Although considerable information regarding the health of the cardiovascular system can be gleaned from a comprehensive patient interview and targeted physical examination, these tools are occasionally inadequate for assessing the patient with or at-risk for CVD. Further diagnostic testing is often required to establish a diagnosis, assess risk, and guide the selection of therapy. The following section provides an overview of some of the most commonly used diagnostic and evaluative modalities in cardiovascular medicine.
The ECG is the oldest diagnostic tool used in contemporary practice, and it remains a critical step in the evaluation of a wide range of clinical presentations, including chest pain or discomfort, palpitations, syncope, dizziness, and signs and symptoms of new-onset heart failure. The ECG provides a real-time map of electrical activity across the heart. As with other muscles in the body, the end-result of these electrical impulses is a muscular contraction. Therefore, prior to learning the fundamentals of ECG interpretation, a basic framework of how electrical impulses in the heart trigger a muscular contraction is required.
Electrical activation in the heart begins within the right atrium, in the sinoatrial (SA) node (Fig. e29-6). The impulse generated by the SA node is then transmitted as a wave of depolarization across the atria and into the atrioventricular (AV) node. From the AV node, conduction flows through the right and left bundle branches, and finally into the His-Purkinje system, where it is transmitted to the ventricular myocardium. Electrical stimulation of cardiac myocytes triggers the entry of calcium into the cell, which leads to further release of calcium from within the cell, culminating in a muscle contraction. Following contraction, myocytes repolarize in preparation for the next impulse.
Electrical System of the Heart. An electrical impulse begins at the sinoatrial node in the right atrium. Depolarization proceeds to the atrioventricular node. Finally, conduction proceeds down the left and right bundle branches, and then into the Purkinje fibers. (Reproduced, with permission, from Barrett KE, Barman SM, Brooks HL, Yuan JJ. eds. Ganong’s Review of Medical Physiology. 26th ed. New York, NY:McGraw-Hill;2019.)
Since the heart is a three-dimensional structure, the wave of electrical depolarization does not travel in a straight line. By placing pairs of positively and negatively charged electrodes called leads at different locations across the chest, a comprehensive map of depolarization and repolarization can be produced. A standard ECG consists of 12 leads (Fig. e29-7). Six leads are placed across the chest wall and are named V1 to V6 (ie, the precordial or chest leads). The remaining six leads (ie, limb leads) are formed by pairs of electrodes on each arm and leg (or more commonly, on each shoulder and each side of the abdomen in the direction of the lower limbs), which monitor the signal on a frontal plane on the chest. Leads I, II, and III monitor the electrical vectors between the right arm and left arm, right arm and left leg, and left arm and left leg, respectively. Leads aVF, aVL, and aVR (ie, the augmented limb leads) monitor electrical vectors from an imaginary point in the middle of the abdomen to the left leg, left arm, and right arm, respectively.
Electrocardiogram Lead Placements. The positions of the unipolar lead placements (V1-V6) are shown. Bipolar leads are also placed on both arms and both legs to obtain tracings for leads I, II, and III, and aVL, aVF, and aVR. (Reproduced, with permission, from Goldman MJ: Principles of Clinical Electrocardiography. 12th ed. Originally published by Appleton & Lange. Copyright © 1986 by McGraw-Hill.).
As electrical activity approaches the positive electrode of each lead (for bipolar leads) or the lead itself (for unipolar leads), a positive deflection is recorded. Current moving away from an electrode produces a negative deflection. Electrical signals are recorded from several leads simultaneously over the course of a few seconds, and these are displayed on an ECG (Fig. e29-8). The bottom three tracings of the ECG are often referred to as the rhythm strip, as they track the same leads over an entire tracing, facilitating rhythm interpretation. Because no two leads are located in the same area, each displays a slightly different electrical signal. Combinations of leads can thus be used to detect a variety of disorders beyond arrhythmias, including the presence of myocardial ischemia and pathologic changes in heart structure, such as chamber enlargement or hypertrophy.
A Normal Electrocardiogram. A normal 12-lead electrocardiogram (ECG). The ECG plots electrical activity on the Y-axis against time on the X-axis. Each lead records for a few seconds and then switches, with the exception of the bottom three, which do not change. These latter three leads constitute the rhythm strip, as they permit a more comprehensive assessment of cardiac rhythm.
