Cardiac Output

CARDIAC OUTPUT

Indications

CO measurement to permit calculation of the SV is one of the primary reasons for PA catheterization. Currently, there are a number of alternative, less invasive methods to estimate ventricular function to assist in goal-directed therapy.

Techniques & Complications

A. Thermodilution

The injection of a quantity (2.5, 5, or 10 mL) of fluid that is below body temperature (usually room tem-perature or iced) into the right atrium changes the temperature of blood in contact with the thermistor at the tip of the PA catheter. The degree of change is inversely proportionate to CO: Temperature change is minimal if there is a high blood flow, whereas tem-perature change is greater when flow is reduced. After injection, one can plot the temperature as a function of time to produce a thermodilution curve (Figure 5–23). CO is determined by a computer program that integrates the area under the curve.Accurate measurements of CO depend on rapid and smooth injection, precisely knowninjectant temperature and volume, correct entry of the calibration factors for the specific type of PA catheter into the CO computer, and avoidance of measurements during electrocautery. Tricuspid regurgitation and cardiac shunts invalidate results because only right ventricular output into the PA is actually being measured. Rapid infusion of the iced injectant has rarely resulted in cardiac arrhythmias.

A modification of the thermodilution technique allows continuous CO measurement with a special catheter and monitor system. The catheter contains a thermal filament that introduces small pulses of heat into the blood proximal to the pulmonic valve and a thermistor that measures changes in PA blood temperature. A computer in the monitor determines CO by cross-correlating the amount of heat input with the changes in blood temperature.

Transpulmonary thermodilution (PiCCO ^® sys-tem) relies upon the same principles of thermodi-lution, but does not require PA catheterization. A central line and a thermistor-equipped arterial cath-eter (usually placed in the femoral artery) are nec-essary to perform transpulmonary thermodilution. Thermal measurements from radial artery catheters have been found to be invalid. Transpulmonary thermodilution measurements involve injection of cold indicator into the superior vena cava via a central line ( Figure 5–24). A thermistor notes the change in temperature in the arterial system follow-ing the cold indicator’s transit through the heart and lungs and estimates the CO.

Transpulmonary thermodilution also per-mits the calculation of both the global-end dia-stolic volume (GEDV) and the extravascular lung water (EVLW). Through mathematical analysis and extrapolation of the thermodilution curve, it is possible for the transpulmonary thermodilu-tion computer to calculate both the mean transit time of the indicator and its exponential decay time (Figure 5–25). The intrathoracic thermal volume (ITTV) is the product of the CO and the mean tran-sit time (MTT). The ITTV includes the pulmonary blood volume (PBV), EVLW, and the blood con-tained within the heart. The pulmonary thermal volume (PTV) includes both the EVLW and the PBV and is obtained by multiplying the CO by the exponential decay time (EDT). Subtracting the PTV from the ITTV gives the GEDV (Figure 5–26). The GEDV is a hypothetical volume that assumes that all of the heart’s chambers are simultaneously full in diastole. With a normal index between 640 and 800 mL/m², the GEDV can assist in determining volume status. An extra vascular lung water index of less than 10 mL/kg is normative. The EVLW is the ITTV

minus the intrathoracic blood volume (ITBV). The ITBV = GEDV × 1.25.

Th us EVLW = ITTV – ITBV. An increased EVLW can be indicative of fluid overload. Thus, through mathematical analysis of the transpulmonary thermodilution curve, it is possible to obtain volu-metric indices to guide fluid replacement therapy. Moreover, the PiCCO^® system calculates SV varia-tion and pulse pressure variation through pulse contour analysis, both of which can be used to determine fluid responsiveness. Both SV and pulse pressure are decreased during positive pressure ven-tilation. The greater the variations over the course of positive pressure inspiration and expiration, the more likely the patient is to improve hemodynamic measures following volume administration.

B. Dye Dilution

If indocyanine green dye (or another indicator such as lithium) is injected through a central venous cath-eter, its appearance in the systemic arterial circula-tion can be measured by analyzing arterial samples with an appropriate detector (eg, a densitometer for indocyanine green). The area under the resulting dye indicator curve is related to CO. By analyzingarterial blood pressure and integrating it with CO, systems that use lithium (LiDCO^TM) also calculate beat-to-beat SV. In the LiDCO^TM system, a small bolus of lithium chloride is injected into the circu-lation. A lithium-sensitive electrode in an arterial catheter measures the decay in lithium concentra-tion over time. Integrating the concentration over time graph permits the machine to calculate the CO. The LiDCO^TM device, like the PiCCO ^® ther-modilution device, employs pulse contour analysis of the arterial wave form to provide ongoing beat-to-beat determinations of CO and other calculated parameters. Lithium dilution determinations can be made in patients who have only peripheral venous access. Lithium should not be administered to patients in the first trimester of pregnancy. The dye dilution technique, however, introduces the prob-lems of indicator recirculation, arterial blood sam-pling, and background tracer buildup, potentially limiting the use of such approaches perioperatively. Nondepolarizing neuromuscular blockers may affect the lithium sensor.

