Non-invasive diagnostic tests (Table 14.1) are under- taken to support the clinical impression of and to quantify the extent of cardiac disease. The investiga- tions may be repeated over time to follow the progress of disease. Information may be supplemented with invasive investigations such as angiography when cor- onary artery disease is suspected.
Electrocardiography
The ECG represents the sum of myocardial voltage changes throughout the cardiac cycle along the vector of each of the leads recorded. It may be recorded from the skin surface, from the endocardium in the cath- eter laboratory or from the epicardium during certain open procedures requiring cardiac mapping. The sim- plicity of acquisition of the surface ECG makes it one of the most frequent tests performed.
The following processes affect the sum of myocar- dial voltage changes (Table 14.2):
the frequency of atrial and ventricular systole the mass of the chambers undergoing
depolarization
the route by which depolarization occurs myocardial perfusion
metabolic influences.
The ECG, in conjunction with markers of myocardial necrosis (e.g. troponin I), helps to establish the diag- nosis of STEMI or NSTEMI.
Exercise ECG
Myocardial oxygen extraction is maximal at rest, so an exercise-induced increase in metabolic demand can only be met by increase in coronary blood flow.
It follows therefore that coronary flow limitation in the setting of coronary artery stenosis has a far greater effect during exercise than at rest, thereby increasing the sensitivity of the ECG to detect ischemia.
Exercise ECG (exECG) testing, using a treadmill or static cycle ergometer, is to investigate “cardiac- type” chest pain or exertional breathlessness. Both treadmill and cycle-based testing are limited to patients with the ability to exercise. In the Bruce protocol both the speed and gradient of the tread- mill are increased at each stage, whereas in the modified Bruce protocol the treadmill speed remains constant during the first three stages. In the Naughton protocol, however, only the treadmill gradient is altered. Heart rate and blood pressure are recorded at each increase in workload and the ECG examined for evidence of ST segment depres- sion. Myocardial ischemia is suggested by the devel- opment of chest pain, ST segment changes, failure to increase blood pressure or arrhythmias. Results are defined as positive, negative, equivocal and uninterpretable.
exECG testing is relatively cheap and can be per- formed in the outpatient setting. It has low specificity
Table 14.1 Classification of non-invasive diagnostic techniques Electrocardiography Ionizing
radiation
Non-ionizing imaging Resting 12-lead ECG CXR Echocardiography Exercise stress test CT MRI
Ambulatory ECG monitoring
Nuclear scintigraphy Intraoperative ECG
monitoring Intraoperative ST segment analysis
Core Topics in Cardiac Anesthesia, Second Edition, ed. Jonathan H. Mackay and Joseph E. Arrowsmith. Published by
Cambridge University Press. © Cambridge University Press 2012.
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and sensitivity (65–70%) for coronary artery disease, although simultaneous transthoracic echocardio- graphic detection of regional wall motion abnormal- ities (RWMA) improves specificity. Low-grade stenoses (<50%) are difficult to detect, as are fixed stenoses with collateral blood flow. Contraindications
toexECG include acute coronary syndromes, severe congestive cardiac failure and severe aortic stenosis. It may also be unsuitable for patients with physical disability, respiratory disease, peripheral vascular dis- ease, left bundle branch block or A-V conduction abnormalities.
Table 14.2 Overview of ECG abnormalities
Abnormality Description Comments
Atrioventricular conduction
First-degree block
PR interval>200 ms One P wave per QRS
Seen in coronary artery disease, acute rheumatic fever, digoxin toxicity and electrolyte disturbance
Second- degree block
Mobitz type I
Progressive lengthening of PR interval
Also known as Wenkebach Usually benign
Mobitz type II
PR interval constant, occasional non- conducted beats
May herald complete heart block
2:1 block: 2 P waves per QRS complex, normal P wave rate
May herald complete heart block Third-degree
block
Normal atrial depolarization, no conducted beats, usually wide QRS with ventricular rate<50/min
Ventricles excited by slow“escape”
mechanism. Myocardial infarction, chronic fibrosis around bundle of His and in right bundle branch block.