The distinguishing features of a normal ECG are shown in Fig. e29-9. The X-axis of the ECG reflects time, and each large box on the ECG graph paper represents 0.2 seconds. The Y-axis reflects the magnitude of the electrical signal in each lead, described in terms of millimeter of deflection above or below the isoelectric baseline. The P wave represents atrial depolarization at the beginning of atrial systole. The PR interval is an isoelectric line from the end of the P wave to the onset of the QRS complex and reflects slowed conduction through the AV node and the delay between atrial and ventricular depolarization. The QRS complex reflects ventricular depolarization and the onset of ventricular systole. The QT interval reflects the delay in ventricular repolarization and the duration of the action potential. Finally, the ST segment and T wave occur during ventricular repolarization at the beginning of ventricular diastole. Normal ranges exist for the amplitude and timing of each wave and segment.
Electrocardiogram Terminology. Terminology used to describe the distinguishing features of a normal ECG. (Reproduced, with permission, from Barrett KE, Barman SM, Brooks HL, Yuan JX-Y. Ganong’s Review of Medical Physiology Examination and Board Review. 26th ed. New York: McGraw-Hill; 2019.).
Interpreting an ECG requires a systematic approach, although no “one-size-fits-all” algorithm exists. Once a method is selected, clinicians are encouraged to use it consistently. A common approach to ECG interpretation is described in Table e29-7. Rhythm identification is often the most challenging aspect of electrocardiography, and complex arrhythmias may be difficult for even the most experienced cardiologists to interpret. The axis reflects the overall net depolarization vector in the frontal plane (gathered from the limb leads), and certain conduction disorders or cardiovascular conditions may shift this axis. Following identification of the axis, the P wave and QRS amplitude are assessed, as prominent voltage may suggest chamber enlargement or hypertrophy. The QRS complex must be carefully inspected for Q waves (an initial downward deflection in the QRS complex), which may be a sign of prior MI. Next, the ST segments and T waves are inspected. In the appropriate context, ST segment depressions may reflect myocardial ischemia, and ST segment elevations may be seen with myocardial injury, although neither of these changes are specific and should therefore be evaluated in context with other clinical information (eg, symptoms). Finally, the intervals are assessed, as abnormalities may be a reflection of electrolyte disorders.
TABLE e29-7Approach to Electrocardiogram Interpretation |Favorite Table|Download (.pdf) TABLE e29-7 Approach to Electrocardiogram Interpretation
Identify atrial and ventricular rates.
Assess P and QRS amplitude/duration.
Review ST Segment and T wave.
Calculate QT/QTc interval.
Another important aspect of ECG interpretation is determining the duration of the QT interval, as prolongation increases the risk of polymorphic ventricular tachycardia and sudden cardiac death.48 The QT interval is measured from the beginning of the QRS complex to the end of the T wave, and represents the time required for ventricular depolarization and repolarization. The QT interval varies based on heart rate, and can be corrected according to Bazett’s formula as follows:
A normal corrected QT (QTc) interval is <440 msec for men and <460 msec for women. Electrolyte abnormalities (eg, hypokalemia) can affect the QT interval, along with ischemia, commonly prescribed medications (eg, atypical antipsychotics, azole antifungals, select antiarrhythmics), and inherited membrane channelopathies. Because the QT interval is comprised of the QRS complex, ST segment, and the T wave, conditions associated with a widened QRS (eg, bundle branch block, ventricular pacing) will also prolong the QT interval. In these scenarios, the standard QTc reference intervals do not apply.
One of the most common reasons for obtaining an ECG is in the evaluation of palpitations, which may occur intermittently and not at a time when they can be evaluated by a healthcare professional. If a determination of heart rhythm is all that is required, a 12-lead ECG may not be necessary and instead ambulatory monitoring may be performed using a portable ECG device known as a Holter monitor. Several ECG leads are placed on the patient’s chest, and these are attached to a small recorder. All heart beats are recorded over a period of time (typically 24-48 hours), and the patient keeps a record of any symptoms that emerge. If symptoms occur and correlate with the presence of an arrhythmia on the monitor, a diagnosis may be made. A Holter monitor will also provide information regarding the minimum, maximum, and average heart rates; the number of premature atrial and ventricular beats; and any ST segment changes.
For symptoms that occur very infrequently, an event monitor may be used. An event monitor is similar to a Holter monitor except that the device may be worn for 14 to 28 days. Unlike Holter monitors, event monitors do not record and store every heartbeat. Instead, when patients experience symptoms, they trigger the device with a button, prompting it to record and store several minutes of heart rate and rhythm data. For even less frequent episodes (ie, those occurring less than once a month) in patients with severe symptoms (eg, syncope), an implantable monitor may be placed under the skin, and data can be recorded for many months.
