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.
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.
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.
Digital palpation of peripheral pulse is the simplest, but the least sensitive indirect technique for determining the blood pressure. Feeling the pulse means feeling the difference between systolic and diastolic arterial blood pressure known as pulse pressure. Caution should be taken when interpreting pulse pressure because the fingers are not good transducers and pulse pressure does not correlate well with blood pressure. It is a misconception to think that good pulse or high pulse pressure indicates normal blood pressure. The two examples below will illustrate this point.
A patient with a systolic blood pressure of 90 mmHg and a diastolic blood pressure of 30 mmHg will have a pulse pressure of 60 mmHg. This pulse pressure will result in feeling a "good" pulse; however, the mean blood pressure of this patient is about 50 mmHg, which is a lower driving pressure to perfuse tissue. Patients with patent ductus arteriosus have bounding pulse and yet their mean blood pressures are typically lower than most animals.
The second example is a patient with systolic blood pressure of 100 mmHg, a diastolic pressure of 70 mmHg, and a pulse pressure of 30 mmHg. The mean blood pressure of this patient is about 80 mmHg. Although the pulse pressure is lower resulting in weaker pulse, the mean blood pressure of 80 mmHg results in driving pressure adequate to perfuse the tissues and organs.
The Doppler ultrasound device consists of a probe with 2 crystals; one transmits an ultrasonic beam through tissue, and the other crystal that receives waves that are reflected back to the probe from the underlying tissue and moving RBC. The signal that is received from moving red blood cells is converted into an audible sound.
The probe is placed over the metacarpal or metatarsal arteries on the palmar or plantar surface of the limbs. The cuff is placed snugly around the limb proximal to the Doppler probe with the width of the cuff approximately 40% of limb circumference. The cuff bladder should extend at least halfway around the limb, on the side of the artery. The cuff should fit snugly and be positioned over the muscular part of the leg. If it is placed around the carpus or hock, the readings will be erroneously high.
To take the blood pressure, the cuff is inflated until the arterial pulsations are no longer heard from the Doppler. The cuff is deflated gradually and the first clear pulse that is heard is the systolic blood pressure. Determination of the diastolic blood pressure using the Doppler is unreliable. A soft tapping noise may be heard at times before the true systolic pressure. The Doppler provides information about the pumping ability of the heart, the presence of blood flow in arteries, and systolic blood pressure. The accuracy of the technique decreases at low pressures. It is important to keep systolic blood pressure above 80 mmHg in dogs. This technique underestimates the direct systolic pressure in cats and a lower systolic blood pressure is acceptable in cats.
The correct placement of cuff is important to get reliable results. If the cuff is placed too loosely, the systolic pressure will be erroneously high and if it is too tight, the systolic pressure will be erroneously low. The size of the cuff relative to the circumference of the leg is also important. If the cuff is wider (> 40%) relative to the circumference of the leg, the systolic pressure will be erroneously low. If the cuff is smaller, the systolic pressure will be erroneously high.
The oscillometric technique involves the use of pneumatic cuff placed around the limb. The cuff placement is similar to when the blood pressure is being taken using the Doppler technique. The cuff is coupled to an oscillometric device which inflates the cuff to suprasystolic blood pressure and then deflates the cuff over 20-40 seconds. During the deflation of the cuff, oscillation develops as a result of arterial pulsation. The oscillation amplitude is registered within the device. Based on an algorithm, the changes in the amplitude of the oscillations as the cuff is being deflated correspond to systolic, diastolic and mean blood pressures.
The technique is based on detecting these oscillations and if the device does not detect the oscillations, it will be not provide any information. This is the reason that this technique does not work consistently in very small patients and in case of severe cardiovascular depression.
The mean blood pressure under anesthesia should be kept above 60 mmHg to ensure perfusion of vital organs.
