Anesthesia monitoring equipment (Proceedings)

Monitoring, for these purposes will consist of checking vitals on a regular basis. The vitals being monitored may be changed based upon the patient's status and the procedure performed. By watching for changes in trends we are able to catch and potentially stop a crisis from happening..

Why? Why do we monitor patients under anesthesia? By monitoring vitals of patients under anesthesia, we are able to facilitate optimum depth of anesthesia with minimal physiological impairment. When patients are deeper under anesthesia they are suffering from greater physiological impairment. By decreasing the concentration of inhalant, we are able to decrease the negative impact from the inhalant. The American College of Veterinary Anesthesiologists is now promoting new goals of monitoring under anesthesia which are less anesthetic morbidity rather than simply less mortality.

How? How we monitor animals under anesthesia will depend on 1) the procedure, 2) the patient, and mostly 3) the equipment available. We will typically monitor heart rate and rhythm, respiratory rate, temperature, blood pressure, oxygen saturation, end tidal carbon dioxide, capillary refill time, and mucous membrane color on all of our patients. We have the ability to run arterial blood gas samples when attainable and necessary.

Blood Pressure

Blood pressure is a product of cardiac output, vascular capacity and blood volume. A decrease in any one will cause a drop in blood pressure. Measurement and support of blood pressure is extremely important because cardiovascular homeostasis is greatly compromised by anesthetic drugs. Compromising cardiovascular homeostasis causes hypotension and bradycardia. Excessive hypotension is a common cause of perioperative morbidity. Since the new goal of the ACVA is to decrease anesthetic morbidity, monitoring blood pressure is a vital part of "safer anesthesia". The goal of blood pressure measurement and support is to keep the mean arterial pressure (MAP) above 60 mmHg to assure adequate perfusion to kidneys and other organs. There are three methods of blood pressure measurement, oscillometric, Doppler, and direct (through an arterial catheter).

Oscillometric blood pressure monitoring uses a machine with a device that detects pulsation under an occlusion cuff placed over an artery. The machine will measure the mean then calculate the systolic and diastolic pressures. The blood pressure cuff used should be measured on the limb it will be used and should be approximately 40% of the circumference of the limb. This machine may not work consistently or even at all on small pets. The petMAP is a type of oscillometric blood pressure monitor. It is a sphygmomanometer that reads out systolic, diastolic, and mean blood pressures. There are two forms of this device the newest one is digital.

Measurement of blood pressure using a Doppler is ideal for small pets, but works on any size animal (assuming appropriate cuff sizes available). The Doppler uses an ultrasonic probe attached to a speaker to provide an audible sound for each pulse beat. Use of a blood pressure cuff and a sphygmomanometer with the Doppler will give systolic blood pressure readings. Sometimes the Doppler can be difficult to hear, this could be due to the patient (vasoconstriction) or the equipment (low battery, faulty probe). Sometimes the Doppler gets feedback from cautery, or from cell phones. For continuous BP measurements during a procedure, the Doppler probe can be attached to patient with tape. Common places where the Doppler will work is any limb or on the tail artery. A Doppler probe can be attached near the heart of exotic patients in order to monitor heart rate, but will not function as a blood pressure monitor.

Arterial catheterization is necessary for direct blood pressure, also known as invasive blood pressure (IBP), monitoring. This requires skills for placement of the catheter, a monitor with IBP capabilities, and a pressure transducer. Similar to oscillometric blood pressure monitoring, IBP will provide the systolic, mean, and diastolic arterial blood pressures. However, this monitor will actually measure the systolic and diastolic blood pressures then calculate the mean. Invasive blood pressure monitoring is different from oscillometric in that it is read out in "real time" not intermittently. Direct blood pressure monitoring is considered the most accurate form of blood pressure. On a multiparameter monitor, the direct blood pressure wave form should match the ECG complexes as well as the pulse oximeter plethysmograph. Other than blood pressure monitoring, the arterial catheter can be used to obtain blood samples.

Above all, with blood pressure monitoring consistency is most important. Switching between different monitors may result in completely different values. Watching trends in the blood pressure is better than just looking at a single value occasionally because it will give a better picture of what is going to happen in the future. The heart rate should be considered when monitoring blood pressure. If the heart rate is high and the blood pressure is low the patient may be hypovolemic. Intervention may be indicated with low blood pressures; an anesthetist should not continue to watch BP trend down without a plan of action. Turning down inhalant gas, providing fluid therapy, and starting a CRI of a positive inotrope are all good responses to low blood pressure. Which treatment to start with, and how much to give/turn down will depend on the patient and the procedure performed.

