3.3 Diagnostic Imaging, ECG, and Device Interrogation

1. Transthoracic Echocardiography: Systolic Function, Remodeling, and Hemodynamics

Transthoracic Echocardiography (TTE) is the premier non-invasive diagnostic imaging modality in heart failure, mandatory for initial evaluation and clinical restaging.

Left Ventricular Ejection Fraction (LVEF) Assessment

The recommended standard for quantifying LVEF is the biplane Simpson's method of discs in apical 4-chamber and 2-chamber views. By contouring the endocardial border at end-diastole and end-systole, the method calculates end-diastolic volume (EDV) and end-systolic volume (ESV) without assuming geometric sphericity:

$\text{LVEF} = \frac{\text{LVEDV} - \text{LVESV}}{\text{LVEDV}} \times 100\%$

This defines the four core heart failure phenotypes:

• HFrEF (Heart Failure with Reduced EF): LVEF ≤ 40%
• HFmrEF (Heart Failure with Mildly Reduced EF): LVEF 41% to 49%
• HFpEF (Heart Failure with Preserved EF): LVEF ≥ 50%
• HFimpEF (Heart Failure with Improved EF): Previous LVEF ≤ 40% with a follow-up measurement > 40% (the 2021 Universal Definition also requires a ≥ 10-point increase)

Other Ways LVEF Is Measured

The outline expects you to recognize ejection fraction reported from several tests:

• Nuclear imaging: Gated SPECT or a MUGA (radionuclide ventriculography) scan gives a reproducible, count-based LVEF that is useful for serial monitoring, such as during cardiotoxic chemotherapy.
• Cardiac MRI: The reference standard for volumes and LVEF; late gadolinium enhancement also shows scar, infiltration (amyloidosis, sarcoidosis), and myocarditis.
• Cardiac CT: Can estimate LVEF from ECG-gated images while it evaluates coronary anatomy.
• Left ventriculography during cardiac catheterization: Contrast injected into the left ventricle provides an LVEF estimate at the time of coronary angiography.

Values differ modestly between modalities, so trend LVEF with the same test when possible before relabeling a phenotype.

Ventricular Dimensions and Geometric Remodeling

Echocardiography distinguishes adaptive and maladaptive architectural remodeling by assessing left ventricular end-diastolic diameter (LVEDD), wall thickness, and relative wall thickness (RWT):

$\text{Relative Wall Thickness (RWT)} = \frac{2 \times \text{Posterior Wall Thickness (PWT)}}{\text{Left Ventricular End-Diastolic Diameter (LVEDD)}}$

• Concentric Remodeling: Normal LV mass index with elevated RWT (> 0.42).
• Concentric Hypertrophy: Increased LV mass index with elevated RWT (> 0.42); classic hallmark of chronic systemic hypertension and aortic stenosis.
• Eccentric Hypertrophy: Increased LV mass index with normal or decreased RWT (≤ 0.42); characterized by chamber dilation out of proportion to wall thickness, typical of ischemic cardiomyopathy, mitral regurgitation, and dilated cardiomyopathy.

Right Ventricular (RV) Size and Systolic Performance

Because the thin-walled right ventricle is sensitive to afterload changes induced by left heart congestion, RV dysfunction is an independent predictor of cardiogenic shock and death. Key echocardiographic RV parameters include:

1. Tricuspid Annular Plane Systolic Excursion (TAPSE): M-mode measurement of the longitudinal excursion of the lateral tricuspid annulus toward the apex. TAPSE < 17 mm defines RV systolic dysfunction.
2. RV Tissue Doppler Systolic Velocity (S'): Peak systolic velocity of the lateral tricuspid annulus. S' < 9.5 cm/s indicates impaired RV contractility.
3. Fractional Area Change (FAC): Percentage change in RV area from diastole to systole. FAC < 35% denotes systolic failure.

