Read the German translation: Konventionelle und Doppler Echokardiographie für die Diagnose des kongestiven Herzversagens

Introduction

Left-sided congestive heart failure (l-CHF) is a common and often fatal clinical syndrome in dogs and cats characterized by cardiac dysfunction, neurohormonal activation, sodium and water retention, and elevation of left ventricular (LV) filling pressure (LVFP). It occurs most often secondary to degenerative mitral valve disease (MVD) and dilated cardiomyopathy (DCM) in dogs and hypertrophic cardiomyopathy (HCM) in cats. Early recognition of CHF or compensated heart disease with elevated filling pressure (FP) is of clinical importance. CHF can be suspected by clinical signs including lethargy, exercise intolerance, and weakness and more specifically tachypnea, labored breathing, pulmonary crackles, jugular venous distension, and an S3 gallop although reliability of such signs may be limited. Thoracic radiography is the most commonly applied method for the diagnosis of CHF and is considered the clinical "gold standard" in small animals. However, clinical signs may be confused by concurrent disease. Moreover, radiography is of unspecified sensitivity and specificity, especially in the setting of combined heart and lung disease, and may suffer from considerable observer variation. In addition, thoracic radiography may add stress to already compromised patients to the point that this technique may not be recommended in the initial evaluation of all patients with dyspnea. Finally, sedation or even anesthesia of dogs and cats prior to radiography is generally required in some countries making the repeated acquisition of chest films in hemodynamically unstable patients challenging. Therefore, a simple but quantifiable noninvasive method that predicts volume status and elevated FP could not only refine the diagnosis, but also promote the early recognition of CHF, advance optimal medical management, and facilitate therapeutic monitoring.

The development of cardiogenic pulmonary edema is predicted largely by the magnitude of volume overload and resulting increase in FP. The recent introduction of novel Doppler echocardiographic (DE) techniques has sparked considerable interest in the noninvasive prediction of FP using DE. One variable, the ratio between peak early transmitral flow velocity (Peak E) to peak early tissue Doppler mitral annulus velocity (Peak Ea; E:Ea) has gained the most attention in the prediction of LVFP in dogs and people but has, to the authors knowledge, not yet been sufficiently evaluated in cats with heart disease. Moreover, previous validation studies in experimental dogs performed by the authors challenged the use of E:Ea as a reliable indicator of FP. Instead, variables such as the isovolumic relaxation time (IVRT) and the ratio between Peak E to IVRT (E:IVRT) have outperformed E:Ea and other commonly-used DE variables in the prediction of elevated FP. In a recently completed clinical study in dogs with naturally acquired MVD and DCM, the latter findings were confirmed. The following paragraphs will summarize the diagnostic utility of commonly used DE variables of LVFP in the prediction of CHF in dogs and cats.

Conventional Echocardiographic Indices of CHF

Conventional echo indices such as chamber dimension, shortening fraction (SF), wall thickness, or EPSS have limited value in the assessment of l-CHF. Although LA size is most often considerably increased in dogs and cats with CHF, exceptions exist and the specificity of such finding is rather poor. The latter is particularly true in dogs with chronic MVD. There is no general relationship between SF, wall thickness, or EPSS and CHF status.

Doppler Echocardiographic Variables of CHF

Transmitral Flow

Traditional indices used to estimate FP include, but are not limited to, peak E velocity, E:A ratio, DT_E, and class of LV diastolic function based on E:A (Class 1: Normal filling; Class 2: Relaxation-delay pattern; Class 3: Pseudonormal filling; and Class 4: Restrictive filling). However, in addition to FP other major determinants of diastolic filling include LV relaxation, LV suction and untwisting, LV compliance, LA function, filling volume, and heart rate and rhythm. Therefore, the accuracy of transmitral flow variables in the prediction of FP is limited in situations where 1) LV ejection fraction is preserved, 2) filling volume is severely increased as observed secondary to advanced mitral regurgitation ("volume overload-dominant effect on LV filling"), 3) there is a "relaxation delay-dominant effect" on LV filling as seen with myocardial ischemia or severe concentric LV hypertrophy (e.g., in cats HCM), or 4) Doppler filling flow waves are fused or absent (e.g., in animals with sinus tachycardia, atrial fibrillation, or severe LA dysfunction). Recent studies done by the authors in healthy anesthetized dogs and cats, in experimental dogs with rapid pacing-induced heart failure, in dogs with MVD or DCM, and in cats with HCM revealed only modest correlations between LVFP or CHF status and single DE variables of transmitral flow. However, Class of LV diastolic function using E:A and E:Ea in combination was superior to most other variables of LV filling in the prediction of l-CHF in dogs and cats with heart disease.

