Dynamic Assessment of the Heart: Echocardiography in the Intensive Care Unit

BSE

ACCF/ASE

Indicated

Chest pain with haemodynamic instability

Assessment of the presence of complications following MI

Persistent hypotension of unknown cause

Suspected pericardial tamponade

Suspected or established PE to inform decision regarding thrombolysis

Infective endocarditis (characterise valvular lesions/complications

Appropriate

Hypotension/haemodynamic instability of uncertain or suspected cardiac aetiology

Acute chest pain with suspected MI, inconclusive ECG during pain

Suspected complication of MI

Respiratory failure/hypoxaemia of uncertain aetiology

Guide therapy of known acute PE

Severe chest trauma with suspected cardiac injury

Uncertain

Assessment of volume status of critically ill patient

Inappropriate

To establish diagnosis of acute PE

Routine evaluation of mild chest trauma

BSE British Society of Echocardiography, ACCF American College of Cardiology Foundation, ASE American Society of Echocardiography, PE pulmonary embolism, MI myocardial infarction

7.3 Focused Versus Detailed Echo Examination in ICU

Characteristics unique to the critically ill patient and the ICU environment make the bedside TTE examination different to a ‘standard’ echocardiography exam. Factors such as mechanical ventilation and limitations in positioning the patient in the ideal left lateral decubitus for echocardiography can preclude the acquisition of good quality images in certain windows. The effect of acute pathology, inotropic medication and intrathoracic pressure in normal physiology further compounds the interpretation of spectral Doppler haemodynamic values.

In common with other imaging techniques, a limitation of echocardiography is operator experience and skill. The performance and interpretation of a full echo examination in ICU requires significant training and expertise.

There is however evidence that bedside-focused screening TTE can positively influence management of the critically ill patient and can uncover unsuspected cardiac abnormalities, prompting more detailed investigations [9].

Several protocols are currently in existence for focused echocardiography in the critically ill patient. The Focus-Assessed Transthoracic Echocardiography (FATE) protocol [10], the Focused Echo Evaluation in Life Support (FEEL) [11], the Focused Assessment with Sonography in Trauma (FAST) [12] and The Focused Intensive Care Echo (FICE) are examples of commonly used protocols.

The common denominator to these protocols is a systematic examination in the context of the clinical situation. The FICE protocol has been specifically designed for the assessment of the unstable critically ill patients

7.4 Standard Basic Windows

A focused echocardiography examination of the critically ill patient should include two-dimensional (2D) images of the following views or windows (Fig. 7.1), (Table 7.2):

Table 7.2

Basic transthoracic echocardiographic windows, tips for obtaining each view and utility of each window

Windows	Obtaining this view	Utility
Parasternal long axis (PLAX)	Third to fourth intercostal space, left parasternal border. Transducer index marker points to right shoulder	LV size and function RV size Pericardial and left pleural effusion Aortic dissection Aortic and mitral valve
Parasternal short axis (PSAX)	Third to fourth intercostal space, left parasternal border. Transducer index marker points to left shoulder	LV /RV size and function Pericardial effusions
Apical four chamber (A4C)	Fourth to fifth intercostal space, midclavicular line. Index marker points towards left	LV/RV size and function Mitral and tricuspid valves Atrial size (RA, LA) Pericardial effusion
Subcostal four chamber	Transducer flat in epigastrium below ribs. Index marker points towards left	LV/RV size and function Mitral and tricuspid valves Atrial size Pericardial effusion Useful in mechanically ventilated patients
IVC subcostal	Subxiphoid, index marker points to head. Transducer tilts to the left of the patient	IVC size and respiratory variation

Table 7.3

Normal left ventricle end-diastolic diameter (LVEDD) dimensions

	Male	Female
Normal (mm)	42–59	39–53
Mild (mm)	60–63	54–57
Moderate (mm)	64–68	58–61
Severe (mm)	≥69	≥62

Parasternal long axis (PLAX)
Parasternal short axis (PSAX)
Apical four chamber (A4C)
Subcostal four chamber (SC) and the inferior vena cava (IVC)

Fig. 7.1

Focused echocardiography transthoracic windows

7.5 Goals of a Focused Critical Care Echo Examination

A systematic examination using the standard views described above aims to answer the following questions:

What is the Left Ventricle (LV) size and function?
What is the Right Ventricle (RV) size and function?
What is the fluid status (preload), and is there evidence of preload dependence?
Is there a pericardial effusion? Is the effusion causing cardiac tamponade?

Answering these questions at the bedside provides the clinician with non-invasive information to diagnose and guide treatment for the haemodynamically unstable critically ill patient. Accurate Doppler-based measurements are challenging in the critically ill and often mechanically ventilated patient. Therefore, focus should be placed in a systematic acquisition of good quality 2D images. Any abnormality should be confirmed in at least two windows.

7.5.1 Left Ventricular Assessment

Accurate analysis of left ventricular size function is an essential first step when evaluating a critically ill patient presenting with shock.

The aim of the echo examination will be to determine if the left ventricle is: (Table 7.3)

Small
Normal size
Dilated: mild, moderate or severe

The LV diameter is best measured in the PLAX window at the level of the tip of the mitral valve [13]. The left ventricle end-diastolic diameter (LVEDD) is the largest cardiac dimension and should be obtained shortly before systole begins. This corresponds to the beginning of the QRS complex or the frame just after mitral closure.