Consider pacing Intra-ventricular
conduction
Left bundle branch block
QRS>120 ms, late R-waves in I, aVL and V5–6, no septal Q waves, deep S in V1, tall R in V6, associated with T wave inversion in lateral leads
Best seen in V6
Always pathological, prevents further interpretation of ECG
Left anterior hemi-block
QRS 100 ms, marked left-axis deviation, deep S in II, III, q in IaVL
Left posterior hemi-block
QRS 100 ms, right-axis deviation Right bundle
branch block
QRS>120 ms, RSR in V1–2, dominant R and inverted T in V1, usually normal axis
Best seen in V1
May indicate RV problems, may be normal variant
Bifasicular block: RBBB with left anterior hemiblock
Ischemia ST depression>2 mm
Infarction Raised ST segments,þ/Q waves (>3 mm,>30 ms), normalization of ST segments, T wave inversion
Hypertrophy RA Peaked P wave
LA Bifid P wave
RV RAD, tall R in V1, T wave inversion V1–2, deep S V6,RBBB
LV R in V5/V6>25 mm or R in V5/6þS in V1/2
>35 mm
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Figure 14.1 First-degree heart block (left) and 2:1 heart block (Mobitz II) (right).
Figure 14.2 Wenckebach phenomenon (Mobitz I). The P-R interval gradually increases.
Figure 14.3 Third-degree (complete) heart block.
V2 V5
V2 V5
Figure 14.4 Left bundle branch block (upper) and right bundle branch block (lower).
Table 14.3Limitations to the resting surface ECG Temporal changes in the ST segment may be missed with a single recording
Small non-Q wave infarcts may fail to meet diagnostic criteria
Large transmural infarcts may obscure additional electrocardiographic events
The sensitivity to detect ischemia is limited by the position of the exploring electrode
Posterior cardiac events are often missed
Resting ECG may be normal even in the setting of three- vessel coronary artery disease
May fail to reflect the effect of exercise on myocardial perfusion
ECG changes lag behind changes in diastolic and systolic dysfunction with ischemia
Prone to skeletal muscle myopotentials obscuring changes
Chapter 14: Non-invasive diagnostic tests
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Table 14.4 The Bruce treadmill protocol and an example of one of the many modifications described.
The target heart rate for the test is 220 minus patient age. For example, the test would be halted (regardless of ECG changes) for a patient aged 50 when a heart rate of 170 had been achieved
Bruce protocol Modified protocol
Stage Speed (mph)
Grade (%) Speed (mph)
Grade (%) Duration (min)
1 1.7 10 1.7 0 3
2 2.5 12 1.7 5 3
3 3.4 14 1.7 10 3
4 4.2 16 2.5 12 3
5 5.0 18 3.4 14 3
6 5.5 20 4.2 16 3
7 6.0 22 5.0 18 3
8 5.5 20 3
9 6.0 22 3
10 6.5 24 3
Figure 14.5 Exercise ECG summary print out from a patient undergoing low workload with significant ST segment changes. The depression of the ST segment is measured 80 ms after the J point. The maximum ST segment depression was 2.25 mm in lead V5.
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Ambulatory ECG
Ambulatory ECG monitoring, also known as Holter or continuous 24 hour recording, is a method used to aid the diagnosis of chest pain, palpitations or syn- cope that occur intermittently during normal daily activities. The ECG electrodes are applied to the chest and attached to a recording device which is carried by the patient for a period of 24–48 hours. Some devices comprise a patient-activated event monitor. Patients are required to complete a log of events to aid analysis and correlate physical activities with contemporan- eous ECG changes. The recorder is interrogated to produce a printout or computer analysis of events.
Longer-term (up to 6–12 months) implantable ECG recorders are used to increase event detection rate. They may record data continuously or intermit- tently. Integral electrodes are placed subcutaneously in the left subclavian position. The Reveal
®
device isan example of a patient-activated intermittent loop recorder (Figure 14.6). When activated, data acquired in the minute before activation (the prodrome) and during symptoms are stored. The digital storage of data is limited to 45 min. The device may be interro- gated transcutaneously and therefore may be left in situ if required. Both insertion and removal of the recorder can be conducted under local anesthesia.
