Assessment of the Right heart
The right heart assessment clinically and echocardiographically is not a very important part of mainstream cardiology. In the ICU, however, acute right heart failure is common and assessment of the right heart is sometimes the only way to diagnose pulmonary embolism, a common diagnosis in the critically ill. The presence of RV dysfunction is independently associated with an increased mortality. It is also important to diagnose right heart failure in the ICU as its management is completely different from that of left heart failure.
In ICU echocardiography, the right heart is assessed along three lines:
Detecting enlargement of the RA and the RV
Assessment of RV function
Estimation of pulmonary artery pressures
Detection of RA and RV enlargement
Since the assessment of the right heart does not entail a separate set of views, it is done as part of the usual view sequence. Hence the signs of RA, RV enlargement will be described in the various views.
Parasternal long axis view
If the RV cavity is visualised well on the parasternal long axis view, its largest diameter can be measured with a caliper. If this exceeds 30mm, it indicates a dilated RV. The key however, is getting a plane the right ventricular cavity is well represented. This might be easier to achieve by using a lower intercostal space than the usual PLAX and angulating the beam a little toward the left shoulder. Failure to do this might result in underestimating the RV internal diameter.
Parasternal short axis view
Typically this view shows the round LV with the thin, crescenteric RV cavity anterior and to the right of it hugging the round LV contour. The RV cavity cross section is smaller than the LV cavity cross section.
When the RV dilates, 2 things are noticed. First, the RV cross section becomes as large as, or larger than the LV cross section. Secondly, the RV cavity becomes more rounded with the IVS moving into the LV cavity during part (early diastole) or whole of the cardiac cycle. This causes the typical round cross section of the left ventricle to become "D-shaped".
Apical 4 chamber view
Normally, the right ventricular cavity is smaller than the left ventricular cavity in this view. If the view is frozen in end diastole and the cavities traced with a caliper, the areas of the two ventricular cavities can be recorded. A ratio of RV/LV area of 0.6 to 1.0 suggests mild enlargement of the right ventricle, while a ratio of 1 to 2 suggests severe and >2 suggests extreme enlargement of the RV. However, it is tedious to do and if the RV is not opened out enough by turning the probe to achieve an exact 4-chamber view, the RV area may be underestimated. More practically, the RV can be said to be significantly enlarged if the RV cavity appears to be as large as or larger than the LV cavity on this view. In addition, the RV loses its usual triangular shape and becomes more oval. In severe RV enlargement, the RV apex may extend beyond the LV apex.
Video 1. The RA and RV are grossly dilated compared to the LA and LV
In the subcostal 4 chamber view, a comparison of the areas of the RV and LV cavities can be made as described in the A4C view, either by measurement or visual gestalt. In addition, this view affords a good look at the RV free wall. RV free wall thickness more than 0.5cm at end-diastole suggests RV hypertrophy, which is a response to chronic pressure overload.
The subcostal view also affords a good view of the interatrial septum. Normally the IAS is curved, bulging into the RA. However, in patients with right heart overload, the RA is enlarged and the IAS curves towards the LA (see video).
Video 2. Subcostal view showing IAS bulging into the LA, with ballooning of the floor of the fossa ovalis. The round chamber on the left is the right atrium, with the partially visible left atrium to the right
25% of the population have a probe patent foramen ovale. In such patients, an increase in RA pressures can cause a small right to left shunt through it. To look for this, a CFI box is placed over the IAS in the subcostal view. Flow can usually be appreciated moving away from the probe into the LA (see Fig.4).
Sometimes a contrast echo with 10ml of agitated saline may be needed to demonstrate the right to left shunt.
Technique of Contrast Echocardiography
Contrast echoes are most easily done using agitated saline as a contrast medium. For this 2, 10ml syringes are connected to a 3-way connection which is connected to the central or peripheral line.
9.5ml of saline and 0.5ml of air is taken in one of the syringes. This saline and air is rapidly pushed alternatingly from one syringe to the other through the 3-way, so that microbubbles form in the saline. It is better to use luer lock syringes as the non lock syringes tend to disconnect during the agitation process.