C. Pulse Contour Devices

Pulse contour devices use the arterial pressure tracing to estimate the CO and other dynamicparameters, such as pulse pressure and SV variation with mechanical ventilation. These indices are used to help determine if hypotension is likely to respond to fluid therapy.

Pulse contour devices rely upon algorithms that measure the area of the systolic portion of the arte-rial pressure trace from end diastole to the end of ventricular ejection. The devices then incorporate a calibration factor for the patient’s vascular compli-ance, which is dynamic and not static. Some pulse contour devices rely first on transpulmonary ther-modilution or lithium thermodilution to calibrate the machine for subsequent pulse contour measure-ments. The FloTrac (Edwards Life Sciences) does not require calibration with another measure and relies upon a statistical analysis of its algorithm to account for changes in vascular compliance occurring as a consequence of changed vascular tone.

D. Esophageal Doppler

Esophageal Doppler relies upon the Doppler prin-ciple to measure the velocity of blood flow in the descending thoracic aorta. The Doppler princi-ple is integral in perioperative echocardiography, as discussed below. The Doppler effect has been described previously and is the result of the apparent change in sound frequency when the source of the sound wave and the observer of the sound wave are in relative motion. Blood in the aorta is in relative motion compared with the Doppler probe in the esophagus. As red blood cells travel, they reflect a frequency shift, depending upon both the direction and velocity of their movement. When blood flows toward the transducer, its reflected fre-quency is higher than that which was transmitted by the probe. When blood cells move away from the transducer, the frequency is lower than that which was initially sent by the probe. By using the Doppler equation, it is possible to determine the velocity of blood flow in the aorta. The equation is:

Velocity of blood blow = {frequency change/ cosine of the angle of incidence between the Doppler beam and the blood flow} × {speed of sound in tis-sue/2 (source frequency)}

For Doppler to provide a reliable estimate of velocity, the angle of incidence should be as close to zero as possible, since the cosine of 0 is 1. As theangle approaches 90°, the Doppler measure is unre-liable, as the cosine of 90° is 0.

The esophageal Doppler device calculates the velocity of flow in the aorta. As the velocities of the cells in the aorta travel at different speeds over the cardiac cycle, the machine obtains a measure of all of the velocities of the cells moving over time. Mathematically integrating the velocities represents the distance that the blood travels. Next, using nor-mograms, the monitor approximates the area of the descending aorta. The monitor thus calculates both the distance the blood travels, as well as the area: area × length = volume.

Consequently, the SV of blood in the descend-ing aorta is calculated. Knowing the HR allows cal-culation of that portion of the CO flowing through the descending thoracic aorta, which is approxi-mately 70% of total CO. Correcting for this 30% allows the monitor to estimate the patient’s total CO. Esophageal Doppler is dependent upon many assumptions and nomograms, which may hinder its ability to accurately reflect CO in a variety of clinical situations.

E. Thoracic Bioimpedance

Changes in thoracic volume cause changes in tho-racic resistance (bioimpedance) to low amplitude, high frequency currents. If thoracic changes in bioimpedance are measured following ventricular depolarization, SV can be continuously determined. This noninvasive technique requires six electrodes to inject microcurrents and to sense bioimpedance on both sides of the chest. Increasing fluid in the chest results in less electrical bioimpedance. Mathematical assumptions and correlations are then made to calculate CO from changes in bioimpedance. Disadvantages of thoracic bioimpedance include susceptibility to electrical interference and reliance upon correct electrode positioning. The accuracy of this technique is questionable in several groups of patients, including those with aortic valve disease, previous heart surgery, or acute changes in thoracic sympathetic nervous function (eg, those undergoing spinal anesthesia).

F. Fick Principle

The amount of oxygen consumed by an individual (Vo₂) equals the difference between arterial and venous (a–v) oxygen content (C) (Cao₂ and Cvo₂) multiplied by CO. Therefore

Mixed venous and arterial oxygen content are easily determined if a PA catheter and an arterial line are in place. Oxygen consumption can also be calculated from the difference between the oxygen content in inspired and expired gas. Variations of the Fick principle are the basis of all indicator–dilution methods of determining CO.

