Monitoring the anesthetized patients (Proceedings)
There are many physiologic variables that can be monitored in anesthetized small animal patients. The major goal of monitoring an anesthetized patient is to ensure adequate oxygen delivery to the tissue. Appropriate oxygen delivery to the tissue needs the proper functioning of the cardiovascular and respiratory systems. Therefore, the scope of this presentation is limited to the monitoring of the cardiovascular and respiratory systems. It is important to remember that oxygen delivery depends on ventilation, cardiac output, distribution of cardiac output, and hemoglobin concentration and functional state of hemoglobin. Clinical skills alone will miss significant changes in the cardiopulmonary function of the anesthetized patients during anesthesia. Equipment that are important in monitoring anesthetized patients include electrocardiograph, blood pressure monitor, pulse oximeter, and capnograph. The use of these monitors, the common misconceptions, indications, and tips when using these monitors will be discussed.
Electrocardiograph monitors only the electrical activity of the heart. It does not indicate the mechanical function of the heart. Its main role is to continuously assess heart rate and rhythm in anesthetized dogs and cats. It is very useful in identifying life-threatening dysrhythmias. It is essential during cardiopulmonary resuscitation (CPR). Without an electrocardiograph, the exact steps necessary for return to spontaneous circulation will not be known. It is important to remember that normal heart may show dysrhythmias during anesthesia.Finding on the electrocardiograph should always be followed up by the presence of the pulse. Normally looking ECG in anesthetized patients without a pulse is known as pulseless electrical activity and this patient needs CPR.
Electrograph has limitation because a normal ECG does not guarantee adequate blood pressure, tissue perfusion or cardiac function. It is a misconception to state that normal ECG guarantees life or normal cardiovascular function.
A 3-electrode system is used during anesthesia. One of the leads serves as the ground lead. Lead II (right front leg-left hind leg) is primarily used. There are esophageal stethoscopes in the market that also serve as connectors to the electrodes of an ECG machine.
Arterial Blood Pressure
Blood pressure is the most commonly used clinical parameter to assess perfusion of tissues; however, bear in mind that blood pressure is not equivalent to blood flow (cardiac output). Cardiac output is the product of heart rate and stroke volume. Blood pressure is the product of cardiac output and systemic peripheral resistance. A blood pressure may be increased but it does not mean that the blood flow to the tissues is also increased. Blood pressure may have gone up because of increased peripheral resistance. However, a decrease in blood pressure is frequently the basis for changing the anesthetic management to prevent a life-threatening situation. During maintenance with inhalant anesthetic, arterial pressure decreases progressively with increasing depth of anesthesia. In essence, blood pressure provides more information about the depth of anesthesia.
There are two ways to monitor blood pressure: indirect or non-invasive and direct or invasive. The invasive blood pressure monitoring involves inserting a catheter into a peripheral artery. The catheter is then attached to a pressure transducer using a rigid pressure tubing. The pressure transducer converts the mechanical movement to an electrical signal and an oscilloscope converts the signal to blood pressure reading. This is the golden standard of blood pressure monitoring. It provides beat-to-beat information about the blood pressure and is very useful in very sick patients. It is not practical for routine anesthesia cases.