Coronary Angiography and Invasive Diagnostics

Introduction

Coronary artery disease (CAD) remains one of the most significant contributors to morbidity and mortality worldwide. Accurate diagnosis and assessment of CAD are crucial for guiding management, selecting appropriate interventions, and improving patient outcomes. While non-invasive imaging modalities such as stress testing, CT angiography, and nuclear imaging provide valuable information, invasive coronary angiography remains the gold standard for visualizing coronary anatomy and detecting significant stenosis.

Beyond angiography, modern catheter-based techniques such as intravascular ultrasound (IVUS), optical coherence tomography (OCT), and fractional flow reserve (FFR) have transformed our ability to assess plaque morphology, vessel dimensions, and the physiological significance of stenoses. Together, these tools enable a comprehensive evaluation of both the structural and functional aspects of CAD.

This article provides an in-depth exploration of coronary angiography and invasive diagnostics, covering indications, intravascular imaging modalities, physiological assessment, and interpretation of angiographic findings.

Indications for Coronary Angiography

1. Diagnostic Indications

Coronary angiography is primarily performed to evaluate the presence, severity, and extent of CAD. It is indicated in several clinical scenarios:

Acute Coronary Syndromes (ACS)
- ST-elevation myocardial infarction (STEMI): Urgent primary percutaneous coronary intervention (PCI) following angiography is the standard of care.
- Non-ST-elevation ACS (NSTEMI/unstable angina): Angiography helps stratify risk and identify lesions requiring revascularization.
Stable Ischemic Heart Disease (SIHD)
- Persistent angina despite optimal medical therapy.
- High-risk findings on non-invasive stress testing.
- Unclear or inconclusive non-invasive results in symptomatic patients.
Sudden Cardiac Arrest or Life-Threatening Arrhythmias
- When ischemia is suspected as a trigger.
Pre-Surgical Evaluation
- Prior to valvular or aortic surgery to rule out concomitant CAD.
Congenital and Structural Heart Disease
- Coronary anomalies, anomalous origins, or fistulas.

2. Therapeutic Indications

Angiography is often coupled with PCI. Indications for revascularization include:

Significant stenosis (≥70% in epicardial vessels, ≥50% in left main coronary artery).
Functionally significant lesions confirmed by FFR or other physiological assessment.
Complex multivessel or left main disease requiring planning for PCI vs. coronary artery bypass grafting (CABG).

3. Contraindications

While there are no absolute contraindications in life-threatening emergencies, relative contraindications include:

Severe renal insufficiency (due to contrast nephropathy risk).
Uncontrolled bleeding or coagulopathy.
Severe allergy to iodinated contrast.
Advanced comorbidities limiting life expectancy.

Intravascular Imaging: IVUS and OCT

Coronary angiography provides a lumenogram, a two-dimensional silhouette of the contrast-filled lumen. However, it cannot fully characterize plaque morphology, vessel wall composition, or subtle dissections. To overcome these limitations, intravascular imaging techniques—particularly IVUS and OCT—are used.

1. Intravascular Ultrasound (IVUS)

Principle: IVUS uses a miniaturized ultrasound transducer mounted on a catheter tip to generate cross-sectional images of coronary arteries.

Applications:

Plaque burden and vessel remodeling: IVUS measures vessel dimensions, plaque area, and remodeling index.
Stent optimization: Ensures proper stent expansion, apposition, and absence of edge dissections.
Assessment of ambiguous lesions: Helps clarify severity when angiography is inconclusive.
Left main disease: Particularly valuable where angiography may underestimate lesion significance.

Advantages of IVUS:

Good tissue penetration (up to 10 mm), allowing visualization of vessel wall and plaque burden.
Useful in guiding complex interventions such as chronic total occlusion (CTO) PCI.

Limitations of IVUS:

Lower spatial resolution (~100–150 μm) compared to OCT.
More difficult to identify fine details like thin fibrous caps.

2. Optical Coherence Tomography (OCT)

Principle: OCT uses near-infrared light to provide high-resolution images of coronary arteries (10–20 μm). It requires contrast flushing to clear blood from the imaging field.

Applications:

Plaque characterization: Differentiates lipid-rich, fibrous, and calcified plaques.
Detection of vulnerable plaques: Identifies thin-cap fibroatheromas (TCFA) prone to rupture.
Stent assessment: OCT is superior for detecting stent malapposition, edge dissections, tissue prolapse, and neoatherosclerosis.
Assessment of intraluminal thrombus: High resolution allows detection of thrombi missed by angiography.