A resting ECG may be normal in patients with CAD, as blood flow may be adequate for meeting the heart’s metabolic needs even in the face of significant obstructive lesions.49 However, physical exertion produces an increase in heart rate, myocardial contractility, and BP, leading to increased myocardial oxygen demand.50 In the presence of a fixed coronary stenosis, oxygen delivery will be unable to keep up with demand, and myocardial ischemia may occur, manifesting as chest pain or abnormalities on ECG.
Based on these principles, stress testing is a common modality for detecting CAD in patients with anginal symptoms in the absence of an MI. If obstructive CAD is detected, patients may benefit from risk factor reduction, pharmacologic therapy, and potentially coronary angiography and revascularization. Although chest pain is the most common reason for stress testing, other established indications are listed in Table e29-8. Stress testing is not routinely recommended as a screening tool for CAD in asymptomatic patients.
TABLE e29-8Common Indications for Stress Testing |Favorite Table|Download (.pdf) TABLE e29-8 Common Indications for Stress Testing
Chest pain suggestive of angina
Risk assessment in patients with unstable angina or non-ST segment acute coronary syndrome
Preoperative risk assessment in patients undergoing noncardiac surgery
Assessment of severity in valvular heart disease
Determining role for coronary revascularization in left ventricular dysfunction
Clinical significance of arrhythmias or bradycardias
In the United States, most stress tests are performed on a treadmill rather than a bicycle. Although patients may be accustomed to walking or jogging on a treadmill, stress testing differs from a standard exercise regimen. The most commonly used treadmill protocol is the Bruce protocol, which begins at a speed of 1.7 mph (2.7 km/h) and an incline of 10%. The speed then increases by 0.8 mph (1.3 km/h) and the grade increases by 2% every 3 minutes for a total of 21 minutes.51 During the test, the ECG is monitored continuously, and BP is measured every 3 minutes. Patients are also routinely asked about chest pain symptoms. Most stress tests are symptom-limited, that is, patients exercise until they develop dyspnea that limits further activity. Other indications for early termination of the test include the development of arrhythmias or drastic changes in BP. In properly selected candidates under appropriate supervision, ECG stress testing is safe, with serious complications (eg, ventricular arrhythmias, MI) occurring at a rate of only one event per 10,000 studies.52
Patients must achieve 85% of the age-predicted maximum heart rate in order for a stress test to be diagnostic.53 If patients cannot achieve this threshold despite maximal exercise, the study is considered “nondiagnostic,” even if no ECG abnormalities are observed. As a result, the presence of CAD cannot be definitively excluded, and alternative testing should be considered.
Development of chest pain, especially if accompanied by ECG abnormalities, is highly specific for CAD.54 A positive stress test is defined as a change on the ECG of 1 mm of down-sloping or horizontal ST segment depressions in two contiguous leads persisting 80 msec after the QRS complex.55 ST segment depressions can occur during exercise or in the recovery period after the test. If ST segment elevation occurs in a patient without baseline Q waves, profound myocardial ischemia is likely present, and the study should be terminated immediately so that urgent cardiac catheterization may be performed.
When certain ECG abnormalities are present at baseline, such as left ventricular hypertrophy with prominent repolarization abnormalities, the development of ST segment depression loses both sensitivity and specificity.55 Additionally, the presence of a left bundle branch block renders the ST segments uninterpretable, and treadmill testing should not be performed.
Stress testing also suffers from the limitations common to all diagnostic studies in that there may be both false positive and false negative results, and the likelihood of a positive test is dependent on the pretest probability of disease. Consequently, stress testing should not be the sole factor in determining whether further testing (eg, coronary angiography) should be performed. Instead, multiple data points should be considered, including traditional ASCVD risk factors and the nature of the patient’s symptoms.
For patients who are unable to exercise (eg, orthopedic limitations, other factors that may limit ambulation) or who cannot achieve the target heart rate, pharmacologic stress testing may be performed instead. In a pharmacologic stress test, ischemia is induced by the administration of an agent that increases myocardial oxygen demand or reduces oxygen supply, resulting in a perfusion defect. Since pharmacologic stress testing is always performed in conjunction with additional imaging studies such as echocardiography or radionuclide imaging, this modality will be discussed in further detail in the section on nuclear cardiology later in this chapter.