Pulmonary Artery Systolic Pressure (PASP)

PASP is estimated non-invasively using continuous-wave Doppler interrogation of the tricuspid regurgitant (TR) jet velocity and applying the modified Bernoulli equation:

$\text{PASP} = 4 \times (V_{\text{TR}})^2 + \text{Estimated Right Atrial Pressure (RAP)}$

Right atrial pressure is derived from the inferior vena cava (IVC) diameter and its percentage collapse during normal inspiration:

• IVC ≤ 2.1 cm with > 50% inspiratory collapse: Normal RAP (~3 mmHg).
• IVC > 2.1 cm with < 50% inspiratory collapse: Severely elevated RAP (~15 mmHg).
• Intermediate scenarios: RAP ~8 mmHg.

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2. Diastolic Function and Intracardiac Filling Pressures

Elevated left ventricular filling pressure is the common physiological endpoint of symptomatic heart failure regardless of LVEF.

DIASTOLIC PRESSURE INTERROGATION:

1. Mitral Inflow Insonation (Pulsed-Wave Doppler at Leaflet Tips):
   - E Wave = Early rapid diastolic filling velocity
   - A Wave = Late diastolic filling velocity from atrial contraction
   - E/A Ratio normal: 0.8 to 1.5; Restrictive (Elevated LAP): > 2.0

2. Tissue Doppler Imaging (TDI at Mitral Annulus):
   - e' Velocity = Early diastolic annular tissue relaxation speed
   - Septal e' < 7 cm/s or Lateral e' < 10 cm/s indicates impaired myocyte relaxation

3. The E/e' Ratio (Hemodynamic Surrogate for PCWP):
   - E / e' Ratio < 8  --> Normal LV Filling Pressures (PCWP < 12 mmHg)
   - E / e' Ratio 8-14 --> Indeterminate / Gray Zone
   - E / e' Ratio > 14 --> ELEVATED LV FILLING PRESSURES (PCWP > 15 mmHg)

4. Structural Correlates of Chronic Diastolic Stress:
   - Left Atrial Volume Index (LAVI) > 34 mL/m²
   - Peak Tricuspid Regurgitant Velocity > 2.8 m/s

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3. Chest Radiography: The Hydrostatic Congestion Continuum

In acute decompensation, chest radiography (CXR) visualizes the anatomical progression of hydrostatic pulmonary congestion driven by rising pulmonary capillary wedge pressure (PCWP).

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4. 12-Lead Electrocardiography: Conduction, Ischemia, and Dyssynchrony

Every heart failure patient requires a baseline 12-lead ECG, repeated with each change in clinical status.

QRS Duration and Conduction Abnormalities

Normal ventricular depolarization occurs in < 120 ms. Prolongation of the QRS complex (≥ 120 ms) reflects intraventricular conduction delay and abnormal electrical activation, causing mechanical dyssynchrony between the interventricular septum and LV free wall.

Left Bundle Branch Block (LBBB) and CRT Response

• ECG Criteria for LBBB:
  1. QRS duration ≥ 120 ms (complete bundle branch block).
  2. Broad, notched or slurred R waves in lateral leads (I, aVL, V₅, V₆) with absence of Q waves.
  3. Wide, deep rS or QS complexes in right precordial leads (V₁, V₂).
  4. ST-T wave vector discordant with the primary QRS deflection.
• Predicting Cardiac Resynchronization Therapy (CRT) Benefit: Large randomized clinical trials (MADIT-CRT, RAFT, COMPANION) established that the magnitude of CRT benefit correlates directly with QRS morphology and width:
  • Class 1 Indication: Patients with LBBB morphology and QRS duration ≥ 150 ms in sinus rhythm with LVEF ≤ 35% and NYHA class II, III, or ambulatory class IV symptoms on optimal GDMT experience the most dramatic reverse remodeling, symptom reduction, and survival extension.
  • Class 2a/2b: Non-LBBB morphology (e.g., Right Bundle Branch Block or nonspecific IVCD) or QRS between 120 and 149 ms derive significantly less benefit.