Transthoracic echocardiography
Transthoracic echocardiography (TTE) is relatively quick and straightforward to perform, and provides qualitative and quantitative assessment of cardiac structure and function.
Although TTE is an extremely useful tool, it does have several limitations. Certain patient factors reduce echogenicity and therefore make image quality poor or unobtainable. These include obesity, pulmonary
emphysema, interference from the ribs, an inability to lie in the lateral position, and the presence of surgi- cal drains and anterior chest wall dressings. Findings are relatively operator-dependent and technical diffi- culties may limit both the quality and completeness of an examination. Spatial resolution is restricted to one wavelength (0.3 mm at 5 MHz) and depth resolution to 200 wavelengths (60 mm at 5 MHz). TTE is poor at imaging posterior structures and Doppler can only be used to estimate flow accurately if the angle of incident ultrasound is<20. (See Figure 24.2.)
Stress echocardiography
Echocardiography can be combined with exercise testing or dobutamine-induced (40–60 µg kg1 min1) stress in patients unable to exercise to allow qualitative assessment of ventricular perform- ance with increase in heart rate. Occasionally, pacing or atropine is required to obtain an adequate HR response. Imaging dynamic changes in LV outflow obstruction in hypertrophic obstructive cardiomyopathy (HOCM) is also possible. Stress echocardiography is more sensitive than stress ECG at detecting ischemia. The value of the test rests in the visualization of functional changes with increased myocardial demand.
Figure 14.6 The Medtronic Reveal
®
implantable ECG recorder.The recording electrodes are on the left.
SHORT-AXIS PLANE AO
PA
RA
RV LV
LA
LONG-AXIS PLANE FOUR-CHAMBER
PLANE
Figure 14.7The thee basic image planes used in transthoracic echocardiography. The long axis extends from the LV apex through the A-V plane. The short axis lies perpendicular to the long axis. The four-chamber view lies perpendicular to both the long and short axis planes, and includes the LV apex, RV, LA and RA.
Chapter 14: Non-invasive diagnostic tests
85
Contrast echocardiography
Contrast agents can be employed to improve image quality by delineating the border between endocar- dium and ventricular cavity. Sonographic contrast agents are suspensions of microspheres filled with perfluorocarbon gas which enhance image resolution by acting as intravascular tracers. The insonated gas bubbles pulsate, with compression occurring at the peak of the ultrasound wave and expansion at the nadir. SonoVue
®
is a stabilized aqueous suspension of sulfur hexafluoride (2.5 µm microbubbles) within a shell of polyethylene glycol (macrogel 4000). After peripheral intravenous injection of 0.2–0.4 ml, the microbubbles traverse the pulmonary vascular bed to opacify the ventricular cavity. Further refinements allow quantification of coronary microcirculation by assessment of subendocardial opacification. This is particularly powerful during stress echocardiographyas a reduction in opacification is often easier to appre- ciate than a new RWMA. Allergic reactions to contrast agents have occurred and therefore, contrast is contra- indicated in patients with recent unstable cardiac symptoms, a recent (<7 days) coronary intervention, class III and IV heart failure or serious arrhythmias.
Further reading
Chung EK, Tighe D.Pocket Guide to Stress Testing. London:
Wiley Blackwell; 1997.
Garcia MJ.Non-Invasive Cardiovascular Imaging:
A Multimodality Approach. London: Lippincott Williams and Wilkins; 2009.
Leeson P, Mitchell ARJ, Becher H.Echocardiography (Oxford Specialist Handbooks in Cardiology). Oxford:
Oxford University Press: 2007.
Otto CM.Textbook of Clinical Echocardiography, 4th edition. Philadelphia: Saunders; 2009.
Figure 14.8 Still echo images taken at rest (left) and during dobutamine-induced stress (right) demonstrating significant ventricular dilatation. The patient had significant LAD stenosis.
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Section 3 Chapter