Cardiology (Full Guide)

Objectives

  1. Understand the normal anatomy of the heart as it appears in all 4 ultrasound views
  2. Understand the utility of ultrasound as a way of assessing normal or abnormal cardiac function
  3. Be able to quickly and consistently obtain all 4 ultrasound views of the heart
  4. Lean to recognize common pathologic cardiac conditions with ultrasound

Introduction
Cardiac ultrasound (US) is one of the most difficult US modalities to learn, but proficiency in this scan can mean the difference between life and death. It is therefore vital that physicians be familiar with normal and pathologic cardiac US findings.

Normal Cardiac Anatomy
It is necessary to understand the normal anatomy of the heart in order to accurately interpret the US images. Each US view of the heart will show the heart in a different angle. Understanding the path of blood flow and the orientation of the heart within the mediastinum allows for easy and rapid translation of cardiac US images. The heart is composed of four chambers: the left and right atria and the left and right ventricles. Blood flows freely into the right atrium from the superior and inferior venae cavae. Blood then passes through the tricuspid valve to enter the right ventricle. When the right ventricle contracts it expels blood through the pulmonic valve and into the pulmonary arteries which carry blood to the lungs. The pulmonary veins return blood to the left atrium of the heart where the blood passes through the mitral valve to enter the left ventricle. Blood then flows through the aortic semilunar valve into the aorta and out to the body.

The base of the heart is located midline in the body with its apex (opposite the base) pointing left and caudally with the majority of the heart’s mass located left of the midline. The heart is surrounded by the sac-like pericardium that provides lubrication and anchoring for the heart as it beats. There are two layers to the pericardium, a parietal and a visceral layer.

Indications for Cardiac Ultrasonography
The following are indications for performing a cardiac US:
•    Acute coronary syndromes
•    Clinically suspected heart failure
•    Unexplained hypotension
•    Pulseless electrical activity (PEA) cardiac arrest
•    Cardiomegaly on physical examination or chest x-ray (CXR)
•    Malignant arrhythmias
•    Cardiac procedures, such as pericardiocentesis

Technique of Cardiac Ultrasound
There are four important US views of the heart that will be addressed individually. It is necessary to use the phased array transducer so as to “shoot” the sound waves between the ribs that guard the heart anteriorly. One key to learning cardiac US is to remember the proper position of the transducer for each view. In cardiac US, the position of the indicator does not always follow the rule of pointing towards the patient’s right or head. Each view requires a different transducer position and necessitates practice to achieve mastery.

Parasternal Long Axis View
The patient should be lying supine, or if necessary due to difficulty visualizing the heart, in the left lateral decubitus position. Place the probe in the 4th left intercostal space (LICS) adjacent to the sternum and point the indicator toward the patient’s right shoulder. Some will find it easier to start on the sternum and slide to the 4th LICS. Once in the correct location, make slight adjustments, rock the probe back and forth, and fan until the desired image comes into view.

In this location the right heart is most superficial such that in the US image the right heart is sitting on top of the left heart. The important structures to identify in this view are the right ventricle, left ventricle, left ventricular outflow tract, aortic valve, aorta, mitral valve, left atrium, and pericardium.

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Parasternal Short Axis View
The patient should be lying supine, or if necessary due to difficulty visualizing the heart, in the left lateral decubitus position. From the parasternal long axis view, rotate the probe 90° clockwise so that the indicator is now pointing toward the patient’s left shoulder. Now the heart is visualized in a plane perpendicular to the plane of the parasternal long axis. Rock and fan until a clear cross-section view of the left ventricle is achieved.  Angling the probe up towards the base of the heart (opposite of the apex of the heart) will bring the aortic valve into view. The aortic valve is composed of three cusps which, when they come together, form the “Mercedes Benz” sign (US visualization looks undeniably like the Mercedes Benz emblem). Fanning the probed away from the base, towards the apex, will bring the mitral valve into view.  The mitral valve is said to look the  like the flopping of a fish’s mouth inside the left ventricle. Fanning further towards the apex allows identification of the papillary muscles supporting the mitral valve inside the left ventricle.

