Arterial Catheter Placement

Goals/Objectives

Indications
Techniques
Complications

Monitoring of the Heart and Vascular System

David L. Reich
Alexander J.C. Mittnacht
Gerard R. Manecke
Joel A. Kaplan

From Kaplan JA, Reich DL, Savino JS: Kaplan's Cardiac Anesthesia: The Echo Era, 6th edition (Saunders 2011)

Key Points

1.
Patients with severe cardiovascular disease and those undergoing surgery associated with rapid hemodynamic changes should be adequately monitored at all times.
2.
Adequate monitoring is based on specific patient, surgical, and environmental factors.
3.
Standard monitoring for cardiac surgery patients includes invasive blood pressure, electrocardiography, central venous pressure, transesophageal echocardiography, urine output, temperature, capnometry, pulse oximetry, and intermittent blood gas analysis.
4.
Additional monitors used in specific patients includes a pulmonary artery catheter, left atrial pressure, cardiac outputs, or central nervous system monitoring such as the bispectral index (BIS), regional oxygen saturation, or cerebrospinal fluid pressure.
5.
The American Society of Anesthesiologists has published recommendations for use of pulmonary artery catheters.
6.
The Society of Cardiovascular Anesthesiologists and the American Society of Echocardiography have published recommendations for intraoperative transesophageal echocardiography.
7.
Evidence-based data on the relation of monitoring to clinical outcomes in cardiac anesthesia are hard to obtain because of difficulties in conducting large prospective trials.
8.
Minimally invasive and noninvasive techniques for hemodynamic assessment continue to be developed, with increasing functionality and accuracy. They will likely play an expanded role in the care of cardiac surgery patients.

Hemodynamic Monitoring

For patients with severe cardiovascular disease and those undergoing surgery associated with rapid hemodynamic changes, adequate hemodynamic monitoring should be available at all times. With the ability to measure and record almost all vital physiologic parameters, the development of acute hemodynamic changes may be observed and corrective action may be taken in an attempt to correct adverse hemodynamics and improve outcome. Although outcome changes are difficult to prove, it is a reasonable assumption that appropriate hemodynamic monitoring should reduce the incidence of major cardiovascular complications. This is based on the presumption that the data obtained from these monitors are interpreted correctly and that therapeutic decisions are implemented in a timely fashion.

Many devices are available to monitor the cardiovascular system. These devices range from those that are completely noninvasive, such as the blood pressure (BP) cuff and electrocardiogram (ECG), to those that are extremely invasive, such as the pulmonary artery catheter (PAC). To make the best use of invasive monitoring, the potential benefits to be gained from the information must outweigh the potential complications. In many critically ill patients, the benefit obtained does outweigh the risks, which explains the widespread use of invasive monitoring. Transesophageal echocardiography (TEE), a minimally invasive technology, provides extensive hemodynamic data and other diagnostic information. Standard monitoring for cardiac surgical patients includes BP, ECG, central venous pressure (CVP), TEE, urine output, temperature, capnometry, pulse oximetry, and intermittent arterial blood gas analysis ( Box 59-1-1 ). The next tier of monitoring includes PACs with thermodilution cardiac output (CO), other CO monitors, indices of tissue oxygen transport, and cerebral monitoring (cerebral oximetry and processed electroencephalography; Box 59-1-2 ). Rarely, left atrial pressure (LAP) catheters may still be utilized. The interpretation of these complex data requires an astute clinician who is aware of the patient's overall condition and the limitations of the monitors.

