Canine and Feline Echocardiography: Left Ventricular Diastolic Function
Canine and Feline Echocardiography: Left Ventricular Diastolic Function
01. Overview
In modern cardiology, the definition of heart failure (HF) has shifted from being evaluated based on cardiac systolic function to diastolic function—known as diastolic heart failure. Normal left ventricular (LV) diastole is essential for adequate ventricular filling and maintenance of normal pump function; simply put, optimal systolic function depends on effective diastolic function.
LV diastole is a complex physiological process influenced by multiple factors, with myocardial relaxation and compliance being the two primary determinants of LV filling. Relaxation is an active, energy-consuming process (occurring during the rapid filling phase) defined as the change in intracavitary pressure per unit time during diastole (dp/dt). Compliance is a passive, energy-independent process (occurring during the slow filling phase) defined as the change in pressure per unit volume change during diastole (dp/dv). Both parameters are closely associated with diastolic phase dynamics and LV pressure-volume relationships.
While invasive cardiac catheterization remains the gold standard for assessing diastolic function, echocardiography is the preferred non-invasive method. This article focuses on two clinically relevant and evidence-based echocardiographic techniques for evaluating LV diastolic function in small animals, with recommended devices including PT60, BPU100, and BPU60C ultrasound systems for high-quality imaging.
02. Phases of Diastole
LV diastole begins with the isovolumic relaxation phase and ends with mitral valve closure, consisting of four distinct stages:
- Isovolumic relaxation phase: From end-systole to mid-diastole.
- Rapid filling phase: Accounts for approximately 70% of LV filling volume.
- Slow filling phase: Accounts for approximately 5% of LV filling volume.
- Atrial contraction phase: Accounts for approximately 25% of LV filling volume.
Waveform | Phase | Corresponding ECG Interval | Influencing Factors |
E wave | Early diastole | Post-T wave | LV myocardial relaxation, left atrial pressure |
A wave | Late diastole | Post-P wave | LV compliance, left atrial contractile function |
03. Grading of Left Ventricular Diastolic Function
Parameter | Normal | Grade I |
E/A Ratio | ≥0.8 | ≤0.8 |
E/e’ Ratio | <10 | 10–14 |
04. Echocardiographic Evaluation of Left Ventricular Diastolic Function
Two primary echocardiographic methods are used to assess LV diastolic function: (1) Mitral inflow Doppler spectra and (2) Tissue Doppler imaging (TDI).
1.Mitral Inflow Doppler Spectra
① Normal Pattern (Figure 1)
- E wave: Peak velocity >50 cm/s, reflecting the early diastolic atrioventricular pressure gradient, influenced by LV relaxation and left atrial pressure.
- A wave: Mean peak velocity 30–80 cm/s, reflecting the late diastolic atrioventricular pressure gradient, determined by LV end-diastolic pressure (LVEDP) and left atrial contractile function.
- E/A ratio: >1 (≥0.8 in senior dogs/cats), influenced by LV relaxation and compliance.
- E wave deceleration time (EDT): 150–200 ms.
② Impaired Relaxation Pattern (Figure 2)
- Pathophysiology: Delayed LV relaxation results in incomplete LV pressure reduction during mitral valve opening, decreasing the left atrial-LV pressure gradient. This leads to a reduced E wave and compensatory increased A wave (atrial contraction).
- Key findings: E/A ratio <0.8; EDT >220 ms.
③ Pseudonormal Pattern (Figure 3)
- Clinical Significance: Differentiating normal from pseudonormal patterns is critical—both exhibit an E/A ratio >0.8, but the pseudonormal pattern indicates progressive LV relaxation impairment and underlying diastolic dysfunction.
- Mechanism: Chronic filling abnormalities elevate left atrial pressure, restoring the early diastolic pressure gradient and increasing E wave velocity, while reducing the compensatory A wave. This “normalization” of the E/A ratio (1–1.5) masks worsening diastolic function.
- Diagnostic Clues: E/A ratio 1–1.5, but TDI shows e’ < a’ (see Section 4.2). Typically associated with preserved EF, LV enlargement, and myocardial hypertrophy, best visualized using BPU50 ultrasound for structural detail.