Isovolumic Relaxation Time (IVRT)

The IVRT as an index of relaxation has been shown to be linearly related to tau, the time constant of LV isovolumic relaxation in dogs and cats but is also influenced by a multitude of other factors including preload, afterload, heart rate, rhythm, and age. Therefore, IVRT will represent the net effect of many determinants of which relaxation is only one. Whereas a mild elevation of FP in dogs shortens tau (makes relaxation better) but is not associated with shortening of IVRT, moderate and severe elevation of FP prolongs tau (makes relaxation worse) but shortens IVRT in a linear manner. Shortened IVRT is by definition an integral part of restrictive LV filling, a transmitral flow pattern considered specific for advanced diastolic dysfunction, high LVFP, and CHF. That is, high FP may minimize the effect of relaxation on IVRT turning it into a more specific indicator of FP. This is particularly evident in dogs with DCM and MVD. In contrast, cats with moderate and severe HCM may still have prolonged IVRT despite a significantly elevated FP due to the relaxation-delay dominant influence on LV filling commonly observed with hypertrophied, stiff ventricles. However, to the author's experience, most cats with HCM and l-CHF have either low normal (40-50 ms) or shortened (< 40 ms) IVRT.

Pulmonary Venous Flow (PVF)

When FP is elevated, operating chamber compliance of the LV is reduced, and there will be an increased pressure build-up during late diastole given that LA function is not compromised. Thus, a greater force will retard the flow in the pulmonary veins increasing the duration of retrograde flow (AR wave) from the LA into the pulmonary veins and reducing the duration of antegrade flow (A wave) from the LA into the LV. The ratio between both variables (Aduration:ARduration; Adur:ARdur) has been used to identify elevated FP in dogs and cats. In a recent study on canine DCM performed by the author, the Adur:ARdur was found to be one of the single best DE predictors of l-CHF. However, the practical use of Adur:ARdur is hampered somewhat by the fact that it cannot be used if transmitral flow waves are overlapped or if A or AR waves are missing. In addition, acquisition of high-quality flow recordings can be challenging in some dogs. Due to the ease, high quality, and good repeatability of recordings, interpretation of PVF signals seems to be of particular importance in cats with LV diastolic dysfunction.

Combined Indices

The rationale behind the use of combined indices such as E:IVRT or E:Ea is to "correct" for the effect of relaxation on a variable that is largely dependent on FP, but also influenced by relaxation. By combining peak E, a variable that is determined mainly by the FP and relaxation with a variable that is more dependent on relaxation, the effect of changes in relaxation on Peak E can be minimized. In a series of previous studies in experimental dogs, dogs with rapid-pacing induced CHF, and dogs with naturally-acquired MVD and DCM, E:IVRT consistently outperformed other DE variables of FP and can, therefore, be recommended for clinical use. The diagnostic value of E:IVRT in cats with HCM needs yet to be determined. The E:Ea ratio has gained considerable interest in people in the prediction of elevated FP with E:Ea < 8 indicating normal FP and E:Ea > 15 indicating elevated FP in the majority of patients. However, studies from our laboratory indicated that only in dogs with DCM but not with MVD E:Ea may be useful to predict the presence or absence of l-CHF. The strong preload-dependency of Ea in hearts with preserved systolic function, as found often in dogs with compensated MVD, may limit the use of E:Ea in the prediction of FP under such circumstances. In contrast, Class of LV diastolic function using both E:A and E:Ea was clinically useful in the prediction of l-CHF in dogs with MVD and DCM and cats with HCM.

Tricuspid Regurgitation (TR) Velocity

In the absence of hemodynamically-relevant pulmonary vascular disease peak TR velocity correlates closely to pulmonary vascular resistance and pulmonary capillary wedge pressure as long as RV systolic function is not reduced. Elevated TR velocity may, therefore, indicate elevated FP. Although increased TR velocity (and thus elevated PA pressure) is observed in many dogs with l-CHF (in > 75%), this variable should not be used in isolation in the diagnosis of CHF. However, a normal TR velocity in a dog with tachypnea makes l-CHF less likely.

Diagnostic cut-off's of echo variables suggestive of elevated LV filling pressure or CHF in dogs and cats.

a. Results from a prospective study in dogs with MVD (n=45) and DCM (n=18) and b. Results from a retrospective study in cats with HCM (n=392) recently performed by the author. PSN, pseudonormal. N.U., diagnostically not useful. Please notice that cut-offs reported represent a tradeoff between sensitivity and specificity of the test and can, therefore, not be used with 100% diagnostic accuracy in some animals. Combination of multiple variables in the assessment of CHF status is recommended by the author.

References

References are available upon request.