2: Evaluation of myocardial ischaemia

Published on 02/03/2015 by admin

Filed under Internal Medicine

Last modified 22/04/2025

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 2603 times

Topic 2 Evaluation of myocardial ischaemia

Topic Contents

Markers of myocardial injury and infarction 6
Myocardial territories supplied by coronary arteries 8
The 17 segment model 9

Regional assessment of myocardial perfusion or wall motion 9
Stress tests 10

Stress protocols 10
Exercise ECG stress test 10
Nuclear imaging 11
Stress echocardiography 13
Cardiac magnetic resonance imaging 14
Coronary calcification imaging 16
Diagnostic adjuncts in angiography and percutaneous intervention 16

Pressure wire 17
Intravascular ultrasound 18

Markers of myocardial injury and infarction

A universal classification system for myocardial infarction (MI) was published in 2007.

It defines five types of MI:

1. Spontaneous MI due to a primary coronary event, e.g. plaque erosion, rupture, fissuring or dissection.
2. MI secondary to ischaemia due to increased oxygen demand or decreased supply.
3. Sudden unexpected cardiac death with pathological evidence of coronary artery thrombus.
4. MI associated with coronary stenting.

a. MI post percutaneous coronary intervention.
b. MI secondary to coronary stent thrombosis.

5. MI associated with coronary artery bypass surgery.

Myocardial damage results in release of proteins into the circulation that can be detected with laboratory assays. These include troponins, myoglobin and CK-MB. Cardiac troponins are currently the preferred biomarkers as their detection in blood is highly specific and sensitive for myocardial damage.

CK-MB is an isoenzyme of creatine kinase, which is present in skeletal and myocardial muscle. An elevated level is usually detectable within 4–6 h of myocardial injury. Reference range is 0–5 ng/mL.

Cardiac troponins are myocardial contractile proteins. An increased value for cardiac troponin is defined as a measurement exceeding the 99th percentile of a normal reference population upper reference limit, measured with a coefficient of variation <10%.

Cardiac troponin T (cTnT) assays have a relatively consistent sensitivity with a cutoff (including 10% coefficient of variation) of 0.03 μg/L. The lowest level of detectability is 0.01 μg/L. cTnT is expressed by skeletal muscle in patients with chronic renal failure, and therefore cTnT measurements during acute presentations must be compared to baseline values in this patient cohort.

Cardiac troponin I (cTnI) assays are more variable and therefore reference to local laboratory assay ranges and coefficient of variation is appropriate. As a guide, a serum cTnI value of >0.5 ng/ml is evidence of acute myocardial injury with significant prognostic implications. Serum cTnI levels <0.01 are normal. cTnI values between 0.01–0.04 ng/ml may reflect myocyte necrosis, but depend on specific sensitivities and variability of the local assay. cTnI values between 0.04 and 0.5 ng/ml are suggestive of acute myocardial injury, and need to be placed in clinical context.

Troponin values may remain elevated for 7–14 days following the onset of infarction.

Myocardial injury and troponin elevation can occur without coronary artery disease. Examples include:

• Acute decompensated heart failure.
• Tachyarrhythmias.
• Myocarditis.
• Cardiac contusion.
• DC cardioversion.

Troponin levels may also be elevated in noncardiac conditions such as in acute pulmonary embolism, renal failure, sepsis and following brain injury or burns.

Myocardial territories supplied by coronary arteries (Figure 2.1)

image

Figure 2.1 Left ventricular coronary artery territories on standard 2D echo views (16 segment).

The 17 segment model (see Figure 2.2)

Regional assessment of myocardial perfusion or wall motion (Figure 2.2)

• The left ventricle is divided into three sections (basal, mid, apical) in its short axis.
• The basal and mid segments are each divided into six segments.
• The apical segment is divided into four segments.
• The true apex (no cavity) is a single segment.
image

Figure 2.2 Recommended 17 segment model for left ventricular cardiac tomographic imaging.

Results can be placed into a polar plot as below (Figure 2.3).

image

Figure 2.3 Polar plot display for the 17 segment model with coronary artery territory and recommended nomenclature.

image

Stress tests

Stress protocols

Several modalities can be used to noninvasively assess coronary artery disease. Many techniques induce a form of cardiac stress to enhance detection of physiologically significant coronary artery disease, and therefore provide functional information which complements anatomic methods of assessment such as coronary angiography.This may be achieved with exercise (typically treadmill or bicycle) or by pharmacological methods or a combination of both.

