Based on the physiology of the cardiovascular system, anesthesia providers need to assess the cardiac rhythm, preload, and afterload, as well as detect cardiac ischemia. Monitoring the status of the cardiovascular system during the administration of anesthesia allows the clinician to promptly detect hemodynamic and physiologic changes and to respond with therapeutic interventions. There are different types of monitoring (noninvasive and invasive) that can be used to evaluate the status of the cardiovascular system. A thorough understanding of the monitoring equipment and the underlying physiology measured by these monitors is important for their safe and effective use.

The electrocardiogram (ECG) is an important tool for monitoring intraoperative arrhythmias and myocardial ischemia. The continuous oscilloscopic ECG is commonly used in the operating room. When cardiac muscle depolarizes, an action potential is created and the resulting electrical activity can be measured (see Chapter 7). The human body is a volume conductor of electricity that is then transmitted throughout. Electrodes capture the electric current generated by the heart. Standard ECG leads are bipolar because they measure differences in electrical potential between electrodes. Intraoperative ECG monitoring systems typically use either a threeelectrode or five-electrode system. Commonly, the five-electrode system is used as it allows better detection of myocardial ischemia. The cables from the electrodes attach to a single cable that plugs into a port on the ECG monitor.

A computer program detects changes in ST segments that deviate from the preset normal values and displays changes on the monitor and creates an audible alert. ST and T-wave changes can be indicators of myocardial ischemia. The monitor has an audible indicator with each QRS complex that allows the clinician to listen for changes in heart rate and rhythm while working on other tasks. Visual analysis of the P wave (if present) and the QRS complex can help diagnose arrhythmias. Most operating room ECG monitors also have the ability to print the ECG on a “strip” of special paper. These strips can also be helpful in the diagnosis of arrhythmias.

The most commonly encountered problem with the use of ECG is interference or artifacts. Electrocautery, patient or lead wire motion, faulty electrodes, or electrodes that do not properly adhere to the patient’s skin can all contribute to artifacts on the ECG tracing. Anesthesia monitors have a filtering mechanism that eliminates some interference, and this can be set up under monitoring mode. However, if the detection of myocardial ischemia is a priority (i.e., patient with a history of coronary artery disease), then the diagnostic mode should be used. To optimize electrical conductance, good contact is important between the patient and the electrodes. Adequate contact can be difficult in areas with hair, sweat, or damaged skin (e.g., burn).

Noninvasive blood pressure (NIBP) monitoring should be performed in all patients receiving anesthesia. It is regularly performed with the use of an oscillometric monitor; however, it can also be measured via palpation and auscultation. Systolic blood pressure measurement through palpation is performed with the application of a cuff to the patient’s extremity. The pulse is palpated while the cuff is inflated until the artery is occluded. The cuff is slowly released, and systolic blood pressure is measured when pulsations are again palpable. This method tends to underestimate the blood pressure, and only systolic
measurements can be made. Blood pressure measurement by auscultation utilizes the same method, except that a stethoscope is used to listen for Korotkoff sounds over the brachial artery (the first audible sound resulting from turbulent blood flow after deflation of the blood pressure cuff). Diastolic and calculated mean arterial blood pressure measurements can be made with the auscultation method.

Oscillometric monitoring is more commonly used in the operating room due to its ease of use and reliability. A single cuff is applied to the patient’s arm and is initially inflated to a level greater than the systolic pressure. The cuff gradually deflates, and there is an electronic pressure sensor that detects oscillations in blood flow. If oscillations are not detected, the computer opens a deflation valve and repeats inflation at a higher pressure. The systolic pressure is when the pulsations start, the mean pressure is when the oscillations are at a maximum, and the diastolic pressure is calculated. The cuff size should be appropriate, and the patient’s arm should fit within the size markings on the inside of the cuff (the width is approximately 40% of the circumference of the arm). If the cuff is too small, the NIBP reading may be falsely elevated, and if it is too large, the NIBP reading may be falsely low. Too frequent measurements for a sustained time period can result in vascular congestion, bruising, and rarely nerve damage. Standards for basic anesthetic monitoring state that every patient receiving anesthesia must have recorded blood pressures every 5 minutes. The automatic devices rely on regular pulse rhythm and volume. If there is irregularity in the patient’s pulse (atrial fibrillation, frequent premature ventricular/atrial contractions), the oscillations may not register and the cuff may continue to cycle repeatedly or not give a blood pressure reading. This may also occur when stroke volume is low (i.e., decreased cardiac output [CO]).

Invasive blood pressure (IBP) monitoring is direct measurement of beat-to-beat changes in blood pressure. Some indications for the use of IBP are frequent blood gas measurements, inaccurate NIBP reading, cardiopulmonary disease, or unstable hemodynamics. This information is displayed both numerically and graphically. It requires the use of an intra-arterial cannula (typically 20 gauge in adults and 22 or 25 gauge in children and neonates) that is placed in a peripheral artery such as the radial, brachial, femoral, or dorsalis pedis. The radial or dorsalis pedis is preferred as these are not end arteries and there is collateral circulation, which decreases the risk of ischemia if thrombosis occurs. Arterial blood pressure varies depending on where it is measured. Peripheral arteries are smaller and have more resistance; this results in a larger wave reflection leading to increased systolic pressure measurement.