Box 59-1-1

Standard Monitoring for Cardiac Surgical Patients

(Invasive) blood pressure
Electrocardiogram
Pulse oximetry
Capnometry
Temperature
Central venous pressure
Transesophageal echocardiography
Urine output
Intermittent arterial blood gas analysis

Box 59-1-2

Extended Monitoring for Patients Based on Case-Specific Factors

Pulmonary artery catheter
Cardiac output measurements
Processed electroencephalography (e.g., bispectral index)
Cerebral oximetry
Tissue oxygenation monitoring
Spinal drain (intrathecal) pressure

Arterial Pressure Monitoring

BP monitoring is the most commonly used method of assessing the cardiovascular system. The magnitude of the BP is directly related to the CO and the systemic vascular resistance (SVR). This is conceptually similar to Ohm's law of electricity (voltage = current × resistance), in which BP is analogous to voltage, CO to current flow, and SVR to resistance. An increase in the BP may reflect an increase in CO or SVR, or both. Although BP is one of the easiest cardiovascular variables to measure, it gives only indirect information about the patient's cardiovascular status.

Mean arterial pressure (MAP) is probably the most useful parameter to measure in assessing organ perfusion, except for the heart, in which the diastolic blood pressure (DBP) is the most important. MAP is measured directly by integrating the arterial waveform tracing over time, or using the formula: MAP = (SBP + [2 × DBP])/3, in which SBP is systolic blood pressure. The pulse pressure is the difference between SBP and DBP.

Anesthesia for cardiac surgery frequently is complicated by rapid and sudden changes in the BP because of several factors, including direct compression of the heart, impaired venous return because of retraction and cannulation of the vena cavae and aorta, arrhythmias from mechanical stimulation of the heart, and manipulations that may impair right ventricular (RV) outflow and pulmonary venous return. Sudden losses of significant amounts of blood may induce hypovolemia at almost any time. The cardiac surgical population also includes many patients with labile hypertension and atherosclerotic heart disease. A safe and reliable method of measuring acute changes in the BP is required during cardiac surgery with cardiopulmonary bypass (CPB).

Numerous methods of noninvasive BP measurement are clinically available. Nevertheless, most of these require the detection of flow past an occlusive cuff, and none generates an arterial waveform suitable for cardiac surgery. Continuous BP monitoring with noninvasive devices is feasible during anesthesia, but these devices have not proved to be suitable for cardiac surgery. Intra-arterial monitoring provides a continuous, beat-to-beat indication of the arterial pressure and waveform, and having an indwelling arterial catheter enables frequent sampling of arterial blood for laboratory analyses. Direct intra-arterial monitoring remains the gold standard for cardiac surgical procedures.

The arterial waveform tracing can provide information beyond timely BP measurements. For example, the slope of the arterial upstroke correlates with the derivative of pressure over time, dP/dt, and gives an indirect estimate of myocardial contractility. This is not specific information because an increase in SVR alone also will result in an increase in the slope of the upstroke. The arterial waveform also can present a visual estimate of the hemodynamic consequences of arrhythmias, and the arterial pulse contour can be used to estimate stroke volume (SV) and CO. Hypovolemia is suggested when the arterial pressure shows large SBP variations during the respiratory cycle in the mechanically ventilated patient. Coriat et al found that TEE-derived left ventricular (LV) dimensions at end-diastole correlated well with the magnitude of SBP decrease during inspiration.

General Principles

The arterial pressure waveform ideally is measured in the ascending aorta. The pressure measured in the more peripheral arteries is different from the central aortic pressure because the arterial waveform becomes progressively more distorted as the signal is transmitted down the arterial system. The high-frequency components, such as the dicrotic notch, disappear, the systolic peak increases, the diastolic trough decreases, and there is a transmission delay. These changes are caused by decreased arterial compliance in the periphery and reflection and resonance of pressure waves in the arterial tree. This effect is most pronounced in the dorsalis pedis artery, in which the SBP may be 10 to 20 mm Hg greater, and the DBP 10 to 20 mm Hg lower than in the central aorta ( Figure 59-1-1 ). Despite this distortion, the MAP measured in the peripheral arteries should be similar to the central aortic pressure under normal circumstances. However, this may not be the case after CPB.