Aside from the ECG, echocardiography is the most commonly used modality to assess cardiac function and the presence of CVD. Echocardiography is a noninvasive test involving the emission of ultrasound waves, which travel through tissue and are reflected back to a transducer probe, where they are processed to construct images of the heart and related structures. In early forms of echocardiography, only one-dimensional (m-mode) ultrasound was available (Fig. e29-10). However, as probe technology improved, two-dimensional echocardiography was developed (Fig. e29-11). Although some devices are now capable of generating three-dimensional echocardiographs, two-dimensional imaging remains the most common modality used.
M-mode Echocardiography. M-mode echocardiography, also known as “ice pick” echocardiography, provides a one-dimensional view of the heart over time. The black areas represent blood in the heart. From top to bottom, the first black area is the right ventricle, and the left ventricle is below. The mitral valve can also be seen in this view. (Reproduced, with permission, from Pahlm O, Wagner GS, eds. Multimodal Cardiovascular Imaging: Principles and Clinical Applications. New York, NY: McGraw-Hill; 2011.).
Two-dimensional Echocardiography. Two-dimensional apical four-chamber view of the heart. The probe is placed at the left ventricular apex, usually in the fifth intercostal space at the mid-clavicular line. (LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.).
Echocardiographic images are broadly classified according to where the probe is placed during the procedure. During a transthoracic echocardiogram (TTE), the transducer probe is placed on several locations on the chest and upper abdomen. A TTE provides information on the chambers, valves, and contractile function of the heart. Consequently, TTE is the imaging modality of choice for the assessment of valvular heart disease, left and right ventricular function, and pericardial effusions. It is also a cornerstone in the evaluation of HF, as left ventricular ejection fraction (LVEF) is often critical in therapeutic decision-making.
A novel application of ultrasound is Doppler echocardiography, in which the flow and velocity of red blood cells can be visualized. Color Doppler images provide a qualitative assessment of turbulent blood flow through the heart valves, which can be subsequently classified as either regurgitation or stenosis. A continuous wave Doppler provides a quantitative measurement of blood velocity (Fig. e29-12), which can then be used to calculate the pressure gradients across the valves and estimate pulmonary artery pressures. A more recent application of Doppler imaging is measuring myocardial tissue velocities, which can be useful in assessing diastolic function and determining prognosis in chronic HF.
Doppler Echocardiography. Continuous wave Doppler, measuring the velocity of blood (in centimeters/second) across the aortic valve.
Echocardiography can also be used to detect intracardiac shunting. Color Doppler interrogation of the interatrial septum may detect left to right shunts, most commonly as a result of atrial septal defects. Additionally, in a procedure known as a bubble study, saline can be agitated to create microbubbles, which are then injected into an intravenous line. In a structurally normal heart, these bubbles will appear in the right ventricle but are subsequently absorbed in the lungs. However, bubbles that emerge on the left side of the heart suggest the presence of a right to left shunt, such as a patent foramen ovale (PFO).
Advantages of TTE are that it is highly portable, reproducible, and images can be obtained in even the most critically ill patients in the intensive care unit or operating room. Additionally, ultrasound waves do not use ionizing radiation, and intravenous access is not required for most procedures. However, occasionally a TTE is unable to provide satisfactory imaging. For example, in trauma or postoperative patients, the presence of lines and tubes as well as chest wall abnormalities may limit acoustic windows. Additionally, ultrasound waves are reflected by air but degrade as they travel through tissue, which may result in poorer image quality in obese patients. Finally, because of the orientation of the heart within the thoracic cavity, some structures located toward the posterior aspect of the heart are poorly visualized with a TTE. Similarly, TTE may not provide the image clarity necessary to evaluate subtle abnormalities, such as valvular vegetations. Fortunately, if information regarding these latter structures is needed, a transesophageal echocardiogram (TEE) may be performed.
During a TEE, the transducer probe is advanced through the mouth and into the patient’s esophagus where ultrasound waves are transmitted through the posterior aspect of the heart. A TEE is commonly used to evaluate for the presence of aortic dissections, left atrial appendage thrombus prior to cardioversion, and valvular vegetations in patients with bacteremia or suspected endocarditis. Because of the close proximity of these structures to the esophagus (especially the mitral and aortic valves), TEE images are of higher quality than TTE. Consequently, TEE is used extensively in the operating room to ensure the proper functioning of newly implanted heart valves and overall heart function in patients undergoing CABG surgery.