Exercise ECG stress test

This test can provide diagnostic and prognostic information in patients with coronary artery disease.

Various standardized protocols are available. The Bruce Protocol is commonly used (Table 2.1).

• 3 minutes in each stage.
• Continuous ECG monitoring.
• Blood pressure assessment at each stage.

Table 2.1 Bruce exercise protocol with estimated metabolic equivalents

image

Target heart rate:

• 220-age.
• 85% target heart rate is deemed a satisfactory response.

Workload is measured by metabolic equivalents (METs) and reflects body oxygen consumption (VO2). Resting VO2 is approximately 3.5 ml/kg/min which is equivalent to 1 MET.

Indicators of ischaemia:

• ST depression (planar or downsloping) ≥ 1mm relative to isoelectric line, 80 ms after the J point*.
• ST elevation.
• Increase in QRS voltage.
• Failure of BP to rise.
• Ventricular arrhythmias.

Advantages:

• Low cost.
• Widely available.

Limitations

• Lower sensitivity in comparison with other stress imaging techniques.
• Does not localise disease distribution.
• Interpretation is limited in a number of conditions including resting ST- abnormalities, left ventricular hypertrophy, paced rhythm.
• More false positives in women.

Nuclear imaging

Nuclear imaging techniques can be used for non-invasive assessment of coronary artery disease. Applications include perfusion assessment, viability assessment and measurement of left ventricular volume and function.

In these procedures, images are created from the emitted radiation of injected radiopharmaceuticals (radionuclides bound to marker molecules).

Single photon emission computed tomography (SPECT)

SPECT describes the use of a rotating gamma camera, which enables reconstruction to create a three dimensional image. Standard views are shown in Figure 2.4.

image

Figure 2.4 Standard left ventricular views in myocardial perfusion SPECT and PET. From: Society of Nuclear Medicine Procedure Guidelines Manual 2002 ‘Orientation for display of tomographic myocardial perfusion data’.

Radiopharmaceuticals in common use are: Thallium-201, Technetium-99m-sestamibi (cardiolite) and Technetium-99m-tetrofosmin (myoview).

After intravenous injection, the distribution of radiopharmaceutical represents regional blood flow. Distribution with stress and rest is then compared.

A perfusion defect that improves at rest indicates inducible ischaemia. A fixed perfusion defect represents infarction.

ECG-gated perfusion SPECT allows additional functional assessment including LV ejection fraction, wall thickening, wall motion and volumes. Technetium is more suited to gated SPECT than thallium.

Thallium-201 (201-Tl)

Potassium analogue. Believed to enter via the sodium–potassium pump of only viable cells and initial uptake is proportional to regional flow. Poorly perfused or infarcted tissue does not uptake up the tracer. However, tracer distribution in the myocardium changes over time and delayed imaging assesses its redistribution. Perfusion defects in ischaemic areas are filled in after approximately 4 hours. Resolution of the defect indicates viable myocardium. Detection of viable myocardium can be enhanced with delayed redistribution imaging or with thallium reinjection.

Planar images can be obtained before SPECT thallium to assess lung tracer uptake. High lung uptake is a marker of poor prognosis with an increased risk of cardiac mortality.

Technetium-99m (99m-Tc)

Sestamibi and tetrofosmin enter cells passively using negative membrane potential and bind to mitochondria within the cell. Unlike thallium, there is no significant redistribution, so separate rest and stress injections are required.

Higher-quality images are produced with technetium than thallium and radiation dose is lower. However, scatter problems can occur as the agent is eliminated by the liver. Tetrofosmin has less hepatic interference than sestamibi.

Pharmacological stress

Dipyridamole and adenosine are vasodilators which demonstrate areas of relative reduction in coronary flow reserve in regions perfused by stenosed coronary arteries.

• Unsuitable if caffeine/ theophylline taken in prior 12 hours.
• Risk of bronchospasm.
• Untoward symptoms include dyspnoea, chest discomfort, headache.