An Echo view is then obtained that permits good visualization of the right and left sided chambers, preferably apical 4 chambered view or subcostal 4 chambered view. The agitated saline is then injected rapidly. The Right atrium and ventricle opacify (becomes white) as the microbibbles stream past the echo window. The left heart, however normally remains black as the microbubbles dissipate within the pulmonary circulation and do not enter the left heart. If there is a right to left shunt, the left heart also opacifies along with the right, distal to the level of the shunt. We do not attempt to trace individual bubbles, but only note the pattern of opacification. A patent foramen ovale is diagnosed if the left atrium opacifies within three cardiac cycles or right atrium opacification. An intrapulmonary shunt takes longer, usually only after 5 cardiac cycles.
In the video below, the right atrium and ventricle have completely opacified with the contrast microbubbles while the left heart remains black.
Video 3. The right sided chambers on the left side of this video are completely white due to microbubbles
A left to right shunt can also be visualized using the same technique provided the machine has a rapid frame rate. The right sided chambers are white due to the microbubbles on the sonogram. If there is a left to right shunt, it will be seen as a bubble free area in the right sided chambers (a dark "space occupying area") as bubble free blood from the left flows into the right side. The level of this space occupying area will depend on the level of the shunt.
Assessment of RV function
A knowledge of RV systolic function may be essential in making hemodynamic decisions, considering thrombolysis in patients with pulmonary embolism and normal blood pressure and in prognostication in RV infarction and chronic cor pulmonale.
A visual gestalt of the contractility of the RV can be made by looking at the 2 major components of RV contraction, excursion of the free wall and the movement of the tricuspid annulus towards the apex. Although many different ways of calculating the ejection fraction of the RV have been described, none is convenient for everyday use.
2 surrogate markers of RV function which have been shown to be reliable are Tricuspid Annular Displacement (TAD) and Tricuspid Annular Peak Systolic Velocity (TAPSV)
In an Apical 4 chambered view, a M-mode cursor line is paced through the lateral tricuspid annulus and a M-mode tracing is obtained and frozen. A sinusoidal line representing the motion of the annulus is noted. Using calipers, the distance from the peak to the trough of this wave is measured (see figure). This distance is the TAD. In this case it is 1.2cm.
A TAD <1.75 suggests RV dysfunction with 80% sensitivity and specificity, and correlates with an Right ventricular EF of <45% measured by radionucleotide ventriculography.
TAD is also affected by LV systolic function and patients with LV dysfunction but normal RV function have TADs between 1.75 and 2cms, while those with normal biventricular function, demonstrate TADs >2.0cms.
Tricuspid Annulus peak Systolic velocity (TAPSV)
Here again, the apical 4 chambered view is chosen, PWD is chosen and Tissue Doppler is activated. The TDI cursor sample volume is placed over the lateral tricuspid annulus in a similar manner to what has been previously described for the mitral valve in Tutorial 6. A doppler trace is obtained and frozen. Normally 2 diastolic excursions below the baseline and one systolic excursion above the baseline are noted. A caliper is used to measure the peak velocity of the systolic excursion. This value is the TAPSV. A TAPSV of <10cm/sec suggests RV dysfunction and correlates with RVEF <50% measured by Simpson's rule method with a sensitivity of 60% and a specificity of 90%.
Assessment of Pulmonary Arterial pressures
The presence of at least a trivial tricuspid regurgitant jet is almost universal. In patients with right heart disease, this increases in severity. Measurement of the velocity off the tricuspid regurgitant jet velocity (v) enables the estimation of pressure gradient (TVpg) between the RV and the RA during systole using the simplified Bernoulli's equation (TVpg = 4v2). Since the RA pressure is measured from central lines in most critically ill patients, it is easy to arrive at the RV systolic pressure (RA pressure + TVpg). In mid systole, in the absence of RVOT or pulmonic stenosis, the pulmonary arterial pressure is the same as the RV systolic pressure.
