Proceeding of the NAVC North American Veterinary Conference Jan. 8-12, 2005, Orlando, Florida

Transcription

1 Proceeding of the NAVC Nth American Veterinary Conference Jan. 8-12, 2005, Orlando, Flida Reprinted in the IVIS website with the permission of the NAVC

2 Veterinary Technician ADVANCED ANESTHESIA MONITORING CARDIOVASCULAR PERFORMANCE Gregy R. Hanson, RVT, VTS (Anesthesia) Veterinary Medical Teaching Hospital University of Califnia, Davis, CA Hemodynamic moniting involves the observation of the cardiovascular system s response to illness, injury and therapeutic intervention. It has become an intricate part of anesthesia case management. The patient s co-existing diseases, almost all anesthesia drugs, and often the operative procedure f which the patient is anesthetized can all have detrimental effects on the system s ability to meet the demands of tissue perfusion and adequate oxygen delivery f the patient. The primary function of the cardiopulmonary system is to transpt oxygen from the lungs to the peripheral tissues to maintain oxygen metabolism. This task relies on the codinated function of numerous physiologic actions: The lungs must be able to remove oxygen from the environment to the plasma; adequate amounts of hemoglobin must be present to transpt oxygen; cardiac output (CO) must provide sufficient flow of oxygenated hemoglobin toward the tissues; arterial blood pressure (ABP) must be adequate to maintain cerebral and conary perfusion pressure; and vasomot tone must be sufficient to maintain peripheral perfusion pressure, but not so excessive as to reduce visceral gan perfusion. Oxygen delivery is determined by oxygen content of arterial blood and cardiac output. Oxygen content of arterial blood relies on pulmonary exchange of oxygen and carbon dioxide and the presence of hemoglobin as a transpt vehicle in the blood. Cardiac output is a flow parameter that is the product of stroke volume and heart rate. Stroke volume is affected by preload, afterload, and contractility. Preload reflects the ventricular filling pressure diastolic stretch, which is an imptant determinant of stroke volume. Afterload is the resistance to ventricular ejection and is primarily affected by systemic vascular resistance (SVR). Contractility is reflective of the fce and velocity with which the ventricle contracts and ejects its volume. Decreased preload, increased afterload, decreased contractility, bradycardia severe tachycardia will all contribute to a decreased in CO. Other facts contributing to decreased CO would include electrolyte imbalances increased blood viscosity. The challenge f the anesthetist is to determine if the cardiopulmonary system is functioning adequately to meet the patient s needs during anesthesia. If not, then how to treat the underlying problem and then determine if the therapy is effective. The cardiovascular system has several parameters that are observed and used to assess and determine adequate function. The imptance of any single parameter to judge overall perfmance must be referenced to previous measurements of that parameter over time and combined with the evaluation of as many other indicats as are available. The physiological significance of each monited parameter must be understood in der to avoid misinterpretation by the anesthetist. The imptance of looking at the whole picture of cardiovascular perfmance by assessing multiple parameters and not basing the evaluation on any single parameter value data point in time can not be understated. Look at the whole picture and use trends rather than single data points on which to base therapeutic decisions. Anesthetic drugs and the operative procedure can significantly reduce cardiopulmonary function. It is the anesthetist s responsibility to monit the system and provide therapeutic suppt as needed. A systematic evaluation of the patient will provide the anesthetist the infmation needed to make prudent decisions regarding anesthesia suppt. Which parameters will allow the anesthetist to determine adequate function? The most basic moniting does not require any special equipment only the use of the anesthetist s senses. Eyes, ears, sense of touch, and the skill to interpret the results are all that are needed. Peripheral and visceral perfusion is regulated primarily by vasomot tone. Vasomot tone can be assessed by mucous membrane col (MM), capillary refill time (CRT), urine output (UO) and temperature gradients between ce body and extremities (TG). Heart rate, rhythm, and stroke volume can be assessed via pulse palpation. The weak, thready pulse associated with hypovolemia is due to a small stroke volume, these patients may be nmotensive depending on other compensaty cardiovascular changes. Pulse amplitude bears little crelation to ABP, but is me reflective of stroke volume. Pink MM coupled with a CRT less than 2 seconds, UO of 1 to 2 mls/kg/hr, and a TG of less than 6 degrees could be interpreted as adequate while in fact the patient may well be hypotensive and suffering from po conary perfusion. These measurements are very subjective, but are the basics when interpreting the patient s hemodynamic status. Oxygen delivery = Cardiac output x Arterial O 2 content Stroke volume x Heart rate Hemoglobin x S a O 2 Preload Afterload Contractility 41