F igure 59-1-1, The waveform of the arterial pressure changes markedly according to the site of the intra-arterial catheter. These changes are shown as a progression from central monitoring (top) through peripheral monitoring (bottom) . These changes are thought to be caused by forward wave propagation and wave reflection. In the periphery, systolic pressure is greater, diastolic pressure is lower, and mean pressure is minimally lower.

Pressure waves in the arterial (or venous) tree represent the transmission of forces generated in the cardiac chambers. Measurement of these forces requires their transmission to a device that converts mechanical energy into electronic signals. The components of a system for intravascular pressure measurement include an intravascular catheter, fluid-filled tubing and connections, an electromechanical transducer, an electronic analyzer, and electronic storage and display systems.

Components of a Pressure Measurement System

Intravascular Catheters

For arterial pressure measurements, short, narrow catheters are recommended (20 gauge or smaller) because they have favorable dynamic response characteristics and are less thrombogenic than larger catheters. Catheters made from Teflon are most widely used because they are softer and less thrombogenic, but they are prone to kinking. An artifact associated with intra-arterial catheters has been designated end-pressure artifact . When flowing blood comes to a sudden halt at the tip of the catheter, it is estimated that an added pressure of 2 to 10 mm Hg results. Conversely, clot formation on the catheter tip will overdamp the system and narrow the pulse pressure.

Coupling System

The coupling system usually consists of pressure tubing, stopcocks, and a continuous flushing device. This is the major source of distortion of arterial pressure tracings. Hunziker studied the damping coefficients and natural frequencies of various coupling systems. All systems were severely underdamped, and most led to systematic overestimation of the systolic arterial pressure.

Transducers

The function of transducers is to convert mechanical forces into electrical current or voltage. Over the years, this has been achieved by several different mechanisms, but today most transducers are of the resistance type. Pairs of resistors are incorporated into a circuit on the arms of a Wheatstone bridge type of electrical circuit. Most modern disposable transducers have a silicone diaphragm into which resistive elements have been etched. The manufacturers have adopted an output standard of 5 µV per volt excitation per 1 mm Hg so that, theoretically, any transducer can be used with any monitor. The dynamic response of bare transducers is usually in the 100- to 500-Hz range. Modern disposable transducers have eliminated many of the difficulties that used to require frequent recalibration because of drifting of the zero point. The major practical problem remaining with transducer systems is improper zeroing relative to the patient.

Analysis and Display Systems

Most modern equipment designed to analyze and display pressure information consists of a computerized system that handles several tasks. These include the acquisition and display of pressure signals; the derivation of numerical values for systolic, diastolic, and mean pressures; alarm functions; internal data storage; automated data transfer to an anesthesia information management system; trend displays; and printing functions. The algorithms used to analyze the pressure information and to provide numerical data vary among manufacturers. Venous pressures, as well as arterial pressures, to a lesser degree, are significantly affected by respiratory fluctuations. Most display systems average hemodynamic parameters over several cardiac cycles to minimize the effects of respiratory variability.

Flush Systems

The arterial catheter should be kept patent with a continuous infusion of normal saline solution (1 to 3 mL/hr). The infusion minimizes thrombus formation and helps prolong the usefulness of the catheter. Heparin is no longer routinely recommended as an additive to flush solutions because of the risk for heparin-induced thrombocytopenia in susceptible patients.

Characteristics of a Pressure Measurement System

The dynamic response of a pressure measurement system is characterized by its natural frequency and its damping. These concepts are best understood by snapping the end of a transducer-tubing assembly with a finger. The waveform on the monitor demonstrates rapid oscillations above and below the baseline (the natural frequency), which quickly decays to a straight line because of friction in the system (damping). The peaks and troughs of an arterial pressure waveform will be amplified if the transducer-tubing-catheter assembly has a natural frequency that lies close to the frequencies of the underlying sine waves of an arterial pressure waveform (typically < 20 Hz). This is commonly known as ringing or resonance of the system ( Figure 59-1-2 ). For an arterial pressure monitoring system to remain accurate at greater heart rates (HRs), its natural frequency should, therefore, be higher, typically more than 24 Hz. In practical terms, longer transducer tubing reduces the natural frequency of the system and tends to amplify the height of the SBP (peak) and the depth of the DBP (trough) values. Boutros and Albert demonstrated that, by changing the length of low-compliance (rigid) tubing from 6 inches to 5 feet, the natural frequency decreased from 34 to 7 Hz. As a result of the reduced natural frequency, the SBP measured with the longer tubing exceeded reference pressures by 17.3%.

