Cardiac investigations

Cardiac investigations continue to progress with enhanced use of nuclear cardiology, MRI and CT imaging. Artificial intelligence (AI) is widely applied in cardiac applications, including coronary calcium scoring, coronary CT angiography, fractional flow reserve CT, plaque analysis, left ventricular myocardium analysis, diagnosis of myocardial infarction, prognosis of coronary artery disease, assessment of cardiac function, and diagnosis and prognosis of cardiomyopathy.¹

Echocardiography

Echocardiography is an extension of ultrasound techniques which allows visualisation of cardiac walls, cardiac structures and in some cases blood flow velocity within the heart. It is widely used for diagnosis and follow up of valvular heart disease, as well as cardiomyopathy and atrial fibrillation. Please see the separate Echocardiography article for more details.

Cardiac catheterisation

Cardiac catheterisation can be used for investigative purposes - including measurement of oxygen saturations and intracardiac pressure; passage of investigative instruments; heart biopsies (for instance in cardiomyopathy); and injection of radio-opaque dye - as well as for passage of valvoplasty and angioplasty balloons.

Left heart catheterisation is via the arterial route, while right heart catheterisation is via the venous route. The separate Cardiac Catheterisation article gives full details of the procedure and its indications.

Coronary artery calcium score²

Coronary artery calcium score is carried out via CT scanning and is used to measure the amount of calcium within the coronary arteries, which correlates with the degree of atherosclerosis. Calcium deposition bears no relationship to plaque instability, and that not all atherosclerotic plaques contain calcium.

While the value of coronary artery calcium score in the management of coronary heart disease and primary prevention has been confirmed, the benefit of moderate values of coronary artery calcium score to predict prognosis with subsequent treatment and non-invasive cardiac imaging is still controversial.

More details on coronary artery calcium scores, their uses and limitations can be found in our separate Coronary Calcium Artery Score article.

Exercise tolerance testing³

In the era before widespread availability of nuclear and other imaging for possible coronary artery disease, exercise tolerance testing was widely used, but is now used in conjunction with other investigations or cardiac imaging that have replaced traditional exercise tolerance testing.

Cardiopulmonary exercise testing (CPET) is a maximal exercise test with concomitant gas exchange analysis that provides an integrative and comprehensive assessment of physiological responses to exercise and cardiorespiratory fitness. In contrast to traditional exercise ECG, this technique enables assessment of the cause of dyspnoea and fatigue to precisely differentiate cardiac from pulmonary disorders.

Full details of exercise tolerance testing can be found in the separate article.

Nuclear cardiology⁴

Myocardial perfusion studies using single-photon emission computed tomography (SPECT) have played a crucial role in the management of coronary artery diseases. Myocardial perfusion positron emission tomography (PET) is also considered an important tool for the assessment of myocardial viability and perfusion.

Cardiac MRI has raised expectations in terms of diagnostic accuracy and achieving high-quality images with little or no ionising radiation exposure.⁵ However, there are concerns about the radiation exposure associated with computed tomography (CT).

One review found that SPECT, cardiac magnetic resonance (CMR) and PET all achieve high sensitivity, although a broad range of specificity was seen. SPECT is widely available and most extensively validated. PET achieved the highest diagnostic performance. CMR may provide an alternative without ionising radiation and a similar diagnostic accuracy as PET.⁶

Myocardial perfusion scintigraphy imaging

• Radionuclide myocardial perfusion scintigraphy (MPS) has become established as the main functional cardiac imaging technique for coronary heart disease (CHD).⁷

• MPS with SPECT uses a radio-pharmaceutical that is taken up into heart muscle in proportion to localised blood flow and stays in myocardial cells whilst scanning is performed. Most use either thallium 201 or technetium ^99m in proprietary compounds.

• Guidance released by the National Institute for Health and Care Excellence (NICE) in 2003 recommended that MPS with SPECT should be used in the following circumstances:⁸

• As a first-line diagnostic tool for the exclusion of coronary artery disease in patients for whom exercise stress testing poses problems in interpreting or where sensitivity is poor. This may include women, patients with cardiac conduction defects (eg, left bundle branch block) and for people who have difficulty using a treadmill for whatever reason.

• As part of an investigation strategy in patients with a lower risk of coronary artery disease (based on risk factor calculations) in the prediction of likely future cardiac events, as a less invasive alternative to coronary angiography.

• In patients with established coronary artery disease with persistent symptoms after a myocardial infarction or after a reperfusion intervention.