The tricuspid regurgitation can be assessed in a number of views. The view that can be used most consistently for this purpose is the Apical 4 chambered view. In the A4C view, a color box is placed over the tricuspid valve and the presence of a turbulent jet flowing away from the probe is noted (see video). In a few patients, the TR jet is better visualized in the modified SAX (RV inflow-outflow view) or subcostal 4 chambered views.
Video 4. A broad jet of severe tricuspid regurgitation is seen
CWD is then activated and the CWD line is placed through the center of the jet. To get reliable velocity estimations, the angle of the jet should not be more than 20° out of alignment with the CWD line. A CWD tracing is then obtained and frozen. The peak velocity of the tricuspid jet is measured (see fig.7 )
As seen in the above figure, most machines would automatically calculate the pressure gradient and would display the RV systolic pressure if the value of the RA pressure has already been entered (in this case RA pressure of 5mmHg + TVpg of 27mmHg gives us a RV or PA systolic pressure of 32mmHg).
Normal pulmonary arterial pressure in a person living at sea level has a mean value of 12-16 mm Hg. Pulmonary hypertension is present when mean pulmonary artery pressure exceeds 25 mm Hg at rest or 30 mm Hg with exercise.
Mean pulmonary artery pressure (mPAP) should not be confused with systolic pulmonary artery pressure (sPAP). A systolic pressure of 40 mm Hg typically implies a mean pressure more than 25 mm Hg.
Roughly, mPAP = 0.61•sPAP + 2.
Therefore, if PA systolic pressure is the only PA pressure estimated during an Echo study, A PA systolic pressure >40mmHg indicates pulmonary hypertension.
It is important to ensure that the Doppler gain is not set too high as this could lead to an overestimation of the jet velocity (see above figure).
When the tricuspid regurgitation jet is trivial and the CWD spectrum is suboptimal, an injection of agitated saline solution (see under "Technique of contrast echocardiography" above) into an arm vein enhances the tricuspid regurgitation velocity signal.
It is also important to remember that higher tricuspid velocities do not indicate more severe tricuspid regurgitation. The velocities are often lower in severe cases as the RA pressure is also high and the pressure gradient across the tricuspid valve therefore is smaller. Additionally, in conditions such as RV infarct and acute RV failure where severe TR is seen, the RV is not capable of producing very high RV systolic pressures, leading to a lower velocity jet. Severity of the tricuspid regurgitation is assessed by the area of the regurgitant jet, the width of the jet origin and how far back into the RA it extends.
Mean and diastolic Pulmonary arterial pressures
The mean and end diastolic pressures in the pulmonary artery are assessed by measuring peak and end-diastolic velocities of the pulmonary regurgitant jet.
First a short axis view at the level of the aortic valve is obtained. The probe is adjusted so that the RVOT and the pulmonary valve are visualized well. The CFI box is placed over the pulmonary valve. Pulmonary regurgitation appears as a small jet from the valve moving towards the probe. This is present in 85% of the normal population as well. A CWD cursor line is placed though the center of this jet and a CWD trace obtained and frozen.
Unless severe, it is a faint signal above the baseline with a slow diastolic decay. Calipers are used to make 2 measurements, a peak velocity and velocity at end-diastole. Pressure gradients for both measurements are calculated automatically by the machine using the simplified Bernoulli's equation. The Diastolic and Mean PA pressures can be then calculated as follows.
Diastolic PA pressure (dPAP) = PRend diastolic PG + RA pressure
Mean PA pressure (mPAP) = PRpeak PG + RA pressure
In fig.9, the PRpeak PG is 29.5 and the PRend diastolic PG is 8.3mmHg. Since this patient had a RA pressure (CVP) of 15mmHg, the Mean PA pressure (mPAP) calculated is 44.5mmHg (29.5+15) and the Diastolic PA pressure (dPAP) calculated is 23.3mmHg (8.3+15).
Mean pulmonary arterial pressure can also be calculated from the acceleration time of the RVOT VTI.
It is also possible to calculate the Pulmonary vascular resistance (PVR) with an echo, though that would be out of the scope of this tutorial.
Identifying patterns of RV overload
The two major patterns of RV overload are pressure overload or volume overload.
