Chapter 5A – Cardiac Investigations

Chapter 5A Cardiac Investigations Echocardiography

Muhammad Sohaib Nazir and Rajan Sharma

Key Points

  • Transthoracic echocardiography (TTE) is a safe, rapid, non-invasive and cheap test in the preoperative period.

  • Echocardiography is the gold standard for assessment of left ventricular (LV) systolic function and valvular heart disease.

  • Patients with severe valvular disease or poor LV systolic function should be evaluated further by specialist cardiologists prior to non-cardiac surgery.

  • Stress echocardiography can be used in the preoperative period to determine the presence and extent of myocardial ischaemia.

  • A normal stress echocardiogram (SE) has excellent prognostic value, and is associated with a less than 1 per cent annual event rate of myocardial infarction or death.

  • Patients with a high ischaemic burden on stress echocardiography require further evaluation and may need to be considered for coronary revascularisation prior to non-cardiac surgery.

  • Both TTE and SE are not required for low-risk patients undergoing low-risk non-cardiac surgery.

Why Echocardiography?

Preoperative, non-invasive testing aims to provide information on three cardiac risk markers: left ventricular (LV) dysfunction, myocardial ischaemia and heart valve abnormalities, all of which are major determinants of adverse post-operative outcomes. Echocardiography is worldwide the most widely available and cost-effective non-invasive technique for evaluating LV systolic function and valve disease. The technique using ultrasound is completely safe. Machines are now available the size of a smartphone, making this technique attractive for scanning in the community when patients are relatively immobile. A full study takes 30 minutes.

Assessment of Left Ventricular Systolic Function

Echocardiography is considered the gold standard for primary evaluation of LV systolic function and has been the technique of choice for major clinical trials evaluating treatments in heart failure. The principal parameter for defining LV systolic function is LV ejection fraction (LVEF). This is the volume of blood the LV ejects with each heartbeat expressed as a percentage of the total volume of blood in the heart in end diastole. LVEF can be measured by number of techniques. The Teichholz method is based on a change in LV short-axis dimension in M mode. This method assumes uniform function of the LV and therefore is inaccurate when regional wall motion abnormalities are present. More reliably the LVEF is determined by a modified biplane Simpson technique which involves planimetry of the endocardial border in end systole and diastole. An assumption is then made that ventricular volume is the sum of the volumes of adjacent sets of discs of varying depth and cross-section area (Figure 5.A.1). The technique requires a good machine and a trained operator and is feasible in 80 per cent of echo subjects. Patients with poor acoustic windows in whom LVEF cannot be reliably assessed (obesity and pulmonary airways disease) should be considered for LVEF determination by contrast echo, cardiac magnetic resonance imaging (MRI) or a nuclear multi-gated acquisition (MUGA) scan. Recent data suggest three-dimensional echocardiography may be superior to two-dimensional echocardiography, but at present this echo modality is limited to a few centres only. Table 5.A.1 demonstrates ranges of LV systolic function according to LVEF. In general, patients with poor LV systolic function present high anaesthetic risk and should be referred on to the heart failure team. Other determinants of LV systolic function are beyond the scope of this chapter.

Figure 5.A.1 Left ventricular (LV) ejection fraction measured from four-chamber view of the left ventricle, in diastole (a) and systole (b).

Table 5.A.1 LV systolic function ranges and corresponding LV ejection fraction.

LV systolic function LV ejection fraction
Normal ≥ 55%
Mild dysfunction 45 to 54%
Moderate dysfunction 36 to 44%
Severe dysfunction ≤ 35%

Legend: LV = left ventricle, % = per cent

Assessment of Left Ventricular Diastolic Function

LV diastolic dysfunction occurs due to impaired relaxation and filling of LV. Echocardiography is crucial for the assessment of diastolic function; however, there is no single echocardiographic parameter that can be used in isolation for the diagnosis of diastolic dysfunction. Therefore, an assessment requires a comprehensive assessment of cardiac structures with two-dimensional echocardiography, pulsed waved Doppler and tissue Doppler.