Pulse oximeter non-invasively transmits light through pulsatile vascular tissue bed at two wavelengths and determines oxygen saturation of hemoglobin. It actually uses the tongue, lip, or digital pad as a "cuvette" containing hemoglobin; but, these areas contain many light absorbers other than arterial hemoglobin, including mucous membrane, soft tissue, venous, and capillary blood. The light absorbers can be divided into two: constant component and pulsating component. The pulse oximeter reports the oxygen saturation of hemoglobin as resulting from the arterial blood because the pulsating component is the result of arterial blood pulsations, almost exclusively. During pulsations, the systolic volume expands resulting in increased absorbance of transmitted light.
Pulse oximeters use 2 wavelengths of light: 660 nm (red) and 940 nm (near infrared). Oxygenated hemoglobin absorbs more infrared light while deoxygenated hemoglobin absorbs more red light. The pulse oximeter measures the pulsating component of the light absorbance at each wavelength and then divides it by the corresponding constant component. The pulse oximeter calculates the ratio of the two pulse-added absorbencies. The ratio happens to be related to the arterial hemoglobin saturation.
Figure 1- Oxygen-hemoglobin dissociation curve
Each pulse oximeter should be calibrated based on experimental data obtained from specific species. The information provided by the pulse oximeter is continuous and in real-time. The results are reliable in anesthetized dogs and cats because the patient is not moving. Movement of the patient leads to erroneous results in some pulse oximeters. During anesthesia, the tongue is a good site for the placement of the probe. In cats, an alternate site is the digital pad. The oxygen saturation should be above 90% because 90% corresponds to an arterial oxygen tension of 60 mmHg, which is hypoxemic. Preferably, it should be above 95% all the time. The conditions that cause low saturation readings or failure to record a pulse include hypoxemia, inadequate peripheral perfusion (poor cardiac output), hypothermia, and peripheral vasoconstriction. It is important to remember that it is a monitor of oxygen transport. It monitors the function of the lungs in transporting oxygen to the arterial blood.
Pulse oximeter is very useful in determining hypoxemia because a normal mucous membrane color does not rule out hypoxemia. It is also important to remember that pulse oximeter cannot detect hypoventilation (high arterial CO2 tension) when the patient is breathing high concentration of oxygen.
Capnography provides information about the adequacy of ventilation of the patient's lungs. In addition, it also tells us about the patient's metabolism and cardiovascular function. Since carbon dioxide is an end-product of metabolism, any change in patient's metabolism during anesthesia will be manifested as a change in expired carbon dioxide (CO2). Increased metabolism, e.g., malignant hyperthermia, will result in increased expired CO2. If an anesthetized patient has severe cardiovascular depression resulting in low cardiac output, the expired CO2 will progressively decrease. The CO2 is delivered to the lungs by blood. With less volume of blood being ejected by the heart, there will be less CO2 to be exhaled.
Capnograph is the instrument that displays both the digital data and capnogram. Capnogram is the plot of the airway CO2 concentration (inspired and end-tidal) over a period of time. Capnometer only provides the digital data on the minimum and maximum values of CO2 during each respiratory cycle. The capnogram provides more useful information about the patient's respiratory function. The digital data on inspired and expired CO2 should be matched on the capnogram for more reliable information.
Normally, the inspired pCO2 should be 0 and end-expired pCO2 between 35 and 45 mmHg. The expired CO2 tension is an indirect measure of arterial CO2 tension. The arterial CO2 tension is higher than expired CO2 tension because of mechanical and physiologic dead space. The difference is generally between 1 and 10 mmHg. For example, if a mainstream capnograph is used in very small patients, the difference between expired CO2 tension and arterial CO2 tension is higher since the sensor creates an additional dead space.
The parts of a normal capnogram are shown in the figure below:
• Phase I - Inspiratory baseline represents the inspiration of fresh gas with no CO2. The baseline should stay at the level that corresponds to zero concentration (mmHg) of CO2.