Volume overload results from Tricuspid or pulmonary regurgitation, ASD, VSD and TAPVC. Volume overload results in enlargement of the Right ventricle and right atrium with little or no increase in free wall thickness. An analysis of the movement of the interventricular septum 9IVS) is important in identifying this form of overload.
Normally, the IVS is oriented and contracts such that it entirely forms a part of the left ventricle. During systole, it thickens and moves towards the center of the left ventricle and during diastole, it moves away from the center of the LV into the RV cavity. In RV volume overload, because the RV intracaviatry pressure is higher tn normal and the duration of systole longer, RV pressure exceeds LV pressure during end systole and early diastole, leading to the IVS being pushed into the LV. This may be maintained during the rest of the diastole. However, at the onset of systole, the sudden increase in LV pressure produced by LV contraction restores the normal transeptal pressure gradient, pushing the IVS towards the RV cavity. This results in IVS flattening and a D-shaped LV only during early diastole.
Pressure overload can be acute or chronic.
Acute pressure overload is a result of massive pulmonary embolism or ARDS (both are forms of acute cor pulmonale - ACP) and is the commonest pattern of RV overload seen in critically ill patients. Echocardiographically, it is indistinguishable from a volume overload picture. RA and RV enlargement is associated with flattening of the IVS in diastole and the absence of RV free wall thickening. As the RV has not had time to hypertrophy, the peak systolic pressures generated rarely exceed 50mmHg in ACP.
Chronic pressure overload is a result of chronic lung diseases such as COPD and ILD, chronic thromboembolism or chronic pulmonary venous hypertension secondary to left heart valvular or myocardial pathologies. In addition to demonstrating RV enlargement, Thickening of the RV free wall is also apparent with increased trabeculation. Increase in interventricular septal thickness and trabeculation also occur but are difficult to make out echocardiographically. Because RV adaptation has taken place, the RV is able to generate very high RV systolic pressures, usually exceeding 50mmHg, sometimes exceeding LV pressures. The interventricular septum therefore is flattened into the LV cavity during the entire cardiac cycle.
Because of the overlap of features with acute overload, it is difficult to diagnose acute on chronic cor pulmonale in the ICU using echocardiography, unless serial RV systolic pressures are available.
All forms of RV overload also affect the LV, reducing diastolic function and necessitating higher filling pressures.
Pulmonary embolism is not uncommon in critically ill patients. It is a complication with a high morbidity and mortality and needs rapid diagnosis and specific treatment. The standard diagnostic algorithms for pulmonary thromboembolism may be difficult to follow in the ICU as they require the patient to be transported for CTPA or V/Q scans, which may not be feasible when they are very sick. Therefore, we may have to rely on the Echo to make a diagnosis before deciding on thrombolysis or catheter based interventions.
The echo picture of pulmonary embolism is that of acute pressure overload described above. In addition, there may be a right to left shunt through a patent foramen ovale. Typically, peak systolic pressures in the pulmonary artery do not exceed 50mmHg unless it has occurred on a background of chronic RV pressure overload.
One also needs to look for evidence of thrombi in transit, in the IVC (fig.9), RA, RV or main pulmonary artery (fig.10). These thrombi are highly mobile and have the appearance of popcorn or a snake (see videos).
Video 5. Short axis view showing mobile thrombus in the RA
Video 6. Subcostal view showing mobile thrombus in the RA
Although they may sometimes be attached to the RA (see video),
Video 7. Thrombus attached to the auricle and wall of the right atrium
The mobile mass comes from en bloc embolization of venous thrombi cast. The presence of such visible thrombi should prompt urgent thrombolysis and the consideration for a catheter based intervention if required.
Therapeutic results are monitored by repeated echocardiography examinations. The next 2 videos demonstrate dissolution of an IVC thrombus and only residual fibrin strand are seen post thrombolysis with Streptokinase.
Video 8. Residual strands of thrombus in the IVC, a longitudinal view
Video 9. Residual strands of thrombus in the IVC, a cross sectional view
The next assessment should be for RV function. The presence of RV dysfunction even in the absence of shock is considered an indication for thrombolysis by some.