Patient monitoring during anesthesia (Proceedings)

The purpose of patient monitoring is to provide information that can be used by the anesthetist and allow for prudent decisions that will maximize patient support and safety. Patient monitoring should be designed to allow the anesthetist to recognize problems at the earliest stages and respond before these early complications can evolve into an anesthesia crisis. It is somewhat controversial among anesthetists and anesthesiologists what parameters are the most important. When designing a monitoring plan, a systemic approach to monitoring works well. Correlate the information gathered during the pre-anesthesia patient assessment and take into account any problems that might arise from the patient's co-existing diseases. Having a list of anticipated problems in mind will help develop a monitoring strategy.. How will you know if the patient is experiencing a complication? If you don't look for it you probably won't know. What do you need to monitor? Which parameters will alert you to danger for the patient?

A systemic approach to monitoring will allow the anesthetist to evaluate the cardiovascular, respiratory and central nervous systems. Decreased cardiopulmonary function produced by anesthesia drugs can have significant effects on oxygen delivery to the tissues of heart, brain, liver, kidneys and other areas of the body. Most common anesthetic complications are related to the cardiovascular and respiratory system, therefore a major focus of physiological monitoring is the cardiopulmonary system.

The primary function of the cardiopulmonary system is to transport oxygen from the environment to the peripheral tissues to maintain oxygen metabolism. This task relies on the coordinated 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 transport 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 coronary perfusion pressure; and vasomotor tone must be sufficient to maintain peripheral perfusion pressure, but not so excessive as to reduce visceral organ perfusion.

The challenge for the anesthetist is to determine if the cardiovascular 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 importance of any single parameter to judge overall performance must be referenced to previous measurements of that parameter over time and combined with the evaluation of as many other indicators as are available. The physiological significance of each monitored parameter must be understood in order to avoid misinterpretation by the anesthetist.. 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 cardiovascular function. It is the anesthetist's responsibility to monitor the patient's physiology and provide therapeutic support as needed. A systematic evaluation of the patient will provide the anesthetist the information needed to make prudent decisions regarding support therapy. Which parameters will allow the anesthetist to determine adequate function?

The most basic monitoring 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 vasomotor tone. Vasomotor tone can be assessed by mucous membrane color (MM), capillary refill time (CRT), urine output (UO) and temperature gradients between core body and extremities (TG). Heart rate, rhythm, and stroke volume can be assessed via pulse palpation. 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 poor coronary perfusion. These measurements are very subjective, but are the basics when interpreting the patient's hemodynamic status.

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 information 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 monitoring we must also ensure the information provided is accurate by using equipment that is dependable and in good working order. The anesthetist must also have a clear understanding of both the equipment and the significance of the information provided by the equipment. The additional data provided by the monitors is intended to make the anesthetist's job easier and provide a more complete picture of the patient's physiological status.

One of the most frequently used cardiovascular monitoring devices is the electrocardiogram (ECG). The ECG provides information regarding the electrical activity of the heart, but it does not demonstrate mechanical function. ECGs are the tool of choice to monitor cardiac rate, rhythm, and diagnose arrhythmias. Unfortunately, ECGs can appear normal when myocardial performance and tissue perfusion are poor. The strength of the ECG is that it provides a way to analyze PQRST wave morphology and observe changes over time, as well as a means to diagnose various arrhythmias. Arrhythmias should be closely monitored and evaluated in conjunction with other cardiovascular parameters to determine their effect on hemodynamic performance. Bradycardia and excessive tachycardia can significantly reduce CO, premature ventricular contractions will also reduce CO and can progress to more detrimental arrhythmias such as ventricular fibrillation.

Coronary 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 for these organs. 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 support of ABP in any anesthetized patient is important to help optimize anesthesia care. Arterial blood pressure 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 nor blood flow.

There are several methods available today to measure ABP. Measurement of ABP requires a mechanical or electronic device. Methodology is available to measure ABP either directly (invasive) or 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. Therefore, systolic blood pressure, pulse rate and rhythm are this methods forte.

Another indirect method to measure ABP is the use of oscillometric blood pressure monitors. These systems use a blood pressure occlusion cuff connected to a monitor device. Oscillometric monitors 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 monitoring 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 prior to inflation. The cuff size should be so the cuff bladder width is equal to forty 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 or 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 monitors 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 or invasive ABP monitoring is considered to be the "gold standard" of blood pressure monitoring. Direct ABP monitoring requires the placement of a catheter in peripheral artery; the catheter is then connected to a transducer system or an aneroid manometer. Common sites for arterial cannulation are the dorsal pedal, femoral, radial, auricular and coccygeal. Advantages to the direct measurement method are: 1) that the results are displayed as a continuous arterial pulse waveform and usually a digital display of systolic, mean, diastolic values 2) the continuous waveform provides beat to beat monitoring of cardiovascular performance and the waveform morphology can be observed and analyzed 3) arterial pressure waveforms 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 more accurate then the indirect methods. Disadvantages to this method of monitoring 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 performance, 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) catheter system disconnects resulting in severe hemorrhage, and 6) accidental arterial injections.