IBP requires the use of an intra-arterial cannula, pressure tubing, a transducer, a microprocessor, a display screen, and a method to zero and calibrate. The cannula is connected to pressure tubing that is stiff and should not contain air (which may decrease resonance and cause damping). The tubing is attached to a transducer that contains liquid and a diaphragm that moves in response to an arterial pressure wave. The mechanical energy is converted to an electrical signal. The transducer is able to calibrate to atmospheric and hydrostatic pressure. “Zeroing” the transducer refers to calibration such that extraneous atmospheric pressures do not factor into the accuracy of the arterial pressure tracing. “Leveling” the transducer eliminates inaccuracy in readings from hydrostatic pressures. The arterial transducer should be positioned at midaxillary line, as placing it high will result in falsely low readings and placing it low will result in falsely high readings. The air-fluid column in the transducer does not need to be level with the arterial catheter. For a patient in the seated position, the transducer should be leveled at the ear; this approximates blood pressure at the circle of Willis. Transducers should be zeroed periodically to ensure accuracy and eliminate drift.

Extra tubing, compliant tubing, stopcocks, and the introduction of air produce damping, which reduces the arterial pressure waveform and underestimates the systolic pressure. Underdamping can result in a falsely high systolic blood pressure. The frequency of the transducer system typically exceeds the frequency of the arterial pulse by 10-fold. The frequency and damping coefficient can be tested by performing a high-pressure flush test. When flushed, the monitor should display an initial horizontal straight line with a high-pressure reading (typically 300 mm Hg). Once the flush is terminated,
the pressure should immediately drop below the baseline and well-defined oscillations will occur. In a dampened system, the pressure will not drop below the baseline or oscillate and there may be a delay in the return of a waveform. The flush test can be performed on central venous lines and pulmonary artery pressure lines as well.

Central veins can be cannulated to measure the central venous pressure (CVP). Catheterization of a central vein may also be used for venous access either because large-bore intravenous access is required for fluid therapy or because of an inability to catheterize a peripheral vein. It is also used in instances that require continuous infusion of caustic drugs (vasopressors) or total parenteral nutrition. Correct catheter placement should be confirmed by free aspiration of blood through the catheter, transducing the catheter, and radiologic evaluation (i.e., chest x-ray). The CVP is measured by transducing a catheter in the vena cava or by using the proximal port of a pulmonary artery catheter (PAC). Each site for central vein cannulation has different advantages and disadvantages. Catheterization of the internal jugular vein has the advantage of a lower incidence of pneumothorax than a subclavian approach. Catheterization of the femoral vein is associated with an increased risk of infection. The right internal jugular vein, as opposed to the left, is more commonly used due to a direct path into the superior vena cava and because of potential injury to the thoracic duct on the left side. Serious complications of central venous catheterization include air embolism, thrombus formation, malposition, hematoma formation, cardiac tamponade, cardiac arrhythmias, and arterial puncture.

Once the central vein has been cannulated, the catheter is attached to a transducer that is placed at the same level as the catheter tip. The transducer converts the mechanical signal to an electrical signal, and a monitor increases the size of the electrical signal for display. The transducer has a flush system attached to a pressurized intravenous solution (300 mm Hg via a pressure bag). The flush device allows continuous (3-5 mL/hr to prevent clot and backflow of blood) and manual flushing of the system.

CVP monitoring is an approximation of right atrial pressures, which indicate blood return to the heart and ejection of blood from the right ventricle. It also correlates with right ventricular end-diastolic pressure (preload) as the tricuspid valve opens during diastole. Normal CVP pressures range from 3 to 8 mm Hg; however, trends and dynamic changes give a better indication of ventricular filling. High CVP pressures can result from several conditions including increased preload or decreased myocardial contractility. Low CVP pressures may be the result of hypovolemia. In addition to the measurement of static venous pressures, plotting how the CVP changes over time (CVP waveform) can yield additional diagnostic information. The CVP waveform is illustrated in Figure 9.1. The “a” wave is generated from atrial contraction and should correspond to the P wave of the ECG. The “c” wave is produced by closure of the tricuspid valve, and the “v” wave is generated when the right ventricle contracts and the tricuspid valve bulges into the right atrium. The “x” descent represents atrial diastole, and the “y” descent occurs during atrial emptying.

Pulmonary artery catheterization allows monitoring of pulmonary artery pressures, wedge pressures, mixed venous oxygenation, and CO/cardiac index. It is typically reserved for patients with cardiopulmonary disease or for surgery in which large hemodynamic changes
are anticipated (i.e., suprarenal aortic aneurysm repair). There is no evidence that the insertion of a PAC improves survival, and it may increase mortality. Risks associated with insertion of a PAC include new-onset right bundle branch block, tachyarrhythmias, infection, rupture of the pulmonary artery, or thrombus formation.