Although a TEE is generally considered to be a safe procedure, it is invasive and therefore carries a small inherent risk.56 Trauma to the esophagus and the vocal cords are rare but known complications of TEE. Contraindications include esophageal pathology that may limit advancement of the probe, untreated esophageal varices, and oropharyngeal or cervical spine conditions that limit neck extension and flexion. Conscious sedation is used for procedural comfort and safety, and appropriate cardiac and respiratory monitoring by specialized personnel are required.
Echocardiography can also be used in conjunction with stress testing to improve its sensitivity and specificity, as ischemia-related changes in systolic or diastolic function may be detected prior to ECG changes or ischemic symptoms. Stress echocardiography can be performed with exercise using a treadmill or bicycle, or pharmacologically, most commonly with dobutamine. Dobutamine is a beta receptor agonist, which induces physiologic changes similar to exercise (eg, increased myocardial contractility and heart rate), leading to increased myocardial oxygen demand. At peak exercise (or peak dobutamine infusion), echocardiogram images are obtained and compared to baseline images. Myocardium that is well-perfused will become hyperkinetic with stress, whereas ischemic segments may appear hypokinetic compared to baseline, and relatively hypokinetic compared to areas receiving adequate blood supply.
Cardiac catheterization (or heart catheterization) is a procedure in which a catheter is inserted percutaneously into a large blood vessel and advanced to the heart, where it is used to obtain diagnostic or evaluative information. Therapeutic interventions may also be performed. Cardiac catheterization can be broadly classified as being left-sided (left heart catheterization or LHC) or right-sided (right heart catheterization or RHC), depending on whether the catheter is inserted into an artery (LHC) or vein (RHC). Selection of LHC versus RHC depends on the information desired. The common indications for each are listed in Table e29-9.57
TABLE e29-9Common Indications for Cardiac Catheterization |Favorite Table|Download (.pdf) TABLE e29-9 Common Indications for Cardiac Catheterization
|Left Heart Catheterization ||Right Heart Catheterization |
Acute coronary syndromes
Cardiomyopathy, with or without symptoms of heart failure
Coronary artery disease, especially if symptomatic and/or high-risk features are present (eg, new systolic dysfunction)
Valvular disease, as a preoperative evaluation prior to surgery or with indeterminate severity
Ventricular arrhythmias, including resuscitated cardiac arrest
Acute decompensated heart failure, with indeterminate volume status, change in clinical condition, and/or to guide therapy (eg, inotropes)
Surveillance endomyocardial biopsy after heart transplantation
Bleeding from the access site is the most common complication of cardiac catheterization and thus coagulopathies and other bleeding diatheses should be corrected prior to the procedure. If radiocontrast dye is to be used, the benefits of cardiac catheterization must be weighed against the risk of worsening preexisting renal dysfunction. Providing adequate hydration and limiting the volume of contrast dye used during the procedure may ameliorate the risk of renal injury.6 Alternative strategies, such as the administration of oral N-acetylcysteine, have not been shown to be effective and are therefore not recommended.58 With the exception of severe anaphylactic reactions, patients with allergies to contrast dye can often undergo cardiac catheterization safely if adequately premedicated prior to the procedure. Regimens for allergy prophylaxis generally consist of a corticosteroid and histamine-receptor blocker (eg, diphenhydramine). The need to provide prophylaxis in patients with shellfish or seafood allergies is a common misconception and is not recommended.58
Other relative contraindications to cardiac catheterization include hemodynamic instability, uncontrolled hypertension, intractable arrhythmias, severe electrolyte derangements, and systemic infections. Because of the invasive nature of the procedure and the risk of complications, cardiac catheterization is typically performed in a procedural unit by a specialized team and may not be available at smaller medical facilities.
Left Heart Catheterization
During LHC, a catheter is inserted percutaneously through an incision in a large peripheral artery, such as the femoral or radial artery, and advanced in a retrograde fashion to the cardiac structure of interest. Access via the radial artery is becoming more common given growing evidence that it is associated with a lower rate of complications.59
The most common indication for LHC is coronary angiography, a procedure in which fluoroscopy (x-ray) is used to diagnose or evaluate ASCVD. During the procedure, a catheter is advanced into the coronary arteries, where contrast dye is injected to visualize the coronary anatomy and detect occlusion of blood flow (Fig. e29-13). A series of images are obtained using multiple views in order to fully characterize defects in blood flow. Areas of stenosis are considered to be significant when the diameter of the arterial lumen has narrowed by ≥70% (≥50% for the left main artery).58
Coronary angiography. Coronary angiography showing complete occlusion of the left anterior descending artery in a patient presenting with ST segment acute coronary syndrome (A), and restoration of blood flow following angioplasty and coronary stenting (B). (Reproduced, with permission, from Crawford MH, ed. Current Diagnosis & Treatment: Cardiology. 5th ed. New York: McGraw-Hill; 2017.).