3 The Nth American Veterinary Conference 2005 Proceedings Anesthetists spend much of their time in direct contact with the patient. As mentioned above, we use our senses to evaluate the patient and analyze the data we perceive through sight, sound and touch. That infmation can be augmented with the addition of electrical and mechanical equipment to give the anesthetist a view of a bigger picture. When additional devices are used to assist our moniting we must also ensure the infmation provided is accurate by using equipment that is dependable and in good wking der. The anesthetist must also have a clear understanding of both the equipment and the significance of the infmation provided by the equipment. The additional infmation provided by the monits is intended to make the anesthetist s job easier and provide a me data to complete the picture of the patient s physiological status. One of the most frequently used cardiovascular moniting devices is the electrocardiogram (ECG). The ECG provides infmation regarding the electrical activity of the heart, but it does not demonstrate mechanical function. ECGs are the tool of choice to monit cardiac rate, rhythm, and diagnose arrhythmias. Unftunately, ECGs can appear nmal when myocardial perfmance and tissue perfusion are po. The strength of the ECG is that it provides a way to analyze PQRST wave mphology and observe changes over time, as well as a means to diagnose various arrhythmias. Arrhythmias should be closely monited and evaluated in conjunction with other cardiovascular parameters to determine their effect on hemodynamic perfmance. Bradycardia and excessive tachycardia can significantly reduce CO, premature ventricular contractions will also reduce CO and can progress to me detrimental arrhythmias such as ventricular fibrillation. Conary and cerebral perfusion is dependent on adequate ABP to maintain sufficient flow. A mean systemic ABP in excess of 60 mmhg. is required to meet minimal tissue perfusion requirements f these gans. Anesthetic drugs and operative procedures can significantly compromise cardiovascular homeostasis in any patient. Since excessive hypotension is often the cause of perioperative complications, the measurement and suppt of ABP in any anesthetized patient is imptant to help optimize anesthesia care. Arterial blood pressure (ABP) is the product of CO and SVR (pressure = flow x resistance). Systolic pressure is the highest pressure in the artery during the cardiac cycle, diastolic pressure is the lowest pressure, and mean is the average of the pressures. Pulse pressure is the difference between systolic blood pressure and diastolic blood pressure. ABP may reflect the overall status of the cardiovascular system, but it does not directly measure blood volume n blood flow. There are several methods available today to measure ABP. Measurement of ABP requires a mechanical electronic device. Methodology is available to measure ABP either directly (invasive) indirectly (non-invasive). An ultrasonic doppler flow probe can be placed over a distal artery of an appendage to detect blood flow, heart rate and rhythm may be assessed by listening to the arterial pulse. With the placement of an occlusion cuff connected to a sphygmomanometer ABP can be determined by inflating the cuff above systolic blood pressure and stopping blood flow, then slowly releasing the pressure until flow is detected under the occlusion cuff with the doppler flow probe. This method is used to determine systolic blood pressure. Attempts have been made to measure diastolic pressures with this system, but measurements are often difficult and inconsistent. Therefe, systolic blood pressure, pulse rate and rhythm are this methods fte. Another indirect method to measure ABP is the use of oscillometric blood pressure monits. Oscillometric ABP measurements use a blood pressure occlusion cuff connected to an oscillometric monit. Oscillometric monits measure the air pressure fluctuations inside a blood pressure occlusion cuff as an underlying artery pulses against it (these fluctuations can be seen with a sphygmomanometer as you release the air pressure of the occlusion cuff the needle will start to bounce representing systolic pressure, the greatest magnitude of needle fluctuations represents mean pressure, and a sudden decrease in magnitude of needle bounce represents diastolic pressure). Many improvements have taken place in this moniting system in recent years, however the accuracy and consistency of this method is often in question. In both the doppler and oscillometric methods of ABP measurement the occlusion cuff size and placement can greatly affect the accuracy of measurements. Occlusion cuffs should be placed with the middle of the cuff bladder centered over the artery, wrapped snugly enough that a finger cannot be placed under it, but not so tight as to occlude blood flow pri to inflation. The cuff size should be so the cuff bladder width is equal to fty percent of the circumference of the limb, at the site of application. Too large a cuff will result in falsely lower pressure readings, while too small too loose a cuff will result in falsely higher readings. Both of these systems offer intermittent ABP readings that may not be exact, but can be reasonably accurate. Both systems have difficulty delivering accurate readings during severe hypotensive episodes and the oscillometric monits can be confused by arrhythmias. The value of these measurements is not the single data point, but is the data trends generated by the readings. Direct invasive ABP moniting is considered to be the gold standard of blood pressure moniting. Direct ABP moniting requires the placement of a catheter in peripheral artery; the catheter is then connected to a transducer system an aneroid manometer. Arterial catheter systems usually consist of a catheter, plastic connecting tubes, stopcock, automatic flush device, heparinized saline reservoir /pressure bag and either an aneroid manometer a transducer connected to an oscilloscope (usually an ECG with a pressure channel). Common sites f arterial cannulation are the dsal pedal, femal, radial, auricular and coccygeal. Advantages to the direct measurement method are: 1) that the results are displayed as a continuous arterial pulse wavefm and usually a digital display of systolic, mean, diastolic values 2) the continuous wavefm provides beat to beat moniting of cardiovascular perfmance and the wavefm mphology can be observed and analyzed 3) arterial pressure wavefms can be used to detect changes in cardiovascular function, but they can also be greatly affected by catheter system artifacts. 4) readings from this system are considered me accurate then the indirect methods. Disadvantages to this method of moniting include the following: 1) the potential difficulty of cannulating an artery. Arterial catheterization can be a daunting technical challenge, especially in small, critical patients that may not have adequate cardiovascular perfmance, 2) prolonged anesthesia time attempting arterial cannulation, 3) the increased risk of infection from catheter placement, 4) arterial thrombosis and compromise to arterial circulation, 5) 42