Temperature

Due to a loss of thermoregulatory mechanisms under anesthesia, temperature should be monitored. Temperature can either be monitored constantly by an oral or rectal probe or by intermittent rectal or ear temps. Since inhaled gases are colder than room air, every attempt should be made to warm patients under anesthesia. The higher a patient's oxygen flow rate, the colder he will get. Hypothermia causes a decrease in MAC (minimum alveolar concentration), so cold pets get deeper faster. Other effects of hypothermia are bradycardia, vasoconstriction, and prolonged recoveries. The bradycardia causes a decrease in cardiac output which is evident by a decrease in BP. Severe hypothermia can also lead to arrhythmias and coagulopathies. During abdominal surgeries if the temperature probe is advanced too far down the esophagus and into the stomach it will most likely register the temperature of the air exposed to the stomach. Devices useful in warming are heating pads, forced air warmers (Bair Huggers), heating lamps (caution that it is not too close to patient), warm bags of fluids (caution that they are not too hot), warming disks (SnuggleSafe), warm bags of fluids around breathing circuit of the machine, fluid line warmer, warm IV fluids (caution that they are not too hot), plastic and bubble wrap to cover tiny pets, and warm blankets. Never assume that there is nothing that can be done to warm a patient, even warming the saline the surgeons flush the abdomen will help to warm the patient.

Pulse Oximeter

A pulse ox measures the percentage of hemoglobin that is saturated with oxygen. It can detect hypoxia before the patient is clinically cyanotic. This is a simple and noninvasive monitor that will provide a calculated heart rate, percent saturation (SpO2), and an audible signal for each pulse. The audible signal typically has different pitches associated with the different percents. A high saturation will have a higher pitch sound than a low saturation. Some pulse oximeters have a graphical display called a plethysmograph. The plethysmograph is based upon flow quality, low flow equates to a low flat wave form. The opposite is true for good pulse quality, a high and pointed wave form is present.

A pulse ox probe works by generating light at two different wavelengths. The amount of light absorbed by hemoglobin depends on its saturation of oxygen, or lack thereof. By calculating the absorption, the processor computes the proportion of the oxygenated hemoglobin. Oxygen saturation should always be greater than 95% (ideally 100%). Arterial oxygenation (PaO2) is what is most important despite what the pulse oximeter reads. However, with cases where an arterial blood gas is not possible, relying on the pulse oximeter to estimate saturation is necessary. A patient breathing 100% oxygen should have a PaO2 around 300mmHg. As the SpO2 drops to around 90%, the PaO2 is closer to 60mmHg.

There are many different probes available ear probes, finger probes, rectal probes. Probes may need to be moved occasionally because they can constrict vessels and restrict blood flow causing a false reading. Gentle massaging of the area can help restore circulation of vessels. Pulse ox probes can be susceptible to movement so it is sometimes hard for them to read if a patient is becoming light or during the recovery period. Some pulse oximeters have difficulty reading when the patient is particularly bradycardic.

Pulse oximeters are dependent upon pulsatile flow. Some examples of a decrease in pulsatile flow are vasoconstriction, hypovolemia, severe hypotension, hypothermia, and some cardiac arrhythmias. When there are simply slight changes in the pulse ox readings, reposition and assess the patient to see if the readings are accurate. If the heart rate is reading inaccurately, reposition and assess the patient to be sure there are not pulse deficits. If the pulse ox started to drop suddenly and steadily, the anesthetist should assess the patient immediately and take supportive measures when necessary. One necessary step could be intermittent positive pressure ventilation (IPPV) until cause determined and rectified. If the cause is determined to be a pneumothorax, thoracocentesis is typically indicated. Low readings are not a sign of anemia. Even patients with low hemoglobin may still have full saturation of the hemoglobin in their few RBCs. Anemic patients however, need to saturate well because they have fewer RBCs to carry the oxygen around the body. If each of the RBCs is not fully saturated with oxygen, the patient may not be able to meet the body's oxygen demands.

ECG

Electrocardiography can be used to monitor heart rate and rhythm. It is essential for diagnosing and treating arrhythmias. The various P, QRS, and T waves represent the depolarization and repolarization of the atria and the ventricles. By knowing the different components of the ECG, diagnosing and treating arrhythmias is possible. When attaching ECG leads, make sure that they each have proper contact with the skin and that they have been wet with alcohol. Patients can get burns if the ECG lead is touching both the patient and the cautery plate while the surgeon is using cautery. ECG machines are not perfect and will sometimes double count heart rates, or show false arrhythmias. This is often due to large T waves counted as beats or by the leads drying out (beats tend to look wide and bizarre). When the machine displays something that may appear to be an arrhythmia, it is best to look at the rest of the picture. Some questions that could be asked are what is the blood pressure, is the arrhythmia the result of the procedure, is the surgeon using cautery, does the patient have pulses during these arrhythmias, and is the arrhythmia occurring regularly. Sometimes printing a strip from the monitor can help to diagnose arrhythmias. Pulseless electrical activity (PEA) is when the patient is deceased but the heart is still conducting electrical activity that can be registered as beats on an ECG. Sometimes this is confused with actual beats conducted by the heart.

Common arrhythmias seen in patients under anesthesia are second degree AV block, sinus arrhythmia, right bundle branch block, ventricular tachycardia, and bigeminy. With all arrhythmias, blood pressure should be taken into consideration before treating.

Second degree AV block is when some of the impulses from the atria are not conducted through to the ventricles. There is an elongation of the P – R interval until there is a blocked beat. There are two types of second degree AV block I & II; an increase in vagal tone is the most common cause of second degree AV block. Treatment, when necessary, is with anticholinergics.