F igure 59-1-2, The fast-flush test demonstrates the harmonic characteristics of a pressure monitoring system (i.e., transducer, fluid-filled tubing, and intra-arterial catheter). In an optimally damped system (A) , the pressure waveform returns to baseline after only one oscillation. In an underdamped system (B) , the pressure waveform oscillates above and below the baseline several times. In an overdamped system (C) , the pressure waveform returns to the baseline slowly with no oscillations.

Damping is the tendency of factors such as friction, compliant (soft) tubing, and air bubbles to absorb energy and decrease the amplitude of peaks and troughs in the waveform. The optimal degree of damping is that which counterbalances the distorting effects of transducer-tubing systems with lower natural frequencies. This is difficult to achieve. The damping of a clinical pressure measurement system can be assessed by observing the response to a rapid high-pressure flush of the transducer-tubing-catheter system (see Figure 59-1-2 ). In a system with a low damping coefficient, a fast-flush test results in several oscillations above and below the baseline before the pressure becomes constant. In an adequately damped system, the baseline is reached after one oscillation, whereas in an overdamped system, the baseline is reached after a delay and without oscillations.

The formulas for calculating the natural frequency and damping coefficient are as follows:

Natural frequency : f_{n} = \frac{d}{8} \sqrt{\frac{3}{π L ρ V_{d}}}

$Natural frequency : f_{n} = \frac{d}{8} \sqrt{\frac{3}{π L ρ V_{d}}}$

Damping coefficient : ζ \frac{16 η}{d^{3}} \sqrt{\frac{3 L V_{d}}{π ρ}}

$Damping coefficient : ζ \frac{16 η}{d^{3}} \sqrt{\frac{3 L V_{d}}{π ρ}}$

where d = tubing diameter; L = tubing length; ρ = density of the fluid; V _d = transducer fluid volume displacement; and n = viscosity of the fluid.

Arterial Cannulation Sites

Factors that influence the site of arterial cannulation include the location of surgery, the possible compromise of arterial flow because of patient positioning or surgical manipulations, and any history of ischemia or prior surgery on the limb to be cannulated. Another factor that may influence the cannulation site is the presence of a proximal arterial cutdown. The proximal cutdown may cause damped waveforms or falsely low BP readings because of stenosis or vascular thrombosis. Surgeons may use the axillary artery as the site of cannulation for CPB in patients who require anterograde selective cerebral perfusion or with a severely diseased ascending aorta. Depending on the surgical technique, possible complications associated with axillary (CPB) cannulation include distal limb ischemia (direct axillary artery CPB cannulation) or limb overcirculation with systemic hypoperfusion (axillary side graft anastomosis with graft cannulation). Most clinicians would choose to monitor the arterial pressure in the contralateral upper extremity, but some have also advocated additional monitoring of the radial artery on the ipsilateral side to detect overcirculation to the arm and to intervene accordingly. Patients presenting for reoperation who have had prior axillary artery cannulation may have some degree of stenosis at the old cannulation site. Sites generally chosen for arterial cannulation are discussed in the following paragraphs.

Radial and Ulnar Arteries

The radial artery is the most commonly used artery for continuous BP monitoring because it is easy to cannulate with a short (20-gauge) catheter and readily accessible during surgery. The collateral circulation is usually adequate and easy to check. It is advisable to assess the adequacy of the collateral circulation and the absence of proximal obstructions before cannulating the radial artery for monitoring purposes.