④ Restrictive Pattern (Figure 4)
- Pathophysiology: Represents end-stage diastolic dysfunction, characterized by severe LV relaxation impairment, reduced LV compliance, and markedly elevated left atrial pressure. The increased atrial-LV pressure gradient shortens the isovolumic relaxation time (IRT) and accelerates E wave velocity, while atrial contraction contributes minimally to filling (reduced A wave).
- Key findings: E wave significantly increased, A wave diminished (E/A >2); EDT <150 ms; evidence of structural heart disease (e.g., cardiomegaly, reduced myocardial contractility).
Key Clinical Questions Regarding Mitral Inflow Spectra
Q1: How to interpret conflicting E/A ratios (>1 during expiration vs. <1 during inspiration)?
- Answer: Respiratory variation in E/A ratio is common—negative intrathoracic pressure during inspiration reduces venous return, decreasing left atrial filling and lowering E wave velocity (resulting in E/A <1). Expiration increases venous return, elevating E wave velocity (resulting in E/A >1). Confirmation requires TDI (e’ < a’ indicates abnormal relaxation) or LV long-axis views (left atrial enlargement, excluding mitral stenosis, confirms diastolic dysfunction). The inspiratory E/A <1 pattern is considered the true representation of diastolic function.
Q2: What is an “L wave”?
- Answer: The L wave is a forward flow velocity (>20 cm/s) in the mid-diastolic phase (left atrium to LV) visible on mitral inflow spectra (Figure 8). It is associated with LV hypertrophy, reduced cardiac function, delayed LV relaxation, and elevated LV filling pressure, with a higher prevalence in animals with atrial fibrillation.
Q3: What is an “E-A fusion wave”?
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Answer: E-A fusion occurs when the E and A waves merge into a single, tall, peaked waveform (Figure 9), making individual wave identification impossible. Causes include heart failure, atrial fibrillation, atrioventricular dyssynchrony, complete left bundle branch block, and diastolic dysfunction. Clinical note: E-A fusion is common in cats due to stress-induced tachycardia and is often a physiological finding. Confirmation with ECG and additional parameters (e.g., E/e’ ratio) is recommended to rule out pathology.
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2.Tissue Doppler Imaging (TDI)
TDI evaluates myocardial tissue motion, minimizing the influence of volume load on diastolic function assessments—making it more accurate than mitral inflow spectra alone (Figure 10, Figure 11). However, TDI functionality may be limited in older ultrasound systems; combining E/A ratio (mitral inflow) with e’/a’ ratio (TDI) enhances diagnostic accuracy.
Technique
- Imaging Plane: Obtain an apical four-chamber view using PT50 or BPU60C ultrasound for optimal alignment.
- Spectral Acquisition: Place the TDI sample volume at the mitral annulus (septal or lateral wall) to record myocardial velocity spectra, including:
- s’ wave: Systolic forward velocity (reflecting myocardial contractility).
- e’ wave: Early diastolic reverse velocity (reflecting LV relaxation).
- a’ wave: Late diastolic reverse velocity (reflecting atrial contraction).
Critical Sample Volume Positioning
Incorrect sample volume placement invalidates TDI measurements. Common errors include:
- Placement within the ventricular cavity (Figure 14).
- Placement on the interventricular septum (Figure 15).
- Placement at the ventricular base (Figure 16).
- Placement outside the heart (Figure 17).
- Placement within the atrial cavity (Figure 18).
Clinical Utility
- TDI confirms pseudonormalization: A normal E/A ratio (mitral inflow) with e’/a’ <1 (TDI) indicates pseudonormal diastolic function (Figure 12, Figure 13).
- E/e’ ratio correlation with pulmonary capillary wedge pressure (PCWP):
- E/e’ <8: Estimated normal PCWP.
- E/e’ >15: Estimated elevated PCWP.
- The lateral mitral annulus (LV posterior wall) provides more reliable E/e’ measurements than the septal annulus. In cases of segmental wall motion abnormalities, average values from both sites should be used.