4 Veterinary Technician catheter system disconnects resulting in severe hemrhage, and 6) accidental arterial injections. While blood pressure is necessary f blood flow, adequate blood pressure does not always equal adequate blood flow tissue perfusion, MM col, CRT, UO, and TG must still be assessed and figured into the whole perfusion picture. Different methods of measurement often produce different results. It is not unusual to have different readings between direct pressure systems and indirect pressure systems. However, the data trends are a me valuable indicat of cardiovascular status than the individual measurements. Central venous pressure (CVP) is the measurement of hydrostatic pressure within the intra-thacic vena cava and is reflective of right atrial pressure in most instances. CVP is used as an indicat of venous blood volume preload status. This measurement can be influenced by: venous tone, changes in intra pleural peritoneal pressure, pulmonary hypertension, obstructive pulmonary disease, pulmonary emboli, constrictive pericarditis and pericardial tamponade. F this measurement a jugular catheter is placed in near the right atrium and coupled with a water manometer column an electronic pressure transducer and oscilloscope. The manometer is referenced with the zero point at the level of the right atrium, often the sternal manubrium is used as a reference point. This system measures low pressures and is prone to difficulties from catheter placement and improper zeroing. When connected to an electronic transducer and monit, wavefm mphology can be observed and assessed. Typically cardiac output is not measured in clinical veterinary anesthesia. CO measurements are me commonly perfmed in the research lab rather than the clinical anesthesia setting. With the advent of less invasive and non invasive measurement systems, it is becoming a practical parameter to be considered during anesthesia case management. CO is a flow parameter that is me relevant to systemic perfusion than a pressure parameter such as ABP. Histically veterinary anesthetists have made assumptions using other data, thought to reflect CO such as arterial pulse quality, wavefm analysis evaluation of oxygen extraction comparing arterial and venous oxygen content, but seldom is output actually measured. The use of Swan-Ganz catheters, Cardiac Output Computers, and thermal dilution techniques has not been a practical approach f clinical veterinary anesthesia. Today however, there are me options to objectively evaluate cardiac output. There are systems available that do not require a pulmonary artery catheter to assess CO, instead they use a peripheral catheter to facilitate IV injection of lithium and an arterial catheter to facilitate collection of blood samples to measure lithium levels in arterial blood and calculate CO. This is still an invasive system, but less invasive then the thermal dilution techniques used in the past. Many veterinary clinics have ultrasound machines in their practice. Ultrasound is a non-invasive method to assess cardiac function and hemodynamics. Ultrasound machines coupled with doppler technology allows f accurate non invasive flow measurements. These methods of moniting cardiovascular perfmance are a look into the future f patient moniting. They will help to complete the picture of hemodynamic moniting f our patients and could help to improve the quality of care and suppt provided to veterinary patients. Hemodynamic moniting is an intricate part of anesthesia care. The cardiovascular system is affected by many variables and its response has profound effects on physiological homeostasis. There are many compensaty mechanisms that help to protect hemodynamic stability, but many of those mechanisms are blunted disabled in the presence of anesthesia drugs. The table below contains hemodynamic parameters commonly monited during anesthesia. Values are presented to help give the anesthetist some guidelines f therapeutic intervention. REFERENCES 1. Haskins SC, Moniting and suppt. The Veterinary Clinics of Nth America Small Animal Practice March Haskins SC Moniting the anesthetized patient. Veterinary Anesthesia 3rd edition, Lumb and Jones Hall LW, Clarke KW, Trimm CM Patient moniting and clinical measurement. Veterinary Anesthesia 10th Edition 4. Lake CL, Hines RL, Blitt CD. Clinical Moniting, Practical Applications f Anesthesia and Critical Care, Saunders

5 The Nth American Veterinary Conference 2005 Proceedings HR (bpm) CRT (s) MM Col MAP CVP (cmh 2 O) PaO2 21% / % PaCO 2 PvO 2 Hct (%) Hgb (g/dl) SaO 2 (%) Nmal values during anesthesia C 70- < 2 Pi > 80 / > F - < 2 Pi > 80 / > Alert values, patient is heading towards trouble C 50-2 Pi < -1 < 80 / < Pa F Pi- Pa < -1 < 80 / < Action values, patient is in danger, intervention is needed C < 50 > 2 Pa < 60 < -2 < 60 / < > 60 < 27 < 20 < 7 < 90 >200 F < 80 > 240 > 2 Pa < 60 < -2 < 60 / < > 60 < 27 < 20 < 7 < 90 44