Ischemic and Hypertrophic Patterns

• Pathologic Q Waves: Width ≥ 0.04 seconds (1 small box) and depth > 25% of following R wave amplitude in ≥ 2 contiguous leads, indicating prior transmural infarction and myocardial scar.
• Left Ventricular Hypertrophy (LVH): Evaluated via the Sokolow-Lyon index (S(V1) + R(V5/V6) ≥ 35 mm) or Cornell voltage criteria (R(aVL) + S(V3) > 28 mm in men, > 20 mm in women).
• Arrhythmias: Atrial fibrillation (loss of organized P waves with irregularly irregular ventricular response) triggers acute decompensation by abolishing atrial contribution to ventricular filling ("atrial kick," which provides 20–30% of cardiac output in stiff ventricles) and inducing tachycardia-mediated cardiomyopathy.

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Cardiac Catheterization: Left and Right Heart

• Left heart catheterization with coronary angiography identifies obstructive coronary artery disease as the cause of new cardiomyopathy and guides revascularization decisions by the heart team. It also measures left ventricular end-diastolic pressure (LVEDP).
• Right heart catheterization measures right atrial, pulmonary artery, and wedge pressures, cardiac output, and pulmonary vascular resistance (see section 2.3). It is used when volume or perfusion status is unclear, for cardiogenic shock, and in advanced therapy or transplant evaluation.
• Nursing priorities after catheterization: Check the access site and distal pulses, watch for bleeding or hematoma, monitor renal function after contrast, and review whether diuretics or metformin were held.

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5. Cardiac Device Interrogation and Ambulatory Sensor Monitoring

Remote monitoring and device interrogation provide objective, continuous physiological data that precede clinical symptoms.

Remote Monitoring and Biventricular Pacing Percentage

For patients with CRT pacemakers (CRT-P) or CRT defibrillators (CRT-D), maintaining a high biventricular (BiV) pacing percentage is mandatory:

$\text{BiV Pacing Target} > 98\% \quad (\text{Ideally } > 99\%)$

Outcomes worsen progressively as the percentage falls below about 98%. Common culprits for lost BiV pacing include:

• Atrial fibrillation with rapid ventricular response (native conduction outpaces the paced ventricular cycle).
• Frequent premature ventricular contractions (PVCs) interrupting scheduled BiV pacing.
• Suboptimal programmed atrioventricular (AV) or interventricular (VV) delay intervals.
• Lead dislodgement, diaphragmatic stimulation, or elevated pacing thresholds leading to loss of capture.

Nursing Intervention: When BiV pacing falls below 98%, the priority is evaluating for atrial arrhythmias, optimizing AV nodal blocking agents (beta-blockers, digoxin), considering catheter ablation of the AV node, or reprogramming pacing parameters.

Thoracic Impedance (The OptiVol Fluid Index)

Implantable devices measure electrical impedance across the lead in the right ventricle to the pulse generator in the pectoral pocket. Electrical current travels more easily through fluid than through aerated lung tissue:

$\text{Pulmonary Interstitial Fluid Accumulation} \implies \text{Decreased Intrathoracic Electrical Impedance}$

When daily impedance drops below the patient's moving baseline, the device accumulates the difference into a fluid index (e.g., OptiVol 2.0). When this index crosses a programmable threshold (typically 60 to 80 Ω-days), an alert is triggered. Importantly, thoracic impedance alerts precede overt clinical weight gain and shortness of breath by 10 to 21 days, providing an early window for outpatient diuretic adjustment.

Ventricular Tachyarrhythmia Burden

Interrogation quantifies episodes of non-sustained ventricular tachycardia (NSVT), sustained VT, and ventricular fibrillation (VF), as well as delivered therapies (anti-tachycardia pacing [ATP] and high-energy defibrillation shocks). Frequent shocks ("ICD storm") or clustering NSVT indicates worsening myocardial ischemia, electrolyte derangements (hypokalemia, hypomagnesemia), or worsening hemodynamic decompensation.