A limitation of angiography is that it does not provide information regarding the physiologic implications of a coronary lesion, such as whether the degree of stenosis present results in myocardial ischemia. For indeterminate lesions (50%-70% stenosis), fractional flow reserve (FFR) may indicate scenarios in which an intervention may be deferred. During FFR, a coronary vasodilator is administered, and the pressure difference across the area of stenosis is used to calculate flow. An FFR value of ≤0.80 (ie, ≥20% reduction in flow) is generally considered hemodynamically significant and thus warrants revascularization.58
Another limitation of angiography is that it does not provide information on the arterial wall, nor details regarding the morphology of a coronary lesion (eg, plaque volume, area, cross-sectional area). In cases where this information may alter treatment, a specialized catheter can be used to perform intravascular ultrasound (IVUS). Like FFR, IVUS may be used to guide decision-making in patients with indeterminate lesions, but it is less commonly used for this purpose due to the added complexity and expense of the procedure. Instead, IVUS is primarily reserved for the assessment of cardiac allograft vasculopathy (CAV), a special type of CAD found in heart transplant recipients, characterized by diffuse narrowing of the vessel lumen rather than the development of discrete lesions.58
If a significant coronary occlusion is detected, a percutaneous coronary intervention (PCI) may be performed to restore blood flow. Restoration of blood flow following PCI can be graded according to Thrombolysis in Myocardial Infarction (TIMI) Grade Flow on a scale of 0 to 3, where 0 represents no flow and 3 represents normal flow. Blood flow receiving a TIMI 1 score represents partial restoration of flow but incomplete filling of distal vessels, whereas a TIMI 2 score represents delayed flow but complete filling of distal vessels. In instances where a patient would benefit from CABG surgery over PCI, LHC may be terminated once the anatomy has been characterized via angiography. The films obtained during LHC are then used by cardiac surgeons to determine the optimal grafting strategy for restoring blood flow.
A ventriculogram may be optionally obtained during coronary angiography, during which the catheter is advanced across the aortic valve and contrast dye is injected into the left ventricle to characterize chamber size and function. Because of the extra dye volume required for ventriculography, it is usually omitted if the data obtained from the procedure is unlikely to alter management, as the risk of dye-related complications is considerably higher and information regarding LVEF can be obtained from other modalities. Fluoroscopy may also be used to view other cardiac structures (eg, cardiac valves, aorta) but this practice has also been largely replaced by other, less invasive imaging techniques. One notable exception is the evaluation of mechanical valve dysfunction, where fluoroscopy remains the imaging modality of choice.
Right Heart Catheterization
During RHC, a catheter is inserted percutaneously through an incision in a large vein, such as the IJ vein, and advanced to the right side of the heart. Several diagnostic procedures can be performed with RHC, the most common of which is an assessment of cardiac hemodynamics. Reference ranges for common hemodynamic values are provided in Table e29-10. When hemodynamic values are needed in an urgent scenario, RHC can be performed at the bedside; otherwise, it is more commonly performed in the cardiac catheterization lab.
