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.
CirculationThe 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.