Anesthetized patients should be continuously monitored. To augment the anesthetist's senses, electronic monitors have been developed to allow for a more precise picture of a patient's status.
Anesthetized patients should be continuously monitored. To augment the anesthetist's senses, electronic monitors have been developed to allow for a more precise picture of a patient's status. Two of the most critical parameters that are monitored during anesthesia are the electrocardiogram and arterial blood pressure. The ECG can give a picture of the cardiac microenvironment and blood pressure monitoring can help to determine is blood flow to organs is adequate.
The electrocardiogram is used as an indicator of electrical activity of the heart. During anesthesia, the most commonly used lead system is the 3 lead system consisting of the right arm, left arm, and left leg. This system is not diagnostic for vector analysis or chamber enlargement. It provides information about cardiac arrhythmias and the myocardial environment. It is important to remember that an ECG does not give information about the mechanical function of the heart. For example during pulseless electrical activity, an ECG may look normal; however there is no forward blood flow.
Rapid analysis of an ECG can be performed by answering the following questions:
1. Is the rate normal or abnormal?
2. Is the rhythm normal or abnormal?
3. If there is an abnormality, is it ventricular or supraventricular in origin?
4. If there is an abnormality, do I need to treat it?
Rate can be classified as normocardic, tachycardic, or bradycardic. Many disease conditions, anesthetic, and analgesic drugs can affect the heart rate of animals. Additionally, because of the different sizes and breeds of animals a practitioner might see, normal variation must be known. Target heart rates for anesthetized animals should be at what approximates a normal heart rate for that animal when asleep.
The rhythm can be determined to be normal or abnormal based upon the presence of the parts and morphology of each beat. The P, Q, R, S, and T should all be present and the atrial and ventricular waves should be of a consistent normal shape.
If an abnormality is identified, determining if it is ventricular or supraventricular in origin can be done by examining the ventricular wave (Q, R, S). Normal and regular morphology of the QRS indicated that the arrhythmia is originating from a supraventricular origin. If the QRS is "wide and bizarre" in shape, the arrhythmia is ventricular in origin.
Deciding if and/or when to treat an arrhythmia, can be determined by understanding which arrhythmia can lead to or are one of the 4 "malignant" rhythms (rhythms in which there is no forward blood flow and leads quickly to death). These four rhythms are:
1. Ventricular fibrillation
2. Pulseless electrical activity
Treatment of arrhythmias obviously depends on what type of arrhythmia is present. Common treatments include lidocaine, procainamide, beta-blockers, beta-agonists, and calcium channel blockers.
Arterial blood pressure measurement and monitoring is one of the most important and least utilized techniques available to determine the status of an anesthetized patient. Arterial blood pressure is the relationship between blood volume and blood volume capacity. It can be described with the following equation:
Arterial Blood Pressure = Cardiac Output X Total Peripheral Resistance
Cardiac Output = Heart Rate X Stroke Volume
Arterial Blood Pressure = Heart Rate X Stroke Volume X Total Peripheral Resistance
It can be therefore seen that changes in heart rate, stroke volume, or total peripheral resistance can alter arterial blood pressure. Any or all of these variables, depending on the patient, can be manipulated to either increase or decrease arterial blood pressure.
Many of the analgesic/anesthetic drugs used in veterinary practice can affect blood pressure, most commonly decreasing it. It is therefore important to have an understanding on how individual drugs affect blood pressure so that changes can be anticipated and treated before detrimental effects occur.
Normal ranges for blood pressure vary widely in dogs and cats. Normal ranges are as follows:
Systolic: 90 to 180 mm Hg
Mean: 60 to 120 mm Hg
Diastolic: 55 to 90 mm Hg
Mean blood pressure is important because it dictates blood flow to and within organs. Most organs in the body have intrinsic auto-regulation systems that maintain appropriate tissue perfusion across a range of mean blood pressure of 60 to 120 mm Hg. When mean blood pressure falls below 60 mm Hg, blood flow is jeopardized and tissue and organ dysfunction can occur.
Measuring blood pressure during anesthesia is generally performed using one of three methods, Doppler, non-invasive oscillometry, or direct blood pressure. The use of a Doppler is done by placing an ultrasonic probe over an artery to hear blood flow through a speaker connected to the Doppler device. A blood pressure cuff is used to occlude the artery until no flow is heard. The cuff is slowly released until blood flow is heard again and the pressure at that time is read on an attached manometer. This method gives only systolic blood pressure. Non-invasive oscillometry is performed by placing a cuff around a limb that is attached to an oscillometric blood pressure machine. The machine inflates the cuff to a preset pressure and then lets the pressure off incrementally. The machine senses the mean blood pressure and computes the systolic and diastolic pressures from the mean pressure. The readings are usually displayed digitally. Direct blood pressure measurement requires the placement of an indwelling arterial catheter. The catheter is connected to an electronic transducer via a fluid line and gives real time blood pressure at the tip of the catheter.
Direct blood pressure measurement is the gold standard to which the other two methods are compared. Individual measurements taken with the Doppler method or the oscillometric method should be viewed with caution as wide variation can occur within individual animals and the area from where the blood pressure is obtained. These two methods tend to be less accurate than the direct method and are best used for assessing trends over time instead of single point in time measurements.