Dipyridamole: Synthetic. Inhibits reabsorption of adenosine and acts as an indirect vasodilator, increasing extracellular adenosine and leading to vascular smooth muscle relaxation. Standard dose 0.56 mg/kg over 4 minutes (up to 0.84 mg/kg). Much longer half life than adenosine. Effects can be reversed with theophylline. Avoid in patients with asthma.

Adenosine: Direct vasodilator. Very short half life. Infuse at 140 µg/kg/minute for 6 minutes. Stimulates A1 purine receptors in SA node and AV node and can cause conduction disturbance.

Positron emission tomography

This radionuclide imaging technique can be applied to the assessment of myocardial perfusion and viability.

The tracers used in this modality emit positrons when they decay. Positrons and electrons collide to form two high-energy photons which move in opposite directions. Paired detectors register the coincident arrival of photon pairs, and from this an image is reconstructed.

• For perfusion assessment a pharmacological stress agent is such as adenosine or dobutamine is required. The perfusion tracer is commonly 13N-ammonia or rubidium-82.
• For viability assessment, a metabolic tracer is used. This is commonly FDG (18-fluorodeoxyglucose). FDG is a glucose analogue which detects metabolic activity. Viability is indicated by areas of myocardium demonstrating reduced blood flow but remaining metabolically active.
• PET has better spatial resolution than SPECT and attenuation problems are less likely.
• Applications include assessment in obese patients and multivessel disease.
• PET is limited by the need for either an on site cyclotron or generator and is not as widely available as SPECT.

Stress echocardiography

• This modality involves transthoracic echocardiographic assessment at rest and with stress. Stress induced regional wall motion changes are then assessed.
• Exercise or pharmacological stress can be used.

Disadvantages of exercise: 90 second time window after peak exercise for optimum sensitivity. Increased ventilation/respiratory excursion makes image acquisition more difficult.

Typical pharmacological agents: dobutamine (+/– atropine), adenosine, dipyridamole.

Dobutamine effects: Increased contractility and heart rate, reduced systemic vascular resistance; possible side effects: palpitations, flushing, arrhythmias, drop in blood pressure.

The dobutamine dose is 5–10 µg/kg/min increased in increments to max 40 µg/kg/min.

Atropine can be given in addition to increase heart rate to target.

Views:

• Parasternal: Short axis.
• Apical: 4 chamber, 2 chamber and 3 chamber.

A number can be assigned to each segment and a wall motion score can be calculated:

1. Normal.
2. Hypokinesis.
3. Akinesis.
4. Dyskinesis.
5. Aneurysmal.

Interpretation: (Figure 2.5)

• Normal contractility at baseline and low dose but abnormal at peak dose = ischaemia.
• Abnormal at rest but improving contractility at low and peak dose = stunned.
• Abnormal at rest, improving at low dose, sustained improvement at high dose = viable and non-ischaemic
• Abnormal at rest, improving at low dose and deteriorating at peak dose (biphasic) = viable and ischaemic
• Abnormal at rest, low dose and high dose = scarred. Advantages: No ionising radiation Limitations: Need for operator expertise, echo windows may be poor
image

Figure 2.5 Short axis view of left ventricle at end systole demonstrating various responses of the anterolateral wall to low and high doses of dobutamine infusion.

Cardiac magnetic resonance imaging (see Figure 2.6)

Magnetic resonance imaging (MRI) involves the application of a strong magnetic field to the body. Radio wave energy is then applied to the area of interest. The resulting signal is then processed to generate an image.

image image

Figure 2.6 Cardiac magnetic resonance imaging. Four-chamber cine images of a normal heart at end diastole (A) and end systole (B). Equivalent end-diastolic views of a heart demonstrating chronic distal septal and apical myocardial infarction at baseline (C) and after late gadolinium enhancement (D).

Techniques used in the assessment of coronary artery disease include the following:

• Assessment of resting ventricular function, wall thickness and regional wall motion
• Dobutamine stress MRI can be performed using standard protocols. Regional wall motion changes in 17 segments are analysed as for stress echocardiography.
• Myocardial perfusion MRI can be performed using standard protocols. Myocardial perfusion at rest and with adenosine is assessed by measuring first pass signals after a bolus injection of gadolinium contrast. Hypoperfused areas have reduced signal intensity and invariably involve the subendocardium if ischaemia is present.
• Delayed imaging using gadolinium allows direct identification of necrosis or scar. Gadolinium contrast distribution is limited to the extracellular space and is retained preferentially both in necrotic tissue and in scarred myocardium. An enhancement study performed 10 minutes after contrast injection assesses contrast elimination. Late gadolinium enhancement (LGE) refers to retention of gadolinium, which appears as a bright signal.