The following pitfalls should be avoided when measuring LVEDD:

Measurements should be taken between the endocardial borders, not the pericardium.
Distance should be measured perpendicular to the long axis of the LV. 2D measurements are preferred over M-mode for this reason.
Avoid including papillary muscles or chordae in the measurements.

The most commonly used methods for assessment of LV function in the critical care setting are:

Visual gestalt or ‘eyeballing’
Ejection fraction (EF)
Fractional shortening (FS)
Cardiac output (CO)

7.5.1.1 Visual Gestalt

Estimation of EF is achieved by ‘eyeballing’ the overall size and contractility of the LV. The thickening and inward movement of the LV walls are assessed in 2D images with no formal measurements required. The use of ‘eyeballing’ by intensivists with basic echocardiography training has been found to have a good level of agreement with LV function estimation performed by experienced echocardiographers [14].

Visual quantification of LV function can be divided in normal, mild-to-moderate and severe systolic dysfunction.

7.5.1.2 Ejection Fraction

Systolic performance of the LV (stroke volume) is dependent on contractility, preload and afterload. Ideally, a marker of contractility should not be affected by loading conditions (preload) or afterload. EF is less dependent on preload than stroke volume. However, EF is significantly affected by conditions with high afterload. Despite these limitations, EF is widely accepted as a measurement of LV systolic function [15] (Table 7.4).

Table 7.4

Normal values for ejection fraction (EF)

Normal	>55 %
Mild	45–54 %
Moderate	30–44 %
Severe	<30 %

Ejection fraction is calculated by subtracting the LV end-systolic volume (LVESV) from the end-diastolic volume (LVEDV) and then dividing by LVEDV.

$\mathrm{E}\mathrm{F}=\mathrm{LVEDV}-\mathrm{LVESV}/\mathrm{LVEDV}$

EF can be calculated using volumes derived from M-mode. Measurements of the LV, LVEDD and LVESD are obtained in the PLAX window by placing the M-mode cursor in a plane that cuts through the septal and posterior walls of the LV just below the tip of the mitral valve leaflets. LVEDD is measured at the onset of the QRS complex, just prior to MV closure. LVESD measurement is timed to the frame with the minimum LV dimension.

EF is then calculated by computer software that uses the Teichholz or Quinones formulas. These linear calculations have several pitfalls; thus, linear EF measurements are not recommended for critical care practice (Table 7.4).

The Simpson method uses 2D images and calculates LV volume by the summation of the volumes of a stack of elliptical discs constructed inside the LV endocardial outline. It is recommended as the method of choice by the American and European societies of echocardiography [13].

The endocardial border is traced using the A4C and/or apical two-chamber view, and the software in the ultrasound machine calculates the volume of the chamber. Measurements are taken at end-diastole and end-systole (Fig. 7.2).

Fig. 7.2

Ejection fraction calculation using M-mode and 2D echocardiography

7.5.1.3 Fractional Shortening

Fractional shortening provides a rough estimate of LV function. It is obtained by entering measured LV diameters into the following formula:

$\mathrm{F}\mathrm{S}=\left(\mathrm{LVEDD}-\mathrm{LVESD}\right)/\mathrm{LVEDD}\times 100\kern1em \mathrm{Normalrange}:\;25-45$

FS is severely limited by regional wall motion abnormalities and non-global functional alterations that can be missed by the single M-mode plane. This constraint limits the use of FS in the critical care setting.

Cardiac Output

Doppler echocardiography is able to obtain non-invasive measurements of cardiac output. The product of stroke volume and heart rate, cardiac output, is widely used in critical care as an indicator of global cardiovascular system function.

The stroke volume, which is the volume of blood ejected by the left ventricle in systole, is received by the proximal ascending aorta. The aorta can be thought of as a cylinder. The volume of the cylinder can be calculated by multiplying the cross-sectional area (CSA) by the height or distance travelled by the fluid in the cylinder. This measurement is commonly performed at the LVOT, where the CSA is the diameter of the LVOT (cm²) measured in the PLAX window (Fig. 7.3), and the height is the integral of velocity versus time of the blood passing through the ascending aorta (Fig. 7.4):

Fig. 7.3

Cardiac output (CO) calculation using Doppler echocardiography. Step 1: LVOT diameter measured in PLAX

Fig. 7.4

Step 2: Pulsed wave (PW) Doppler signal at the level of the LVOT traced for stroke volume calculation

$\mathrm{S}\mathrm{V}=\mathrm{C}\mathrm{S}\mathrm{A}\times \mathrm{V}\mathrm{T}\mathrm{I}\;.C\mathrm{O}=\mathrm{S}\mathrm{V}\times \mathrm{H}\mathrm{R}$

7.5.2 Assessment of the Right Ventricle

The right ventricle functions as a low-pressure chamber that adapts easily to changes in volume loading but that is less able to tolerate acute increments in afterload. The function of the right ventricle can be directly affected by pathologies frequently encountered in the intensive care such as pulmonary embolism and Adult Respiratory Distress syndrome (ARDS) [16]. Mechanical ventilation and volume status can also impact on the normal function of the RV [17]. Right ventricular dysfunction is directly associated with increased mortality in critically ill patients [18].

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