs. This vastly increases the potential for a full and uneventful recovery.

Information can be quantitative or non-quantitative. Quantitative information is information given in the form of numbers, and most likely calculations. Qualitative information is non-numerical, meaning it is demonstrated in the form of non-numerical facts such as color or appearance changes. Qualitative information should be used to supplement quantitative information and should be not relied upon as the sole source of information. Abnormal qualitative information may contradict the quantitative information the instrument gives, warranting further investigation. It is the collection of information that allows the anesthetist to properly assess the anesthetic patient.

Circulation

The objective of maintaining proper circulation is to ensure that blood flow (perfusion) to tissues is adequate. Quantitative measurement for circulation is measuring the blood pressure. Blood pressure is the force exerted by blood flow against arterial walls while the heart is beating. It is a major determinant of organ perfusion and must be maintained within a physiologic range to ensure adequate organ perfusion. Therefore, blood pressure monitoring is imperative in all anesthetized patients since most anesthetic agents can induce hypotension. Treating anesthetic hypotension includes adjusting the depth of anesthesia, and administering fluids and/or drugs to support the cardiovascular system to maintain adequate physiologic blood pressure values. The consequences of prolonged anesthetic hypotension can be fatal if organs such as the brain and kidneys are deprived of adequate blood flow during anesthesia.

The three values that are measured when recording blood pressure are the systolic pressure, which should be above 90mmHg during anesthesia, diastolic pressure, and the mean arterial pressure (MAP), which is the average pressure in the arteries over the cardiac cycle. It is the best indicator of perfusion to vital organs, and this value is usually measured by a monitor. This value should range between 70-90 mmHg during anesthesia, and a MAP below 60 indicates decreased perfusion to internal organs.

Acquiring these values can be done either noninvasively using a Doppler ultrasonic flow detector or an oscillometric flow detector, or directly. Direct blood pressure monitoring involves an arterial catheter connected to transducer or an anaeroid manometer, and is discussed in the "Advanced Anesthesia Monitoring" section of this series.

Non-invasive blood pressure (NIBP) monitoring is easier to learn, the equipment is more affordable, and it is the method used in most veterinary clinics. It is usually an adequate means of monitoring the anesthetized patient. Regardless of the method used, continuous readings should be performed and trends should be monitored as closely as values in determining patient stability. There are two different types of NIBP monitors; Doppler and oscillometric.

Doppler monitoring uses a crystal to detect the ultrasound echo from red blood cells passing through an artery. A cuff is then inflated with a sphygmomanometer until the sound is no longer heard, and pressure is slowly released until the sound returns. The reading on the sphygmomanometer when sound first returns is the systolic blood pressure. Disadvantages of Doppler monitoring include the only value it provides accurately is the systolic pressure, and there is no automatic readout. Advantages include it is reliable with animals that weigh less than 10 kilograms, and the anesthetist has the ability to detect arrhythmias by a change in quality of the pulse heard, and it minimizes the chance of missing cardiac arrest due to electromechanical dissociation since it amplifies the beating heart. Electromechanical dissociation is when an electrocardiogram (ECG) shows a normal reading when the animal is in cardiac arrest, and it can last for up to five minutes. This phenomenon only emphasizes the importance of hands-on monitoring.

Technical errors are usually associated with improper cuff size and patient position. The probe should be distal to the carpus, tarsus, or tail, and level with the heart. The cuff is proximal to the probe, and should measure 40% of the circumference of the area it is to be placed. Too large a cuff will cause an artificially low reading, and too small will cause the reading to be artificially high. Another cause of error is poor signal conduction. It is important to ensure the area for the probe is shaved well and apply a generous amount of ultrasonic gel onto the probe before placing it on the shaved area. Isopropyl alcohol can be applied to the shaved area and wiped off before the ultrasonic gel is applied. This improves conduction and quality of the sound heard through the machine.

Oscillometric monitoring determines blood pressure by placing a cuff with a detector unit around a limb or tail. Oscillations caused by the pulsation of an artery beneath the cuff changes the width of the limb slightly. A computer inflates and deflates the cuff while measuring the changes in intracuff pressure. It then calculates the systolic, diastolic, and mean arterial pressures. The main advantage if this monitor is it gives the systolic, diastolic, and MAP values with automatic readouts. Disadvantages involve accuracy and sensitivity to movement and motion artifact. The disadvantages make this type of monitoring inaccurate in animals with significant hypotension, fast heart rates, and limb edema. It is also inaccurate in patients that weigh less than 10 kilograms because the detector has difficulty reading pulsations in small arteries. It is advisable to use a Doppler with smaller patients.