Loop Recorders and Wearable Cardioverter-Defibrillators

• Implantable loop recorder (ILR): A small subcutaneous monitor that records rhythm for about 3 years. Reports flag atrial fibrillation burden, pauses, and ventricular arrhythmias. It cannot pace or shock, so a recorded pause or VT needs prompt review.
• Wearable cardioverter-defibrillator (WCD): A vest worn during a temporary high-risk period, such as the waiting time before an ICD decision. Downloads show wear time (daily use under about 20 hours limits protection), detected arrhythmias, and any shocks. Nurses reinforce continuous wear and explain the alarms and response buttons.

Wireless Pulmonary Artery Pressure Monitoring (CardioMEMS)

The CardioMEMS sensor is a miniaturized, battery-free, wireless radiofrequency sensor permanently deployed in a distal branch of the descending pulmonary artery via right heart catheterization. The patient takes daily transmissions from home using a sensor-embedded pillow.

• Clinical Parameter: Measures real-time pulmonary artery systolic, mean, and pulmonary artery diastolic pressure (PADP).
• Physiological Basis: PADP closely approximates left atrial pressure and pulmonary capillary wedge pressure (a commonly used individualized target range: 8 to 20 mmHg).
• Decompensation Timeline: PADP rises 2 to 4 weeks prior to any change in daily patient weight or clinical symptoms. In the landmark CHAMPION trial, proactive, clinician-directed titration of diuretics and vasodilators guided by daily CardioMEMS transmissions reduced heart failure hospitalizations by 37% compared to standard symptom-guided management.

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6. Clinical Application Scenario, Diagnostic Traps, and Board Review

Clinical Case Scenario

A 66-year-old male with non-ischemic cardiomyopathy, LVEF 25%, and a CRT-D placed 18 months ago presents to the outpatient clinic with mild exertional dyspnea. Device interrogation reveals: Biventricular pacing percentage has fallen from 99.2% to 84.1%; OptiVol fluid index is 78 Ω-days (threshold 60 Ω-days); and the device log displays 4,200 episodes of atrial tachycardia/atrial fibrillation over the past 3 weeks with a mean ventricular rate of 118 bpm.

Action Plan: The drop in BiV pacing to 84.1% is directly caused by rapid atrial fibrillation competing with and inhibiting device pacing. The elevated OptiVol index indicates subclinical pulmonary fluid accumulation. The nurse promptly coordinates medication adjustment: uptitrating beta-blocker therapy to achieve resting heart rate < 80 bpm (or initiating digoxin), temporarily increasing oral loop diuretic for 5 days, and scheduling an electrophysiology review for potential AF rhythm control or AV junction ablation. Within two weeks, resting heart rate is controlled at 72 bpm, BiV pacing recovers to 99.4%, and OptiVol normalizes without hospitalization.

CHFN Diagnostic Pearls and Exam Traps

• The E/e' Ratio Cutoff: Remember that E/e' > 14 reliably indicates elevated LV filling pressures (PCWP > 15 mmHg). An E/e' < 8 indicates normal filling pressures.
• The TAPSE Threshold: A TAPSE < 17 mm indicates right ventricular failure. Do not confuse it with left ventricular parameters.
• The CRT Response Predictor: The highest-yield candidate for CRT is LBBB with QRS ≥ 150 ms in sinus rhythm. Non-LBBB morphology is far less responsive.
• The 98% BiV Pacing Imperative: Any question citing biventricular pacing below 98% indicates suboptimal therapy requiring aggressive rate control or device reprogramming.
• The Early Warning Sequence: Pulmonary artery pressures (CardioMEMS) and thoracic impedance (OptiVol) rise 2–3 weeks before weight gain and overt dyspnea occur. Daily weights alone catch decompensation late!