Sinus arrhythmia is a fluctuation in heart rate associated with the respiratory rate. An increase in heart rate takes place during inspiration, then during expiration the heart rate decreases. This is a normal resting rhythm in dogs and no treatment is necessary.

Right bundle branch block (RBBB) occurs after an impulse from the atria passes through the Bundle of His. Blockage of the conduction occurs in the right bundle branch. The left ventricle will conduct as usual. The right ventricle will only conduct once an impulse is passed through the myocardium from the left ventricle. This arrhythmia does not need treatment.

Ventricular tachycardia is defined by three or more premature ventricular contractions (VPC) in a row. A VPC is recognized by wide and bizarre QRS complexes with no associated P wave. Possible causes of VPCs are phenobarbital administration, hypoxia, pain, hypercarbia, trauma to the heart, splenic disease, and GDVs. While treatment is not always indicated (based upon HR and BP), lidocaine dosed to effect is the drug most frequently used.

Bigeminy occurs when every other beat is a either a ventricular premature contraction or a sins beat. The most common cause is administration of barbiturates, such as thiopental. Treatment is not necessary unless blood pressure is affected by the arrhythmia.

Capnograph

A capnograph monitors carbon dioxide levels in respiratory gases. It is attached between the patient's endotracheal tube and the breathing circuit and provides continuous measurement of end tidal carbon dioxide (ETCO2), inspired CO2, and respiratory rate. There are two different types of CO2 monitoring, side stream sampling, and main stream sampling.

In arrest situations, capnography is a useful tool in determining the adequacy of CPCR. With lack of circulation, the carbon dioxide in the blood stream will not make it to the lungs to be exhaled. Under anesthesia a capnograph is also useful for determining adequacy of ventilation, airway obstruction, disconnection from breathing circuit, and severe circulatory problems.

The ETCO2 is approximately 2 – 5 mmHg less than arterial carbon dioxide levels (PaO2). Normal ETCO2 levels are 35 – 45 mmHg. Greater than 50 mmHg is considered hypoventilation, and less than 30 mmHg hyperventilation. Some reasons for a high ETCO2 are tachycardia, a large abdomen pressing on the diaphram (c-section or GDV), and hyperthermia. Low ETCO2 could be caused by hypothermia, hyperventilation (by animal or iatrogenic), pain, and decreased cardiac output. A capnograph is a very useful monitor for all critically ill patients.

Capnographs add dead space to the breathing circuit which is not ideal for very small patients. Also, non-rebreathing circuits require such a high flow rate of inhalant gas that the ETCO2 tends to be washed out. Values from a capnograph attached to a non-rebreathing circuit will usually be falsely low for that reason. The respiratory rates reported are not always accurate. The monitor will count a breath as air moving through the end of the tube, if someone were to press on the patient's chest and force air from the lungs a breath would be registered but not because the patient voluntarily took one. This is particularly true during a hemilaminectomy when the surgeons are pressing on the patient's back.

Ventilators

The two different categories of anesthetic ventilators are volume limited and pressure limited. With a volume limited ventilator (Hallowell & Surgivet), the volume delivered with each breath is determined by the operator without regard for the airway pressure. Pressure limited ventilators (Bird) deliver a breath volume determined by the airway pressure as preset by the operator without regard for volume.

When ventilating patients under anesthesia, the tidal volume is typically 10 – 20 mL/kg. The peak inspiratory pressure (PIP) should be less than 20 cmH2O, or it could cause barotrauma. Respiratory rates should be approximately 8 – 12 bpm. Slow deep breaths are more beneficial for alveolar ventilation. Positive pressure ventilation (manual or mechanical) is indicated when a patient cannot adequately ventilate on its own. Ventilation requirements of patients on 100% oxygen may be determined by the following: hypercapnia (ETCO2 >60 mmHg), hypoxemia (PaO2 < 100 mmHg), de-saturation (SpO2 < 95%), any breach of the thoracic cavity (diaphragmatic hernia or thoracotomy), use of paralytic neuromuscular blockers, patients with potentially increased ICP (intracranial pressure), and patients with severe enlargement of the abdominal contents.

The easiest method of determining adequacy of ventilation is with a capnograph. If available, arterial blood gas sampling is an even more accurate assessment of ventilation because it will measure patient's pH, PaCO2, PaO2, and SaO2. Without these monitors, positive pressure ventilation can be performed if the peak inspiratory pressure (PIP), chest excursions, pulse oximeter, and blood pressure are closely monitored.

The biggest contraindication of ventilation is in cases with a closed pneumothorax. Without a place for the air within the chest to go (open chest tubes), positive pressure ventilation could make the condition worse. In some cases, ventilating can decrease arterial pressure and reduce cardiac output. The circulatory changes seen are due to prolonged increases in airway pressures, and decreases in CO2. When ventilating patients under anesthesia, it is important to remember tidal volume guidelines and not to exceed a PIP of 20 cmH2O.