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Posts Tagged ‘oxymetery in cath lab’

Interventions in Eisenmenger  syndrome  or severe PAH in  left to right shunt continues to be a major diagnostic issue.The challenge lies   not only in  assessing whether the progression of PAH can be prevented by  blocking the left  to right shunt , but also  to assess  it’s impact  on  survival.

The factors  involved are

  1. Pulmonary artery pressures
  2. Pulmonary blood flow
  3. Pulmonary vascular  resistance
  4. RV function
  5. Co-morbid /general condition of the patient

While cardiologists worry more about LV , surgeons have different issue .  In left to right  shunts with PAH  RV function bothers them more , as the high pulmonary artery pressure may never allow the surgeons to come off the pump , once the decompression provided by ASD/ VSD  is removed

How relevant is Ohm’s Law in complex shunt with leaky valves and bidirectional shunting ?

The fundamental hemodynamic equation  is derived from  Ohm’s law .How relevant  is  Ohm’s law in Eisenmenger  is not clear.  For decades we have been using complicated calculations with many presumed  and assumed parameters.  The calculation of effective pulmonary blood flow in bidirectional shunt may be most complex equation in clinical  cardiology. One can only imagine how one error could amplifies the other.

The hemodynamic equivalent of  Ohm’s law states

R = Pressure / Flow .The current thinking is  If the PVR is between 6-8 it is operable .

Is it really that simple ?

We know pressures  can be measured with a fair degree of accuracy . Flow  and resistance are  subjected to change in a  moment  to moment basis  .They are  determined by a gamut of  neural and humoral factors.

Ironically , we are not yet clear , whether flow determines  the pressure or pressure determine  the flow .

The right heart blood flow can get complicated by not only bi-directional shunt but also  by pulmonary  and tricuspid regurgitation ,

There is a huge perception problem here .  We are tuned  to think ,  reversibilty of PAH is  same as operability  of shunt lesion . Definitively not !  This is the reason why there is  a vast difference in  ultimate outcome  with  little correlation with PVR !

In  Eisenmenger   physiology  , critical decisions  regarding surgery  are made outside  the cath lab 

  • Good clinical  acumen,
  • A meticulous echocardiography
  • Hard parameters  like  pulmonary  artery diastolic pressure and pulse pressure
  • Above all a  harmonious  Cardiologist – Cardiac surgeon team is vital to plan  this  complex surgery

So, now it would seem  cath studies  are  primarily done for  academic pursuit ,  and  it  rarely helps  in genuine decision-making process.

The following table  synthesized in our hospital (Mainly with  clinical data ) can be a useful tool.

Reference: Learnt in the bedside from poor children of India

We had a situation like this   . A patient was  in class 3 or 4  and calculated PVR was less than 6 Wood units what will you do ?

Never give importance to numbers .  These  patients  will 99% of times won’t survive a shunt closure surgery.

Future development

With  the availability of modern drugs like Nitric oxide, prostocyclins, Sildenafil  analogues  medical management has a potential to improve upon surgical results. Unfortunately large studies are not possible in these population . In the surgical front, fenestrated  VSD closures peri-operative intensive nitric oxide   show some promise.

 Final message

I think  we are about to say a  final   good-bye* to oxymetry  ( or even cath study )  in  the  work up of  PAH  due to shunts.

*Still, pressures of  right heart chambers and pulmonary artery  is vital .Echo can not be expected to provide accurate measure of PA pressure .(Even though there some echo studies  available to calculate  qp/qs and PVR non invasive)

Reference

Pulmonary artery pulse pressure : A simple parameter to assess reversibility  of PAH

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