Advantages of OCT:

Unparalleled spatial resolution.
Best tool for assessing stent-related complications.

Limitations of OCT:

Limited depth penetration (~2 mm).
Requires contrast, which may be challenging in patients with renal dysfunction.

3. Comparison of IVUS and OCT

Feature	IVUS	OCT
Resolution	100–150 μm	10–20 μm (much higher)
Penetration depth	~10 mm	~2 mm
Plaque burden assessment	Excellent	Limited
Stent optimization	Very good	Excellent (more detail)
Requirement	Saline/contrast not needed	Requires contrast flush

Both IVUS and OCT are complementary; IVUS excels at evaluating plaque burden and remodeling, while OCT provides detailed microstructural imaging for plaque characterization and stent evaluation.

Fractional Flow Reserve (FFR)

1. Principle

Coronary angiography identifies anatomical stenoses, but not all visually significant lesions cause ischemia. FFR is a physiological assessment tool that measures the pressure difference across a coronary stenosis during maximal hyperemia.

FFR = (Pd / Pa)
- Pd = distal coronary pressure.
- Pa = aortic pressure.
An FFR value ≤0.80 is generally considered indicative of ischemia-producing stenosis.

2. Clinical Applications

Intermediate lesions (40–70% stenosis): FFR determines whether revascularization is necessary.
Multivessel CAD: Helps identify culprit lesions.
Left main stenosis: Useful when angiography is equivocal.
Post-PCI optimization: Confirms physiological improvement after stenting.

3. Advantages

Provides functional significance rather than just anatomical severity.
Reduces unnecessary stenting of non-ischemic lesions.
Improves long-term outcomes when incorporated into PCI decision-making.

4. Alternatives and Advances

Instantaneous wave-free ratio (iFR): Pressure measurement without hyperemia (adenosine-free). Threshold: ≤0.89.
Other indices: Resting Pd/Pa, diastolic pressure ratio (dPR).

FFR and its derivatives are now considered standard of care for evaluating borderline lesions, ensuring evidence-based revascularization.

Interpretation of Angiographic Findings

1. Basics of Angiographic Imaging

Coronary angiography involves selective cannulation of the coronary ostia, injection of iodinated contrast, and fluoroscopic imaging in multiple projections. Interpretation requires understanding:

Vessel anatomy: Left main (LM), left anterior descending (LAD), left circumflex (LCx), and right coronary artery (RCA).
Lesion location: Proximal, mid, distal, ostial, bifurcation.
Lesion morphology: Discrete, tubular, diffuse, eccentric, calcified.
Severity of stenosis: Percent diameter reduction (visual estimate).

2. Classification of Stenosis Severity

Mild: <50% diameter stenosis.
Moderate: 50–69%.
Severe: ≥70% (≥50% for LM).
Total occlusion: 100%.

3. Lesion Characteristics

Calcification: Impacts procedural success and requires adjunctive techniques (rotational atherectomy, lithotripsy).
Thrombus: Suggestive of acute coronary syndrome.
Dissections: Seen after PCI or spontaneously.
Diffuse disease: Long segments with narrowing, challenging for stenting.

4. Pitfalls of Angiographic Interpretation

Subjectivity: Visual estimation often overestimates stenosis.
Two-dimensional limitation: Misses eccentric plaques and vessel remodeling.
Overlap and foreshortening: May obscure lesion severity.
Physiological relevance: Not all angiographic stenoses cause ischemia.

Hence, angiography should ideally be complemented with FFR, IVUS, or OCT for optimal clinical decision-making.

Integration of Invasive Diagnostics into Clinical Practice

The modern approach to CAD assessment integrates multiple modalities:

Coronary angiography: Defines anatomy and stenosis.
IVUS/OCT: Provides plaque and stent details.
FFR/iFR: Establishes physiological relevance.

This anatomical + functional + structural strategy ensures accurate diagnosis and individualized treatment.

Future Directions

Hybrid imaging: Combining OCT and angiography in real-time.
Artificial intelligence (AI): Automated stenosis quantification and risk prediction.
Molecular imaging: Detecting plaque activity and vulnerability beyond structure.
Non-invasive FFR (FFR-CT): Derived from CT angiography, reducing the need for invasive measurement.