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Apical Four Chamber View
The patient should be lying supine, or if necessary due to difficulty visualizing the heart, in the left lateral decubitus position. To find the apical four chamber view, locate the apical impulse (point of maximal impulse). This will serve as a good starting place for the probe. Place the probe with the indicator pointed toward the patient’s left lateral chest with the probe tilted towards the patient’s right shoulder. It may be necessary to move the probe around, but by fanning and rocking one should be able to see all major structures of the heart in one image. The ventricles sit on top of the atria in this view because the probe is closest to the ventricles at the apex of the heart. The mitral and tricuspid valves are found in between the atria and ventricles.

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Subxiphoid View
The patient must be in the supine position (left lateral decubitus is not acceptable) for the subxiphoid view. A better image is achieved when the patient bends their knees, relaxing the abdominal muscles. The probe should be placed to the right of the midline underneath the xiphoid process with the probe acutely angled toward the patient’s left shoulder. Point the indicator toward the patient’s left side. The probe is placed right of midline in order to utilize the liver as an acoustic window and avoid the left-sided bowel gas that would scatter the US waves. When attempting to achieve this view, it is necessary to apply significant pressure with the probe against the abdominal wall to send the US waves beneath the rib cage. All four chambers should be visible in this view with the right ventricle closest to the probe, thus, at the top of the screen.

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Pathology

Pericardial Effusion
A pericardial effusion is a situation where the pericardial sac is filled with fluid, most often blood, increasing pressure on the heart and the great vessels. The increased pericardial pressure prevents the heart from expanding properly, inhibits ventricular filling and, therefore, dramatically decreases cardiac output. Patients suffering from a pericardial effusion will present with acute onset dyspnea and paradoxical pulse.

On US exam, blood in the pericardial space will appear as a dark, anechoic stripe surrounding the heart. Smaller effusions result in a smaller stripe found on the posterior/inferior aspect of the heart. Larger effusions are more likely to be circumferential, seen in front of and behind the heart.

Fluid in the restricted space of the pericardium causes increased pressure and compresses the heart chambers. The right heart, due to its lower pressure, will begin to collapse.  As the chamber collapses, it is possible to observe a slight inward serpentine diastolic deflection of the right atrial or right ventricular free wall.  In severe effusions, the chamber will experience complete diastolic collapse. Take care to distinguish true diastolic collapse from systolic contraction, as these can easily be mistaken in a tachycardic patient. Real time ECG can help with this distinction. Alternatively, observe the opening of the mitral valve to determine when diastole begins.

Global Left Ventricular Function
The most quantifiable measurement of cardiac function is ejection fraction (EF). It is commonly measured using the Simpson method. The Simpson and other methods used to measure EF are beyond the scope of this book. EF measurements are at times inaccurate, and it is up to the physician to know when to trust them or not. For that reason, it is vital that medical students practice cardiac US scanning and familiarize themselves with the appearance of a normal functioning heart. Then students will be able to recognize abnormal left ventricular function by “eyeballing” it.

Evaluate the heart for the following characteristics:

1. Inward motion of the endocardium.
All parts of the endocardium should contract inwardly in unison during systole. Multiple views will need to be obtained to ensure that no part of the heart is failing to contract. The walls of the endocardium can be described as moving normally, hypokinetically (slower), akinetically (not moving at all), or dyskinetically (moving in the wrong direction).

2. Thickness of the myocardium
The myocardium of the left ventricle should be assessed in multiple views. Any thickness > 15 mm is suggestive of hypertrophic cardiomyopathy. Myocardium that is especially thin (

3. Proper motion of the valves
Color flow Doppler is very useful when evaluating cardiac valves, as it reveals stenotic or insufficient valves. This modality explicitly captures the abnormal movement of blood flowing through the valves. Other valve abnormalities, like prolapse, scarring, calcification, or thickening can also be detected using US.

4. Geometry of the ventricle
Similar to evaluating for wall thickness or wall motion, evaluating for abnormal ventricle geometries can aid in the diagnosis of an acute MI or stable coronary artery disease even before an ECG will pick them up.