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What is the impact of cardiomegaly on lung function ?

Posted Nov 12 2009 10:01pm

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  and become an illegal occupant 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.

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