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Posts Tagged ‘ct ratio’

Let ventricle is an elliptical or ovoid chamber .The pattern of LV enlargement can vary considerably in different pathologies. We know a dilated , globular heart is the typical feature of terminal congestive heart failure. But in the early stages of cadiac enlargement there are some distinct differences in the contour. (Aortic stenotic lesions retain the ellipitical shape till late in the course )

LV enlargement due to mitral regurgitation is somewhat different from aortic regurgitation. A globular configuration occurs more often in severe MR than AR. This is due to the fact, the long axis and short axis ratio of LV  is maintained till late in the course  of aortic valve disease . Cardiac long axis enlargement is more pronounced in aortic valve disease than in MR. The AR jet reaches LV  at a higher pressure gradient (Diastolic pressure of aorta) than mitral inflow velocity . (Often mimic physiological flow with an S3)

For a given degree of regurgitant volume AR will cause more cardiac enlargement than MR. In the same note , one should realise  the LA becomes huge in MR which receives high pressure regurgitant jet . Further ,mitral valve disease is more likely to result in early PAH and that results in right sided chamber enlargement giving the cardiac contour a more globular configuration

Is the cardiac contour different in rheumatic and degenerative(Myxamatous) mitral regurgitation ?

Yes , rheumatic MR results in less enlargement of the base of the heart as the fibrotic process restricts and restrains LV and prevents uncontrolled LV dilatation . In fact , giant LV are often  reported in mitral regurgitation due to mitral valve prolapse than rheumatic MR.

Why the configuration of LV important in the management of cadiac failure ?

The globular configuration of LV implies , the papillary muscles are attached in a disadvantaged angle and keep the free wall stress high. Specialized procedures are required to restore the LV shape especially in secondary to mitral annular dilatation. Isolated aortic valve disease rarely require LV remodeling surgeries , even if AVR is done late stages.

What is the maximum dimension of LV reported in cardiac failure ?

The upper limit of normal for LV diastolic dimension is 5.6cms. In MR it often reaches 6-7 cms . The maximum of 10cm has been reported with AR. An LV beyond this level looses it’s elasticity and likely to be incompatible with survival unless LV reduction surgeries like Batista are performed.

Is secondary valvular cardiomyopathy an accepted entity ?

 The  term cardiomyopathy when originally defined decades ago ,  required exclusion of all known cases of cardiac enlargement. But now we have a more liberal working concept , if the LV enlarges disproportionate to the loading conditions of the valvular lesions  , secondary cardiomyopathy is said to be present. If cadiomyopathy sets in,  the cardiac shape invariably takes in a globular configuration irrespective of the valvular lesions. So, the simple parameter of shape of LV in X ray chest can give us a clue regarding the outcome in valvular heart disease.

Further reading

Also read sphericity index by echocardiography A spherical LV can be easily quantified by echocardiography

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Thorax is a rigid bony box with a fixed space.The intra thoracic organs are snugly arranged within the cavity.The two lungs on either side with the heart in the middle fill the major volume of the mediastinum .In physiological conditions the volume of mediastinum remain almost constant , except for the respiratory swings.

heart lungs pulmonary function test dyspnea cardiomegaly ct ratio

It is to be noted the two major organs inside the thorax has a distinct behavioral pattern. Lung a very pressure sensitive structure tend to collapse whenever confronted with external pressure .This is evident in all cases of large pleural effusion (Note :The heart collapses only in a fraction of patients with large pericardial effusion -ie tamponade) . Similarly in any mediastinal syndromes , first the lung function is affected , logics then dictate , the low pressure venous system to get compressed resulting in SVC /IVC syndrome.Finally the right heart chambers may get interfered with .This is due the dynamic intracardiac pressures that resists any compression from exterior.

So, it is obvious , lung function is affected with raised intrathoracic volume or pressure .The increase in intrathoracic volume can be due to any thing .

cardiomegaly massive dyspnea mechanism lvedp

The volume of heart in cardiac failure can increase very significantly .For a fraction raise of CT ratio there is many fold raise in it’s volume.A CT ratio of 75% can cause a huge ” housing & accommodation ” problem for the lungs on either side . As we have discussed , the lung is passive organ has absolutely no other option but to bow down like a touch me not plant . The lung , reduces it’s ventilatory function impairing the already poor exercise capacity .The terminal respiratory units collapse significantly. This collapse is not visible in x rays as there is no intrinsic obstruction within the airways as happens in lung pathology.

The course of events in progressive cardiomegaly is often silent and heart successfully encroaches the the human breathing space until the heart failure is corrected and normal heart size is restored. Complete reversal of heart size may not be possible always !

A new unrecognized mechanism for cardiac dyspnea ?

Yes,the mechanism of cardiac dyspnea always been centered around elevated LVEDP , lung congestion etc and the resultant stimulation of lung receptors.

Now we realise a reduction in the lung ventilatory capacity may also contribute significantly in every patient with cardiac failure and cardiomegaly.

When a person with single pnemonectomy lead a comfortable life what is the big issue of heart compressing few respiratory segments of a patient ?

It is true a single normally functioning lungs is sufficient for living but what we are dealing here is patients with compromised cardiac function.Recruitment or non recruitment of even few respiratory bronchooles may have a bearing on patients symptoms and exercise capacity.

Final message

Cardiomegaly is not an inert consequence of cardiac failure. It can have important functional impact on the pulmonary ventilatory and perfusion capacity .It should be emphasised this mechanical encroachment on the lung space is over and above the hemodynamic effects on pulmonary capillary circulation .

Youngsters should recognise this fact as this offers one more explanation for cardiac dyspnea. This is not often discussed in the clinical classes.

Reference

http://10.1067/mhj.2000.110282

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