Late gadolinium enhancement can be used to assess viability. Impaired contractility without wall thinning and without late enhancement indicates hibernating viable myocardium. Recovery of function with revascularisation can be predicted by the transmural extent of scarring. Transmural enhancement percentage is inversely related to the likelihood of functional recovery.

Advantages include:

• No ionising radiation.
• Large field of view.
• Good spatial and temporal resolution.

Limitations include incompatibility with ferromagnetic substances and patient tolerance due to claustrophobia. Resolution of magnetic resonance coronary angiography is not sufficient to replace traditional methods of anatomic assessment.

Coronary calcification imaging

Coronary calcification imaging may be used as a non-invasive surrogate marker of overall coronary artery disease burden.

Coronary artery calcification occurs almost exclusively in atherosclerosis. Detection of calcium with CT is used to assess plaque burden. This has been translated into a calcium score: Agatston score (Table 2.2). The score is generated from the product of calcium plaque area and calcium plaque density (Hounsfield number).

Table 2.2 Description of plaque burden according to calcium score

0 No identifiable atherosclerotic plaque
1–10 Minimal plaque burden
11–100 Mild plaque burden
101–400 Moderate plaque burden
Over 400 Extensive plaque burden

The absolute value can also be matched against expected score for age.

Electron beam CT (EBCT) has a rapid scan speed to reduce motion artefact. Multidetector CT scanning (MDCT) can also be used but acquisition times are longer and radiation dose is higher.

• Not all plaque contains calcium and calcium is not a marker for vulnerable plaque.
• There is evidence that calcium scoring can predict risk of future coronary events but there is no evidence that screening improves outcome.

Possible applications:

• Screening of asymptomatic patients with intermediate risk.
• Assessment of patients with atypical symptoms.

Diagnostic adjuncts in angiography and percutaneous intervention

There are now technologies in common use that provide important supplementary information during invasive coronary angiography and can therefore aid decision making.

Pressure wire

The pressure wire study allows physiological assessment of coronary lesions (Figure 2.7).

image

Figure 2.7 Schematic of fractional flow reserve (FFR) measurement in a coronary artery.

The test result is expressed as fractional flow reserve (FFR). It is defined as the ratio of maximum blood flow in the presence of a stenosis, divided by maximum blood flow in the absence of a stenosis. It is the ratio of two measured pressures: coronary pressure distal to the stenosis (Pd) divided by aortic pressure (Pa), under conditions of maximal hyperaemia.

• The discriminant value for a functionally flow-limiting significant lesion is <0.75.
• The FFR of a non-flow-limiting lesion is > 0.80.
• FFR 0.75-0.80 represents an intermediate ‘grey’ zone.

Applications include:

• Assessment of angiographically intermediate lesions.
• Where non-invasive myocardial perfusion data are unavailable.
• Assessment in multivessel disease.

Disadvantages include:

• May cause vessel injury.
• Complications associated with adenosine.
• Sources of confounding include submaximal hyperaemia, microvascular disease, left ventricular hypertrophy and the presence of serial stenoses.

Intravascular ultrasound

Intravascular ultrasound (IVUS) allows coronary artery visualisation during cardiac catheterisation and provides high resolution images of the vessel lumen and wall.

A flexible ultrasound probe is passed through the angiographic catheter into the vessel of interest. It is then pulled proximally, usually with the use of a motorised attachment at a fixed speed, whilst images are obtained in real time.

This modality allows assessment of vessel dimensions and morphology, lesion composition and length.

Applications include:

• Evaluation of left main stem disease, ostial stenoses, complex lesions and other areas of interest not well visualized on angiography.
• Assessment of stent expansion or restenosis.

Disadvantages:

• May cause vessel injury or spasm.
• For vessels >1.5 mm.
• Increases length of procedure.

* J point is the junction of the S wave and ST segment.

Related posts:

7: Pericardial structure and function 9: Pulmonary vascular disorders 3: Preventative cardiology 4: Electrophysiology 8: Adult congenital heart disease 6: Cardiac valvular structure and function