While blood pressure is necessary for blood flow, adequate blood pressure does not always equal adequate blood flow or tissue perfusion, MM color, 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 more valuable indicator of cardiovascular status than the individual measurements.

Central venous pressure (CVP) is the measurement of hydrostatic pressure within the intra-thoracic vena cava and is reflective of right atrial pressure in most instances. CVP is used as an indicator of venous blood volume or preload status. This measurement can be influenced by: venous tone, changes in intra pleural or peritoneal pressure, pulmonary hypertension, obstructive pulmonary disease, pulmonary emboli, constrictive pericarditis and pericardial tamponade. For this measurement a jugular catheter is placed in or near the right atrium and coupled with a water manometer column or 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 monitor, waveform morphology can be observed and assessed.

Cardiovascular monitoring 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 compensatory mechanisms that help to protect hemodynamic stability, but many of those mechanisms are blunted or disabled in the presence of anesthesia drugs.

Cardiovascular parameters for healthy patients during anesthesia

Today we have access to several tools that can provide the anesthetist accurate information and allow for prudent decisions regarding the patient's respiratory system. More in-depth monitoring is available than the simple assessment of mucous membrane color, respiratory rate and effort. While these assessments are important and give us some valuable information about how the patient is breathing, we need more information. What we ultimately need to know are two things; 1) Does the patient have adequate oxygen in its arterial blood? 2) Is the patient ventilating adequately?

For assessment of oxygenation, pulse oximetry provides a non-invasive, continuous estimate of the oxygen saturation of arterial hemoglobin. SaO2 and PaO2 are related by the oxy-hemoglobin dissociation curve. While both pulse oximeter and arterial blood gases (PaO2 specifically) measure the lungs ability to deliver oxygen to the blood the numbers obtained are quite different. Caution should be exercised when interpreting SaO2 values with animals breathing 100% oxygen. Patients with normal respiratory function, breathing 100% oxygen, should have a PaO2 of approximately 500 mmHg. and a SaO2 of 100%; a patient with poor pulmonary function breathing 100% O2 may only have a PaO2 of 200 mmHg. and still a SaO2 of 100%. There is a significant difference in the efficiency of gas exchange between a PaO2 of 500 or 200 mmHg., yet they both still show a SaO2 of 100%. Most pulse oximeters also provide a pulse rate and plethmogram (usually a bouncing bar or graph). Pulse oximeters do not indicate adequate tissue perfusion nor do they indicate adequate ventilation. Signal quality is greatly affected by poor perfusion such as that resulting from hypotension, vasoconstriction or hypothermia. External light sources and electrocautery may interfere with signal reception.

Capnometry is the measurement and display of CO2 concentrations during a respiratory cycle. Maximum inspiratory and expiratory levels are displayed on an analog or digital monitor. Capnometry allows for the continuous monitoring of exhaled carbon dioxide by analyzing samples obtained directly from the airway. Carbon dioxide readily diffuses across the capillary membrane and quickly equilibrates with alveolar gas. As these gases are exhaled, the CO2 at the end of the breath, or the ET CO2 closely approximates arterial CO2. Normal ET CO2 is approximately 1 - 4 mm Hg less than the PaCO2. Factors affecting measurements include: positioning of the sampling port, fresh gas flow, sampling system integrity, breathing circuit leaks, sensor obstruction, and analyzer system calibration

Capnographs are used to display ETCO2.over time. Most use infrared light absorption to measure CO2 levels. Capnographs allow for continuous monitoring of the patient's CO2 levels. ETCO2 is reflective of the patient's PaCO2 usually within a 5mmHg gradient, this gradient can be affected by pulmonary perfusion. Severe hypotension, embolism, shock or hypovolemia, cardiac arrest (if the lung is under-perfused CO2 is not exchanged) can cause significant changes in gas exchange. Capnographs may sample via mainstream or sidestream methods. Capnographs give the anesthetist more than just ETCO2 levels, the graphs often demonstrate a variety of problems that, if left untreated, can be harmful to the patient, such as mechanical malfunction or endotracheal extubation.

Blood gas analysis is considered to be the "Gold Standard" technique used to assess the respiratory system. Blood gas analyzers measure the pH, partial pressure of oxygen (PaO2) and carbon dioxide (PaCO2) in blood. Additionally, they calculate values for plasma bicarbonate concentrations, base excess, and saturation of hemoglobin with oxygen. This information is used to assess acid-base status, gas exchange efficiency and adequacy of ventilation. PaO2 demonstrates the efficiency of gas exchange, while PaCO2 reflects the adequacy of ventilation. To assess the lungs ability to oxygenate blood an arterial blood sample is required.