Echocardiography

Cardiac Ultrasound General Principles (UCG)

Posted by Keti Dalla, Senior Physician in Anesthesia & Intensive Care. Sahlgrenska University Hospital.
Updated 2019-06-12


An echocardiogram, often referred to as a cardiac echo or simply an echo, is a sonogram of the heart. Echocardiography uses standard two-dimensional, three-dimensional, and Doppler ultrasound to create images of the heart. In the case of two-dimensional echocardiography, ultrasonic beams are transmitted sequentially in different directions from the probe (transmitter/probe). The ultrasound machine can calculate the depth of reflection by keeping track of how long the ultrasound waves have been in the body. The reflection points from each ultrasonic beam together build up an image, which is displayed on the ultrasound machine screen. The number of images displayed per second is called frame rate (FR). Frame rate of over 40 frames per second is required to investigate the heart. The higher the FR the better. FR is increased by reducing the depth and sector width.


Doppler Function in UCG

When the ultrasonic waves are reflected against moving objects e.g. red blood cells, it changes the frequency of the sound. The reflected sound has a higher frequency than the broadcast if the object moves towards the transmitter and lower frequency if the item moves from the transmitter. The frequency difference between the transmitted and reflected ultrasound is called Frequency Shift or Doppler Switch (Df).

Df = 2f x V x cosα/C

If the ultrasound beam has the same direction as the blood flow, the angle α = 0 degrees and the cosin of 0 degrees becomes 1. When the angle increases, the cosin becomes less than 1, which causes the rate of blood flow to be underestimated.

With Continuous Doppler (CW), all speeds are recorded along the entire Doppler beam. The continuous Doppler’s strength is that very high speeds can be recorded.

With Pulsed Doppler (PW), speeds are recorded within a certain range, e.g. LVOT (Left Ventricular Outflow Tract).


LAX/Parasternal Long Axis View

The parasternal longitudinal section cuts through the aorta, mitral valve, left ventricle, left atrium and right ventricle. The inner chamber diameter of the left chamber, the thickness of the septum and the back wall thickness are measured in the final diastole when the chamber is the largest. The measurement is done immediately below the tip of the mitral cone. If the transmitter is moved up, some interstitial can often see large portions of the aorta ascendence.


SAX/Parasternal Short Axis View

In the aortic valve plane, the parasternal short axis section cuts through the aortic valve, right and left atrium, right ventricle (RVOT), pulmonary valve and pulmonary trunk (truncus pulmonalis).

The cut must usually be corrected by moving the transmitter caudally to the right ventricle outlet tract, pulmonary valve and pulmonary trunk. The pulmonary flow profile with PW-Doppler can reveal pulmonary hypertension (Fig. 1). Short acceleration time < 95 ms and a “dip” in the flow profile in the latter half of the systole talks for elevated pulmonary artery resistance (PVR).

The tricuspid valve, parts of the right atrium as well as atrial septum can also be seen in this projection.

The mitral valve plan shows the basic parts of the left ventricles. In the papillary muscular level, the left ventricle’s central portion is seen. In systole, all parts of the chamber wall will become thicker and the endocardium should normally move towards the center of the chamber. At volume loading of the right ventricle, septum often bends towards the left ventricle under the diastole. At pressure load of right ventricle (pulmonary embolism, pulmonary hypertension), septum bends to the left even under systole.


Apical Four Chamber View (4Ch)

The apical four chamber view intersects the central parts of the chambers and atrium, as well as mitral and tricuspid valve. Here you can see septum, left ventricular lateral wall and apex. The left ventricle diastolic and systolic volume and the ejection fraction (EC) can be calculated using Simpson’s method by drawing a line along the entire endocardium in diastole and in the systolic when the chamber is at least.

The mitral valve should be assessed for sclerosis, stenosis, prolapse and SAM. The diastolic flow rates above the mitral valve can be registered with pulsed Doppler and provide information about the diastolic function of the chamber. Sample volume is placed at the tips of the mitral valve.

Filling pressure can be estimated using pulsed Doppler (PW) in the mitral valve and in the pulmonary vein.

Mitral valve insufficiency (MI) can be assessed by Color Doppler and Continuous Doppler (CW).

The left and right atrial size can be estimated by drawing the atrium’s wall in systole when the atrium is the largest. The left atrial size is an important parameter in assessing the diastolic function and the degree of mitral valve insufficiency.

The outflow tract and the aortic valve will be seen if the probe is angled anteriorly. Blood flow rate in LVOT is recorded with pulsed Doppler (PW). If the area of ​​the PW curve is drawn, VTI (Velocity Time Integral), the stroke of the blood cells under the systolic, is calculated as a measure of contractility.