Qualitative methods to determine circulation include palpation of the heart beating through the thoracic wall, ausculting the heartbeat with a stethoscope or esophageal stethoscope, palpating a lingual pulse and palpating the arteries. Palpation of the arteries allows for the assessment of pulse pressure, which is a measure of the difference between the systolic and diastolic pressures. The pulse and pulse quality should be palpated regularly during anesthesia, but it should not be used in place of quantitative measurement of blood pressure. An anesthetist that is familiar with their patient's pulse and pulse quality can determine the significance of sudden unexpected changes in the readings from blood pressure monitors and react appropriately.

If measuring the MAP is not possible, the anesthetist can presume that the MAP is likely to be at or above 60 mmHg if the dorsal pedal artery is palpable. Palpating the femoral artery is also a good indicator of perfusion. The patient needs immediate intervention to increase the blood pressure if neither the femoral nor dorsal pedal arteries are palpable.

ECG

The electrocardiogram (ECG) is a graphic representation of electrical impulses as they move through the heart. This type of monitor is used to watch the heart rate and rhythm, which is important in anesthesia because some anesthetic agents predispose the patient to cardiac arrhythmias. Stress before induction and hypoxia can also cause arrhythmias. It should be noted that ECGs indicate nothing about cardiac function; therefore the anesthetist still needs to monitor the blood pressure. Ausculting the patient throughout the anesthetic procedure is also indicated. An esophageal stethoscope is an economical and effective instrument for this.

The readout of the ECG consists of the:

      1. P wave: The P wave represents atrial depolarization (contraction), which is when blood is being pumped from the atria into the ventricles. The P wave should be positive (above the base line), and the size and configuration correlates to atrial activity.

      2. QRS complex: The QRS complex represents ventricular depolarization, which is when blood leaves the heart to travel to the lungs (from the right ventricle) or to the rest of the body (from the left ventricle). It is also when a pulse is generated. The R should be positive, and the size and configuration of the QRS complex correlates to ventricular activity.

      3. T wave: The T wave represents ventricular repolarization (relaxation), which is when the passive filling of the ventricles occurs. This wave can be either positive or negative.

When assessing the rhythm strip, one should first see if the rhythm is a normal sinus rhythm (regular) or some type of arrhythmia (irregular). It should also be noted if the heart rate is normal, fast (tachycardic), or slow (bradycardic). Then assess the P, QRS, and T waves by determining if the intervals are the same distance, and see if each wave is uniform in size and shape. Then observe the relationship between waves by determining if every P wave is followed by a QRS complex, if every QRS complex is preceded by a P wave, and T-waves should be the same distance from the QRS and uniform in size. A progressive increase in T wave size can indicate myocardial hypoxia. When this is seen, anesthesia should be lowered or stopped, and the patient needs to be ventilated. Anesthesia should be discontinued if the wave configuration does not improve.

An ECG will detect an arrhythmia, which is any electrical activity that differs from a normal. The arrhythmia's significance varies with age and preexisting condition. The anesthetist should be able to recognize arrhythmias such as sinus tachycardia, sinus bradycardia, atrioventricular block, ventricular premature contraction (VPC), ventricular tachycardia (V-tach), and ventricular fibrillation (V-fib), and be knowledgeable of treatments. The Electrocardiogram (CS-3)Western Veterinary Conference 2004

Larry P. Tilley, DVM VetMed/Dr. Tilley & Assoc. Santa Fe, NM, USA

Oxygenation

The objective of monitoring a patient's oxygenation status is to ensure oxygen is circulating to tissues because tissues that lack oxygen become ischemic and die. Ischemia greatly increases the potential for mortality from peri- and post-operative complications such as cardiac arrhythmias and sepsis. The qualitative method to determine oxygenation is observing mucous membrane color. Moist, pink gums are considered normal, pale gums can indicate a lack of perfusion due to hypotension, hypothermia, anemia, or vasoconstriction, while white gums indicate severe blood loss. Cyanotic (blue/gray) gums indicate a lack of oxygen, though it is a late sign of hypoxemia. Mucous membrane color can also be assessed by inspecting the penis or vaginal folds. Quantitative analysis includes pulse oximetry, which is measuring the amount of oxygen bound to hemoglobin, and blood gas analysis to measure the partial pressure of oxygen (PaO2). Blood gas analysis is discussed in further detail in the "Advanced Anesthesia Monitoring" section of this series.