LV fills in two stages: through early ventricular filling and late ventricular filling from atrial contraction. Ventricular filling can be measured by obtaining pulsed wave velocities through the mitral valve in which the E wave and A wave represent early and late ventricular filling, respectively (Figure 5.A.2). In a normal, healthy heart, the E velocity is greater than the A velocity and therefore the E/A ratio is more than 1. When the left ventricle becomes stiff as in diastolic heart failure, the E wave is reduced, and therefore the E/A ratio is less than 1. However, in more advanced diastolic dysfunction, the E/A ratio may revert back to a normal ratio – called pseudo-normal, hence the need for evaluation with multiple indices. This includes tissue Doppler velocity of the mitral valve annulus (an assessment of myocardial relaxation), pulmonary venous flow and left atrial volume which provide additional evidence of diastolic dysfunction and its degree.

Figure 5.A.2 Pulsed wave Doppler measurements of the mitral valve outflow demonstrating the E and A wave that are used to assess diastolic function.

Patients with diastolic heart failure tend to be older, female and more likely to have atrial fibrillation (AF) or hypertension. They are less likely to have underlying coronary artery disease, and their prognosis appears to be better than for patients with systolic dysfunction. The prognostic impact of diastolic dysfunction is not well defined in the perioperative period, thus a patient with diastolic dysfunction should undergo similar perioperative care as with systolic heart failure.

Assessment of Valve Disease

Moderate-to-severe mitral regurgitation and severe aortic valve stenosis are associated with major cardiac events. Echocardiography is the gold standard for assessment for patients with valve disease. Detailed description of the role of echocardiography in determining the aetiology and severity of aortic stenosis and mitral regurgitation is beyond the scope of this chapter. Essentially, two-dimensional echocardiography allows determination of aetiology and colour, pulsed wave and continuous wave Doppler allows stenosis and regurgitation severity to be calculated. Quantitative and semi-quantitative techniques are available. Tables 5.A.2 and 5.A.3 describe echocardiographic determination of severity of aortic stenosis (AS) and aortic regurgitation (AR), respectively. Tables 5.A.4 and 5.A.5 demonstrate features of severe mitral stenosis (MS) and mitral regurgitation (MR), respectively. Figures 5.A.3 and 5.A.4 demonstrate echocardiographic features of a patient with severe AS and MR, respectively. A summary of the European Society of Cardiology (ESC) guidelines management of common valvular disease is presented in Table 5.A.6. Generally, patients with severe valve disease undergoing intermediate or high-risk surgery require a multidisciplinary discussion with the cardiology, cardiothoracic surgery and anaesthetic teams to determine risks and benefits of surgery with or without enabling valve procedure beforehand.

Table 5.A.2 Echocardiographic parameters of severity of aortic stenosis. Critical aortic stenosis is most likely with aortic valve area <0.8cm2.

Severity of aortic stenosis Velocity (m/s) Peak gradient (mmHg) Mean gradient (mmHg) Aortic valve area (cm2)
Mild 2.0–2.9 <36 <25 1.5–2.0
Moderate 3–4 36–64 25–40 1.0–1.4
Severe >4 >64 >40 <1.0

Legend: cm 2 = square centimetre, mmHg=millimetre of mercury, m/s=metre per second

Table 5.A.3 Echocardiographic parameters of severity of aortic regurgitation.

Severity of aortic regurgitation Vena contracta width (cm) Jet width / LVOT ratio (%) Regurgitant volume (mL) Pressure half time (ms)
Mild <0.3 <25 <30 >500
Moderate 31–60
Severe >0.6 ≥65 ≥60 <250

Legend : LVOT = left ventricular outflow tract, mL = millilitre, ms = millisecond, cm = centimetre, % = per cent

Table 5.A.4 Echocardiographic parameters of severity of mitral stenosis.

Severity of mitral stenosis Pressure half time (ms) Mean pressure drop (mmHg) Valve area (cm2)
Mild 71–139 <5 1.6–2.0
Moderate 140–219 5–10 1.0–1.5
Severe >220 >10 <1.0

Legend: ms = millisecond, mmHg = millimetre of mercury, cm2 = square centimetre

Table 5.A.5 Direct and indirect echocardiographic features of severe mitral regurgitation. The vena contracta is the narrowest jet of regurgitation. Proximal isovelocity surface area (PISA) is a colour flow phenomenon observed with blood passage through the regurgitant valve, resulting in hemispherical flow convergence and the radius of this is used to quantify mitral regurgitation.

The Nyquist limit is the highest frequency that can be coded at a given sampling rate in order to be able to fully reconstruct the signal.

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Sep 15, 2020 | Posted by in ANESTHESIA | Comments Off on Chapter 5A – Cardiac Investigations
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