G. Echocardiography

Th ere are no more powerful tools to diagnose and assess cardiac function perioperatively than trans-thoracic echocardiography (TTE) and transesoph-ageal echocardiography (TEE). Both approaches are increasingly used in the operative setting. In the operating rooms, limited access to the chest makes TEE an ideal option to visualize the heart. Both TTE and TEE can be employed preopera-tively and postoperatively. TTE has the advantage of being completely noninvasive; however, acquir-ing the “windows” to view the heart can be difficult. Disposable TEE probes are now available that can remain in position in critically ill patients for days, during which intermittent TEE examinations can be performed.

Echocardiography can be employed by anes-thesia staff in two ways, depending upon degrees of training and certification. Basic (or hemodynamic) TEE permits the anesthesiologist to discern the pri-mary source of a patient’s hemodynamic instability. Whereas in past decades the PA flotation catheter would be used to determine why the patient might be hypotensive, the anesthetist performing hemody-namic TEE is attempting to determine if the heart is adequately filled, contracting appropriately, not externally compressed, and devoid of any grossly obvious structural defects. At all times, information obtained from hemodynamic TEE may be corre-lated with other information as to the patient’s gen-eral condition.

Anesthesiologists performing advanced TEE make therapeutic and surgical recommendations based upon their TEE interpretations. Various organizations and boards have been established worldwide to certify individuals in all levels of peri-operative echocardiography. More importantly, individuals who perform echocardiography should be aware of the credentialing requirements of their respective institutions.

Echocardiography has many uses, including:

·        Diagnosis of the source of hemodynamic instability, including myocardial ischemia, systolic and diastolic heart failure, valvular abnormalities, hypovolemia, and pericardial tamponade

·        Estimation of hemodynamic parameters, such as SV, CO, and intracavitary pressures

·        Diagnosis of structural diseases of the heart, such as valvular heart disease, shunts, aortic diseases

·        Guiding surgical interventions, such as mitral valve repair.

Various echocardiographic modalities are employed perioperatively by anesthesiologists, including TTE, TEE, epiaortic and epicardiac ultra-sound, and three-dimensional echocardiography. Some advantages and disadvantages of the modali-ties are as follows:

TTE has the advantage of being noninvasive and essentially risk free. Limited scope TTE exams are now increasingly common in the intensive care unit (Figure 5–27).

two-dimensional image planes and windows to mentally recreate the three-dimensional anatomy. The ability to interpret these images at the advanced certification level requires much training.

Epiaortic and epicardiac ultrasound imaging techniques employ an echo probe wrapped in a sterile sheath and manipulated by thoracic surgeons intraoperatively to obtain views of the aorta and the heart. The air-filled trachea prevents TEE imaging of the ascending aorta. Because the aorta is manipulated during cardiac surgery, detection of atherosclerotic plaques permits the surgeon to potentially minimize the incidence of embolic stroke. Imaging of the heart with epicardial ultrasound permits intraoperative echocardiography when TEE is contraindicated because of esophageal or gastric pathology.

Three-dimensional echocardiography (TTE and TEE) has become available in recent years (Figure 5–29). These techniques provide a three-dimensional view of the heart’s structure. In particular, three-dimensional images can better quantify the heart’s volumes and can generate a surgeon’s view of the mitral valve to aid in guiding valve repair.

Echocardiography employs ultrasound (sound at frequencies greater than normal hearing) from 2 to 10 MHz. A piezoelectrode in the probe trans-ducer converts electrical energy delivered to the probe into ultrasound waves. These waves then travel

through the tissues, encountering the blood, the heart, and other structures. Sound waves pass read-ily through tissues of similar acoustic impedance; however, when they encounter different tissues, they are scattered, refracted, or reflected back toward the ultrasound probe. The echo wave then interacts with the ultrasound probe, generating an electrical signal that can be reconstructed as an image. The machine knows the time delay between the transmitted and the reflected sound wave. By knowing the time delay, the location of the source of the reflected wave can be determined and the image generated. The TEE probe contains myriad crystals generating and pro-cessing waves, which then create the echo image. The TEE probe can generate images through mul-tiple planes and can be physically manipulated in the stomach and esophagus, permitting visualization of heart structures (Figure 5–30). These views can be used to determine if the heart’s walls are being delivered adequate blood supply (Figure 5–31). In the healthy heart, the heart’s walls thicken and move inwardly with each beat. Wall motion abnormalities, in which the heart’s walls fail to thicken during sys-tole or move in a dyskinetic fashion, can be associ-ated with myocardial ischemia.