TABLE e29-10Reference Ranges for Common Hemodynamic Valuesa |Favorite Table|Download (.pdf) TABLE e29-10 Reference Ranges for Common Hemodynamic Valuesa
|Hemodynamic Parameter ||Reference Range |
|Heart rate (HR) ||60-110 bpm |
|Cardiac output (CO) ||4-6 L/min (0.07-0.1 L/s) |
|Cardiac index (CI) ||2.8-4.2 L/min/m2 (0.047-0.07 L/s/m2) |
|Right atrial pressure (RAPb ||2-6 mmHg |
|Right ventricular pressure (systolic/diastolic) ||15-30 / 8-15 mmHg |
|Mean pulmonary arterial pressure (mPAP) ||10-20 mmHg |
|Pulmonary arterial pressure (PAP, systolic/diastolic) ||15-30 / 5-15 mmHg |
|Pulmonary vascular resistance (PVR) ||150-250 dynes·sec/cm5 (15-25 Mpa·s/m2) |
|Pulmonary capillary wedge pressure (PCWP) ||6-12 mmHg |
|Left atrial pressure (LAP) ||4-12 mmHg |
|Left ventricular pressure (systolic/diastolic) ||100-140 / 4-12 mmHg |
|Mean arterial pressure (MAP) ||70-100 mmHg |
|Systemic arterial pressure (systolic/diastolic) ||100-120 / 60-80 mmHg |
|Systemic vascular resistance (SVR) ||800-1,200 dynes·sec/cm5 (80-120 Mpa·s/m2) |
Although the routine use of invasive hemodynamic monitoring in patients with ADHF does not improve outcomes over clinical assessment alone, subsets of patients may still benefit from the information it provides, such as those patients whose volume or perfusion status remain unclear despite a thorough clinical evaluation or those whose status does not improve despite appropriate therapy.11,57,60,61 An assessment of cardiac hemodynamics is also essential to determining patient eligibility for advanced therapies, such as mechanical circulatory support and cardiac transplantation, as well as in the diagnosis and evaluation of pulmonary hypertension.57,62
During an assessment of cardiac hemodynamics, a special balloon-tipped catheter known as a pulmonary artery (PA) catheter or Swan-Ganz catheter is advanced through the right atrium, right ventricle, and into a small branch of the PA, where pressures in each of these areas is measured (Fig. e29-14). The catheter can also be used to administer fluids and medications, and a thermistor near the distal end of the catheter measures changes in temperature. The catheter may be inserted to obtain a single set of measurements and then removed, or it may be sutured in at the access site and left inside the patient to guide titration of vasoactive medications.
Right Heart Catheterization. During right heart catheterization, a pulmonary artery catheter is inserted into a large peripheral vein and advanced through the right atrium and right ventricle and into the pulmonary artery. (Illustration by Wikipedia contributor Chikumaya, distributed under a CC BY-SA 3.0 license. https://commons.wikimedia.org/wiki/File:Pulmonary_artery_catheter_english.JPG. Accessed December 14, 2018.).
After being advanced into a small branch of the PA, the balloon at the end of the catheter can be inflated to transiently occlude the artery, producing what is known as the pulmonary capillary wedge pressure (PCWP), also known as the pulmonary artery occlusion pressure. In the absence of significant pulmonary vein stenosis or mitral valve disease, the PCWP provides an estimate of left atrial pressure, which is approximately equal to the left ventricular end-diastolic pressure (LVEDP). The LVEDP is a surrogate measure of cardiac preload, and can be used as an assessment of a patient’s overall volume status.
Other key hemodynamic parameters that can be obtained during RHC are cardiac output (CO) and systemic vascular resistance (SVR). In practice, the CO is often normalized for body surface area and reported as the cardiac index (CI). Estimates of CO can be made using either the Fick or thermodilution methods. According to the Fick principle, CO can be calculated by dividing total oxygen consumption within a unit of time by the arteriovenous oxygen difference. Although the Fick method has historically been considered the gold standard for determining CO, prior methods involved direct measurements of oxygen consumption. Since these are rarely used in contemporary practice, estimations and assumed constants are substituted into the equation, making it less reliable in some patients. For example, the Fick method is less accurate in patients with disorders that affect oxygen diffusion and transport, such as pulmonary disorders or anemia.
An alternative to the Fick equation is the thermodilution method, during which CO can be estimated by measuring a change in blood temperature as it flows between two points. During the thermodilution procedure, a small bolus of cold saline at a known temperature is infused from a proximal port on the PA catheter and the resulting change in blood temperature is measured at the thermistor on the distal end of the catheter. Greater changes in temperature between the two points are indicative of slower blood flow and thus reduced CO. As with the Fick method for determining CO, the thermodilution method is also subject to confounding by coexisting conditions, such as intracardiac shunts or regurgitant blood flow. Consequently, the most appropriate method for determining CO may vary by scenario or clinician preference.