• Phase II - Expiratory upstroke occurs shortly after the inspiration ends. The upstroke is caused by the rapid washing out of the fresh gas in the anatomic dead space and then replacement by CO2-rich alveolar gas. The upstroke should be steep.
• Phase III- Expiratory plateau represents the continuous exhalation. This line will be perfectly horizontal if ventilation and perfusion are perfectly matched, i.e., CO2 concentration will be constant throughout the exhalation phase. If the ventilation and perfusion are not perfectly matched, the CO2 slowly increases as gas from lung areas with lower ratio of ventilation to perfusion (V/Q) reaches the sampling site.
• Phase IV- Inspiratory downstroke occurs shortly after the expiration ends and represents the rapid washing out of the CO2 by the fresh gas as inspiration starts. This downstroke should be steep.
Abnormalities and some physiologic changes are presented in the following capnograms:
This capnogram (Fig 3) can be seen in anesthetized dogs. What is the significance of this capnogram? Answer: It is called cardiogenic oscillation and considered physiologic. This is due to the heart beating against the lung during the expiratory phase. Each oscillation represents a heartbeat.
This capnogram (Fig 4) has an abnormality on the inspiratory baseline. What is it? What are the problems during anesthesia that can result in this abnormality?
Answer: The inspiratory baseline is above 0 mmHg. Anytime the inspiratory baseline is above 0 mmHg, there is rebreathing of CO2. The possible causes of rebreathing of CO2 include exhausted soda lime, malfunctioning expiratory unidirectional valves, and inadequate oxygen flow when using a non-rebreathing circuit.
Figure 5 features a problem that involves the expiratory upstroke. What are the possible causes of this problem?
Answer: There is a slanting of the expiratory upstroke. This indicates an airway obstruction. The obstruction can be in the endotracheal tube, breathing circuit, and animal's airway.
This capnogram (Figure 6) is taken from an anesthetized dog being mechanically ventilated. What does the capnogram indicate at this time?
Answer: The animal is breathing spontaneously despite the controlled ventilation.
This capnogram (Fig 7) has an abnormality on the inspiratory downstroke. Remember it should be steep. What can cause this abnormality?
Answer: This slanting inspiratory downstroke is due to malfunctioning inspiratory unidirectional valve in a circle breathing circuit. If the inspiratory valve is working, the CO2 does not return to the inspiratory hose. If the inspiratory valve is defective, the CO2 returns to the inspiratory hose when the animal exhales. When the animal inhales, the CO2 that is in the inspiratory hose is taken in by the animal preventing a steep inspiratory downstroke.
This capnogram (Fig 8) shows an abnormality seen in anesthetized patients. Can you identify the problem?
Answer: The expired CO2 is high with a normally looking capnogram. This indicates hypoventilation.
This capnogram (Fig 9) is from an anesthetized dog with the ventilation being mechanically controlled. What is the abnormality?
Answer: The expired CO2 is low (20 mmHg) with a normally looking capnogram. This indicates that the patient is being overventilated. The ventilator should be reset by decreasing the respiratory rate and maintaining the same tidal volume. Both tidal volume and respiratory rate can also be decreased at the same time in severe hyperventilation.
This capnogram (Fig 10) does not show any waveforms. What does this indicate?
Answer: Possible causes of a capnogram showing a flat line are: cardiac arrest, disconnection, esophageal intubation, malfunctioning capnograph, and complete airway obstruction.
This capnogram (Fig 11) was taken from an anesthetized cat using the mainstream analyzer. The arterial CO2 of this cat was 50 mmHg. What is the abnormality? What causes this abnormality?
Answer: There is a great difference between the expired CO2 and arterial CO2 (about 25 mmHg). This indicates a high dead space caused by the mainstream analyzer.
When monitoring the respiratory function of an anesthetized patient, it is important to remember that normal respiratory rate does not always mean normal arterial CO2 and expired CO2. Remember also that capnography not only monitors ventilation but also cardiovascular function and the metabolism of the anesthetized patient.