• Results:⁹
  

  • Infarction causes matched perfusion defects during stress and at rest. Reversible ischaemia shows defects during stress which re-perfuse at rest.

  • Severity, extent and number of reversible defects are a good prognostic indicator. A normal study implies a risk of an adverse cardiac event of less than 0.5% per annum.

  • Less sensitive in multiple small vessel coronary disease - eg, diabetes mellitus.

• Infarct-avid imaging:
  

  • Can be used in the diagnosis of myocardial infarction.

  • Commonly uses technetium^99m stannous pyrophosphate concentrating in damaged myocardial cells for 'hot-spot' scanning.

  • Scans are best performed 24-96 hours after myocardial infarction. Alternatively, cold-spot scanning can be used where non-viable heart muscle does not take up the radio-pharmaceutical.

  • Unfortunately, because of the delay, this method is of no value in identifying patients suitable for thrombolytic therapy.

Cardiac single photon emission computerised tomography⁷

• Cardiac SPECT is widely used and non-invasive nuclear imaging for investigating CHD.

• SPECT is appropriate for all aspects of detecting and managing CHD, including diagnosis, risk assessment and stratification, assessment of myocardial viability, and evaluation of left ventricular function.

• Hybrid images combining the functional imaging of SPECT and CT coronary angiography enable improved diagnosis, risk stratification, and treatment planning for patients with suspected coronary artery disease.

Radionuclide ventriculography - multiple gated acquisition scans¹⁰

• Scintigraphy can be used to estimate ventricular ejection fraction as an indicator of left and right ventricular function.

• In first-pass method, technetium ^99m pertechnetate is injected as an IV bolus and its passage through the heart recorded every second. The change in radioactivity recorded with time is related to the ejection fraction.

• With the gated cardiac blood pool method, the patient's own red blood cells are labelled with technetium ^99m pertechnetate. Regional wall motion studies can also be performed with this method.

• Indications:
  

  • Can assess both left ventricular and right ventricular function after myocardial infarction.

  • Left ventricular ejection fraction measurement.

  • Monitor anthracycline cardiotoxicity.

• Uses:
  

  • Monitor treatment response in cardiac failure and cardiomyopathy.

  • Useful for serial measurements during anthracycline therapy.

  • Reliable in unechogenic subjects. Otherwise the procedure carries a high radiation dose and echocardiography is preferable for most patients.

  • Cardiac dysrhythmias may interfere with results.

  • One study found that this method contributed to a significant improvement in risk stratification and secondary prevention strategy in patients with known coronary artery disease.¹¹

Magnetic resonance imaging¹²

• Cardiac magnetic resonance imaging has evolved to become an indispensable tool in cardiology. It is a non-invasive technique that enables objective and functional assessment of myocardial tissue. It avoids the use of potentially nephrotoxic contrast agents or radiation.

• It has an important part in diagnosing congenital heart disease as well as disorders of the pericardium, cardiac tumours, atrial or ventricular thrombus, pericardial thickening, myocardial hypertrophy and valvular disease.

• Cardiac MRI is emerging as an important imaging tool in the assessment of heart failure with preserved ejection fraction.¹³

• It may also be used to image the coronary vessels and disease of the aorta, including aortic dissection.

• Stress cardiac MRI is a non-invasive option for the diagnosis of coronary artery disease. The advantages of cardiac MRI are that it does not pose the radiation burden associated with SPECT, can also assess left and right ventricular dimensions, viability, and cardiac mass, and may also avoid the need for invasive diagnostic coronary angiography in patients with intermediate risk factors for coronary artery disease.

• It is contra-indicated for many prosthetic valves and for patients with cardiac pacemakers.

• The slowness of scanners available at most centres limits its usefulness but the introduction of new faster techniques will allow breath-hold and real time scans.

Computerised tomography

• Cardiac CT has several advantages over cardiac MRI. It is much faster and can accommodate patients with implanted devices. It is the imaging modality of choice for the assessment of vascular rings or slings.¹⁴

• It is mainly used to demonstrate aortic dissection, pericardial thickening and fluid, cardiac tumours and coronary calcification. CT with IV contrast is the most reliable and practical technique for the investigation of aortic dissection.

• Multidetector CT angiography (CTA) has become a widely accepted examination for non-invasive evaluation of the heart and coronary arteries.¹⁵

• NICE recommends an immediate computerised tomography pulmonary angiogram (CTPA) for patients with suspected pulmonary embolism.¹⁶