The Doppler effect is routinely used in echocar-diographic examinations to determine the heart’s function. In the heart, both the blood flowing through the heart and the heart tissue move relative to the echo probe in the esophagus or on the chest wall. By using the Doppler effect, it is possible for echocardiographers to determine both the direction and the velocity of blood flow and tissue movement.

Blood flow in the heart follows the law of the conservation of mass. Therefore, the volume of blood that flows through one point (eg, the left ventricular outflow tract) must be the same volume that passes through the aortic valve. When the path-way through which the blood flows becomes nar-rowed (eg, aortic stenosis), the blood velocity must increase to permit the volume to pass. The increase in velocity as blood moves toward an esophageal echo probe is detected. The Bernoulli equation (pressure change = 4V²) allows echocardiographers to determine the pressure gradient between areas of different velocity, where v represents the area of

maximal velocity ( Figure 5–32). Using continuous wave Doppler, it is possible to determine the maxi-mal velocity as blood accelerates through a patho-logic heart structure. For example, a blood flow of 4 m/sec reflects a pressure gradient of 84 mm Hg between an area of slow flow (the left ventricular outflow tract) and a region of high flow (a stenotic aortic valve).

Likewise, the Bernoulli equation permits echo-cardiographers to estimate PA and other intracavi-tary pressures, if assumptions are made.

Assume P₁>> P₂

Blood flow proceeds from an area of high pres-sure P₁ to an area of low pressure P₂.

The pressure gradient = 4V², where V is the maximal velocity measured in meters per second.

Thus,

4V²= P₁− P₂

Th us, assuming that there is a jet of regurgitant blood flow from the left ventricle into the left atrium and that left ventricular systolic pressure (P₁) is the same as systemic blood pressure (eg, no aortic ste-nosis), it is possible to calculate left atrial pressure (P₂). In this manner, echocardiographers can esti-mate intracavitary pressures when there are pressure gradients, measurable flow velocities between areas

of high and low pressure, and knowledge of either P₁ or P₂ (Figure 5–33).

The Doppler principle is also used by echo-cardiographers to identify areas of abnormal flow using color flow Doppler. Color flow Doppler cre-ates a visual picture of the heart’s blood flow by assigning a color code to the velocities in the heart. Blood flow directed away from the echocardio-graphic transducer is color-coded blue, whereas that which is moving toward the probe is red. The higher the velocity of flow, the lighter the color hue (Figure 5–34). When the velocity of blood flow becomes greater than that which the machine can measure, flow toward the probe is misinterpreted as flow away from the probe, creating images of turbu-lent flow and “aliasing” of the image. Such changes in flow pattern are used by echocardiographers to identify areas of pathology.

Doppler can also be used to provide an estimate of SV and CO. Similar to esophageal Doppler probes

previously described, TTE and TEE can be used to estimate CO. Assuming that the left ventricular outflow tract is a cylinder, it is possible to measure its diameter (Figure 5–35). Knowing this, it is pos-sible to calculate the area through which blood flows using the following equation:

Area = πr²= 0.785 × diameter²

Next, the time velocity integral is determined. A Doppler beam is aligned in parallel with the left ventricular outflow tract ( Figure 5–36). The veloci-ties passing through the left ventricular outflow tract are recorded, and the machine integrates the veloc-ity/time curve to determine the distance the blood traveled.

Area × length = volume

In this instance, the SV is calculated:

SV × HR = CO

Lastly, Doppler can be used to examine the movement of the myocardial tissue. Tissue veloc-ity is normally 8–15 cm/sec (much less than that of

blood, which is 100 cm/s). Using the tissue Doppler function of the echo machine, it is possible to dis-cern both the directionality and velocity of the heart’s movement. During diastolic filling, the lat-eral annulus myocardium will move toward a TEE probe. Reduced myocardial velocities (<8 cm/s) are associated with impaired diastolic function and higher left ventricular end-diastolic pressures.

Ultimately, echocardiography can provide com-prehensive cardiovascular monitoring. Its routine use outside of the cardiac operating room has been hindered by both the costs of the equipment and the training required to correctly interpret the images. As equipment becomes more readily available, it is likely that anesthesia staff will perform an increas-ing number of echocardiographic examinations for hemodynamic monitoring perioperatively. When questions arise beyond those related to hemody-namic guidance, interpretation by an individual credentialed in advanced perioperative echocar-diography is warranted.