The ulnar artery provides most blood flow to the hand in about 90% of patients. The radial and ulnar arteries are connected by a palmar arch, which provides collateral flow to the hand in the event of radial artery occlusion. Palm showed that if there is adequate ulnar collateral flow, circulatory perfusion pressure to the fingers is adequate after radial arterial catheterization. Some clinicians perform the Allen test before radial artery cannulation to assess the adequacy of collateral circulation to the hand.

The Allen test is performed by compressing the radial and ulnar arteries and exercising the hand until it is pale. The ulnar artery is then released (with the hand open loosely), and the time until the hand regains its normal color is noted. With a normal collateral circulation, the color returns to the hand in about 5 seconds. If, however, the hand takes longer than 15 seconds to return to its normal color, cannulation of the radial artery on that side is controversial. The hand may remain pale if the fingers are hyperextended or widely spread apart, even in the presence of a normal collateral circulation. Variations on the Allen test include using a Doppler probe or pulse oximeter to document collateral flow. If the Allen test demonstrates that the hand depends on the radial artery for adequate filling, and other cannulation sites are not available, the ulnar artery may be selected.

The predictive value of the Allen test has been challenged. In a large series of children in whom radial arterial catheterization was performed without preliminary Allen tests, there was an absence of complications. Slogoff et al cannulated the radial artery in 16 adult patients with poor ulnar collateral circulation (assessed using the Allen test) without any complications. An incidence of zero in a study sample of only 16 patients, however, does not guarantee that the true incidence of the complication is negligible. In contrast, Mangano and Hickey reported a case of hand ischemia requiring amputation in a patient with a normal preoperative result for the Allen test. Thus, the predictive value of the Allen test is questionable. Alternatively, pulse oximetry or plethysmography can be used to assess patency of the collateral arteries of the hand. Barbeau et al compared the modified Allen test with pulse oximetry and plethysmography in 1010 consecutive patients undergoing percutaneous radial artery cannulation for cardiac catheterization. Pulse oximetry and plethysmography were more sensitive than the Allen test for detecting inadequate collateral blood supply, and only 1.5% of patients were not suitable for radial artery cannulation.

Another infrequently used method of radial arterial catheterization involves percutaneous insertion of a long catheter to obtain a central aortic tracing of arterial pressure. No complications were attributed to these catheters in a series of patients. The advantage of a central arterial tracing is the increased accuracy compared with radial arterial pressure in patients with low-flow states or after CPB. Although reasons for the difference between central and peripheral measurements of BP are not entirely clear, after CPB, they were transiently present in 17% to 40% of patients in several studies. Kanazawa et al suggested that a decrease in the arterial elasticity is responsible for instances in which lower radial artery pressures (compared with aortic pressures) are observed after CPB. When the palpated central aortic pressure is high despite a low radial arterial BP value, the central aortic pressure also may be temporarily monitored using a needle attached to pressure tubing placed in the aorta by the surgeon until the problem resolves. Alternatively, a femoral arterial catheter may be inserted.

Chest wall retractors that are used during internal mammary artery dissection may impede radial arterial pressure monitoring in cardiothoracic procedures in some patients. The arm on the affected side may have diminished perfusion during extreme retraction of the chest wall. If the left internal mammary artery is used during myocardial revascularization, the right radial artery could be monitored to avoid this problem. Alternatively, a noninvasive BP cuff on the right side could be used to confirm the accuracy of the radial artery tracing during periods of chest wall retraction.

Monitoring of the radial artery distal to a brachial arterial cutdown site is not recommended. Acute thrombosis or residual stenosis of the brachial artery will lead to falsely low radial arterial pressure readings. Other considerations related to the choice of a radial arterial monitoring site include prior surgery of the hand, selection of the nondominant hand, and the preferences of the surgeons and anesthesiologists.

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