A pulse oximeter is one of the most common monitors used in veterinary medicine. Advantages include it is noninvasive, generally easy to use, and affordable. The monitor is usually attached to unpigmented tissues such as the tongue, ear, lip, vulva, rectum, prepuce, and digits. It uses infrared and visible light absorption in the tissues to calculate the percentage of oxygen saturation in arterial hemoglobin (SpO2) while measuring the pulse rate of the patient. The normal range for SpO2 is between 95% and 100%, indicating that the patient's hemoglobin is adequately oxygenated.

Causes of inaccuracies include pigment, motion, probe placement, insufficient or excessive moisture, pale or injected mucous membranes, and fluorescent lighting. When used properly pulse oximeters are known to be accurate in the 80-100% saturation range, and action must be taken when the saturation is less than 90% for this can indicate severe shock, anemia, or significant hypoxemia. The first response is to ensure ventilation is adequate. Then evaluate respiratory rate (RR), ensure the airway is clear and that there are no interruptions of the flow of oxygen, and note the tidal volume if the patient is being ventilated as it may need to be adjusted. Mechanical or intermittent positive pressure ventilation, oxygen supplementation, and correcting ventilation-perfusion mismatches (such as changing patient position) may be needed. Also assess the patient's capillary refill time (CRT), pulse quality, arterial blood pressure, mucous membrane color, and temperature because vasoconstriction, poor perfusion, hypovolemia, or hypothermia can cause decreased blood flow. Improving cardiac output, warming the patient, and repositioning the sensor may help improve the reading.

Temperature monitoring

Anesthetic patients frequently become hypothermic due to reduced heat production, open body cavities, intravenous fluids, and prolonged anesthetic periods where the shivering response is inhibited. General anesthesia adds to the potential for complications because it changes the set point for thermoregulation, making the patient unable to compensate for changes in body temperature. Hypothermia can cause complications such as an increased oxygen demand from shivering, cardiac changes, and coagulation deficiencies. Since these complications prolong recovery and increase mortality, perioperative monitoring of body temperature is strongly encouraged.

Prevention of hypothermia includes minimizing patient heat loss. This can entail the following; covering the patient's non-surgical areas, placing a warm blanket or towel on the surgical and prep tables, wrapping limbs in commercial bubble wrap, the use of low flow gas techniques because oxygen is a cooling gas, keeping preparation time to a minimum, and minimizing surgical time when it is feasible.

There are several warming methods that can be used during the patient's perioperative period to maintain body temperature. They include circulating warm water blankets, warm water bottles, and convective warm air devices. Electric heating pads are not recommended because they increase the risk of iatrogenic thermal injuries due to the pad's uneven distribution of heat and the patient's inability to recognize excessive heat and move off of it. It is important to provide adequate padding between the patient and any external heat source due to the potential for iatrogenic thermal burns.

Though hyperthermia, an increased body temperature, is a less common sequelae of general anesthesia, and true malignant hyperthermia (MH) syndromes are rare, it is still important to watch for since there is a disruption of the normal processes for thermoregulation. Hyperthermia can also be iatrogenically induced with certain drug combinations or with the overzealous use of external heat.

Treating hyperthermia entails identifying the underlying cause and correcting it immediately. Remove all external warming devices and begin to cool the patient by increasing oxygen flow. The application of cool water or alcohol to the footpads may help increase evaporative cooling. Applying cold packs (not directly on the animal), and administering cool intravenous fluids can also help decrease the temperature. Cool enemas and cool abdominal lavage are more aggressive methods of correcting hyperthermia. The body temperature must still be monitored continuously regardless of the cooling method(s) used. Cooling sources must be removed before normal body temperature is achieved because it is often difficult to anticipate if the body temperature will continue to drop.

References

Battaglia. Andrea. Small Animal Emergency and Critical Care: A Manual for the Veterinary Technician. W.B. Saunders Company. 2001.

Devey, Jennifer, Crowe, Dennis (Tim). Practical Monitoring Techniques and Equipment. Proceedings Western Veterinary Conference. 2002.

Levensaler, Amy. Anesthesia for Veterinary Technicians (Edited by Susan Bryant), Monitoring: Pulse Oximetry and Temperature, and Hands-On. Wiley-Blackwell. 2010.

McKelvey, Diane, Hollingshead, Wayne. Veterinary Anesthesia and Analgesia, Third Edition. Mosby. 2003.

Tefend, Mary. Blood Pressure Monitoring: What You May Not Know. Proceedings American College Veterinary Internal Medicine Forum 2003.

Tilley, Larry. The Electrocardiogram. Proceedings Western Veterinary Conference 2004.