LVOT area ALVOT = π x r2

The equation for the stroke volume will be:

SV = ALVOT x V TI LVOT.

Aortic insufficiency is seen by color Doppler. Grading of aortic insufficiency and aortic stenosis is done using the continuous Doppler (CW).

The right chamber is usually smaller than the left ventricle. In this intersection, the mobility and size of the right chamber can be estimated. The systolic pressure difference between right ventricle and right atrium is calculated by detecting the maximum speed of tricuspidal insufficiency by means of CW.

ΔPmax = 4 x V TI

Systolic right ventricular pressure, which equals systolic pulmonary pressure in the absence of pulmonary stenosis, is calculated if CVP is added to ΔPmax

PAsyst = ΔPmax + CVP

ΔP = 4V TI 2 (V TI is the maximum rate of tricuspid insufficiency)


Apical Two Chamber View (2Ch)

This image appears if the probe is rotated about 60 ° in relation to the apical four chamber view (slit on the probe directed towards the patient’s left shoulder). Here, the anterior and inferior walls of the left ventricles are visualized, as well as the mitral valve and left atrium.


Apical Two Chamber View with Aorta (3Ch)

Apical two-poster image with aorta is achieved by rotating the probe approximately 120 ° relative to the apical four chamber view (the slit on the probe should be directed towards the patient’s right shoulder). The intersection cuts through the aortic valve, the mitral valve, the anterior part of the septum and the left ventricle’s inferolateral wall. Color doppler and CW can study possible mitral valve and aortic valve insufficiency. With PW in LVOT, V TI LVOT can be calculated and stroke volume.


Subcostal Four Chamber View

This image cuts through the same parts as the apical four chamber image (the slit on the probe should be directed to the left side of the patient). Collection of pericardial fluid can be easily judged from the subcostal view.

Inferior caval vein (vena cava inferior) is seen by turning the probe slightly counterclockwise (the slit upwards). The following estimates can be made:

At normal CVP < 5 mmHg, the inferior caval vein diameter is < 20 mm and in conjunction with valsalva control (sniffing) the diameter decreases by > 50%.

At CVP approximately 10 mmHg, inferior caval vein is < 20 mm and in the case of insufflation /sniffing, the diameter decreases < 50%.

At high CVP > 15 mmHg, inferior caval vein is (IVC) > 20 mm and in the case of insufflation/sniffing, the diameter decreases < 50%.


Hypovolemia

Detecting patients in circulatory shock that is volume dependent is an important issue in intensive care. Echocardiography should always be integrated into the overall clinical picture. From Frank Starling’s curve, it is seen that volume delivery (i v fluids) may be important for a patient in the first part of the curve (preload dependent) or dangerous to another patient on the flat part (preload independent). In suspicion of hypovolemia, static parameters should be supplemented with dynamic measurements to distinguish “responders” from “non-responders”.


Static Parameters

  • IVC diameter < 10 mm may predict a positive response to fluid supply.
  • IVC diameter > 20 mm without breathing variation excludes a responder.
  • A small and hyperkinetic left ventricle is a useful predictor for fluid response.

Dynamic Parameters

  1. 20 % variation in VTI (Velocity Time Integral) during an inhalation exhalation circle discriminates responders from non-responders.
  2. If the increase in VTI (with PW in LVOT) is more than 15% after a bolus infusion of fluid it indicates that the patient is a responder.
  3. In the case of patients with spontaneous breathing or intubated patients who trigger the ventilator, an increase of SV > 12% after PLR (passive leg raising) predicts an increase of SV> 15% after fluid delivery.

When will volume supply be harmful? Examination of PW in the pulmonary veins and mitral valve with a burden of filling pressure can help the assessment. Non-responders: Increase of mitral E or E/A by more than 10%, decrease in S/D ratio and increase of VTI < 10% after bolus infusion of fluid. These measurements indicate a slight change in the stroke volume after fluid supply and a tendency for increased filling pressure.



Reference Values

Women

  • The left ventricle diastolic diameter 4.1-5.1 cm
  • The left atrial surface ≤ 21cm2
  • VTI ≥ 15.5 cm

Men

  • The left ventricle diastolic diameter: 4.6-6.0 cm
  • The left atrial surface ≤ 25 cm2
  • VTI ≥ 14.3 cm

S/D

  •  40-59 years: 0.8-1.6
  • ≥ 60 years: 0.8-2.0

References

  1. Otto: Textbook of Clinical Echocardiography
  2. Arne Olsson: Ekokardiografi
  3. Daniel de Backer : Hemodynamic monitoring using Echocardiography in Critically Ill