Once known, CO can be combined with the mean arterial pressure (MAP) and central venous pressure (CVP) to calculate SVR, an estimate of the resistance force imparted by the systemic vasculature. Because arterial impedance is often the greatest contributor of resistance against the ejection of blood out of the left ventricle, the SVR is commonly used as a surrogate for cardiac afterload. The use of CO, MAP, and CVP to calculate SVR is depicted in the following equation:
Another common use of RHC is in the diagnosis and evaluation of pulmonary hypertension (PH), which is defined as a mean PA pressure >20 mmHg. If elevated due to excess volume, PA pressures will improve and often normalize following reductions in PCWP. However, if PA pressures remain elevated despite a high-normal PCWP (≤15 mmHg) and PVR is also elevated (>3 Wood units), a diagnosis of pre-capillary PH can be made.62 Further evaluation is then required to distinguish among the various types of pre-capillary PH. In patients with pulmonary arterial hypertension (PAH), a specific type of pre-capillary PH, RHC may be used to determine eligibility for high-dose calcium channel blocker therapy (termed vasoreactivity testing) as well as to guide the titration of disease-modifying therapies. Readers are referred to Chapter 45, “Pulmonary Artery Hypertension,” for a more comprehensive discussion of these topics.
Patients being evaluated for cardiac transplantation must undergo RHC to determine the reversibility of elevated PA pressures. During a “vasodilator challenge,” an agent such as sodium nitroprusside is administered until PA systolic pressures are ≤50 mmHg or PVR is ≤3 Wood units (≤240 dynes∙sec/cm5 [24 Mpa·s/m2]). Patients unable to achieve these parameters without compromising SBP (<90 mmHg) are deemed ineligible for cardiac transplantation and may require a left ventricular assist device.63 Recipients of a heart transplant must also undergo routine RHC for the purposes of endomyocardial biopsy (EMB), a procedure in which samples of myocardial tissue are obtained from the septal wall of the right ventricle and evaluated for the presence of graft rejection. Less commonly, an EMB may be used in nontransplant recipients to diagnose otherwise unexplainable causes of myocardial dysfunction, such as myocarditis.
Finally, an electrophysiology (EP) study may be performed via RHC and/or LHC. During an EP study, impulses generated by an electrode catheter are used to map the conduction pathways of the heart and thus diagnose or assess cardiac arrhythmias. Whether RHC or LHC is the most appropriate route for the study depends on the type of arrhythmia and its location. Some arrhythmias are amenable to ablation, a procedure performed during an EP study in which a scar is made to correct an abnormal conduction pathway. Scars may be made by burning the tissue using radiofrequency current or by freezing (cryoablation).
Chest Radiography (x-ray)
A chest radiograph or x-ray is a noninvasive imaging test that provides visual information about the heart, lungs, and related structures within the thoracic cavity. The primary indication for a chest x-ray is in the differential diagnosis of acute dyspnea (eg, pulmonary edema due to HF vs pneumonia). A chest x-ray is also recommended as part of the initial evaluation of patients suspected as having new or worsening HF even in the absence of dyspnea, as it can provide important information regarding the size and shape of the heart as well as other disorders that may contribute to HF.11 The two standard views for a chest x-ray are the posteroanterior (front) and lateral (side) views.
Compared to an echocardiogram, a chest x-ray is not as helpful for visualizing structures within the heart and is therefore most often used to depict its overall size and shape. Relative heart size can be approximated in the posteroanterior view by the cardiothoracic ratio (CTR), which is the maximal horizontal diameter of the heart divided by the maximal horizontal diameter of the thorax. A CTR <0.5 is generally considered normal whereas higher values indicate cardiac enlargement (ie, cardiomegaly). An elevated CTR may also be present in the setting of a pericardial effusion. Dilation of specific heart chambers can also be visualized with chest x-ray. Left ventricular enlargement is visualized as inferior and lateral displacement of the cardiac apex on posteroanterior view whereas right ventricular enlargement is most optimally viewed as intrusion into the retrosternal space.
Visualization of the lung fields is also important in the evaluation and management of pulmonary edema. Pulmonary edema due to an underlying cardiogenic etiology is most commonly evidenced by bilateral opacities (sometimes referred to as a “bat wing” pattern) (Fig. e29-15). Bilateral pleural effusions are common and cause blunting of the costophrenic angles. Other findings characteristic of pulmonary edema include septal lines (also called Kerley B lines) seen at the bases of the lungs, thickening of the interlobar fissures, and peribronchial cuffing. Serial chest x-rays can be used to assess changes in pulmonary edema over time, complementing other information used in the management of dyspnea.
Chest Radiograph (x-ray) in Acute Decompensated Heart Failure. Left ventricular enlargement can be visualized by displacement of the cardiac apex. Bilateral opacities provide evidence of diffuse intestinal pulmonary edema and Kerley B lines are annotated with black arrows. (Reproduced, with permission, from Elsayes KM, Oldham SAA, eds. Introduction to Diagnostic Radiology. New York: McGraw-Hill; 2014.).