How to calculate pulmonary vascular resistance without even touching a catheter ?
Posted Dec 27 2010 1:19pm
Calculating the pulmonary vascular resistance (PVR) has been a big head ache for all those involved in pediatric and (for many )in adult cardiology as well . The complex formulas , the delicate oximeter samples, the catheters, a sick child , an arrhythmia prone right ventricle , restless staff nurses , and finally the mathematics ! all make it a dreaded exercise .
Echo is a great physiological tool . . . It is now been used over 50 years . It is our earnest belief , Doppler can measure the flow and pressure any where within the heart however dynamic the chambers may be !
Then ,why can’t we have a simple formula by this non invasive method to calculate PVR ?
Yes , Dr Abbas et all from the desert hospital of Arizona raised this question and reported a new equation to calculate PVR.
Their hypothesis is as simple as this . . .
Pressure = Resistance X Flow
Resistance = Pressure /Flow
Pulmonary vascular resistance = PA Pressure/PA FLOW
Substitute PA pressure with TR jet
Substitute PA flow by RVOT VTI ( Velocity time integral )
And we get
This is the simplest way to arrive at PVR at the bedside .
Is it validated ?
Then,why it is not being followed widely ?
It is a too simple method to use ! That is the biggest excuse ! We are tuned to think , a complex parameter can not be measured in a simple manner . Any thing simple must be wrong !
But the reality is . . .
Cath calculations are much more complex with so many variables which can get terribly wrong .
The irony about this hypothetical science of PVR is , we do not know which is gold the standard ? In fact , none can be a standard . So , to label PVR derived by echo , as an inferior modality can not be accepted .It is all the more funny , as we are trying to define a new formula with the help of flawed and battered parameter namely the cath derived PVR .
Abbas’s formula is indeed a realistic way of arriving at PVR by echocardiogram. If only we measure it routinely /serially in as many patients as we can , a new data base will be created .Which can later be proven as a fact.It is suggested every cath lab should try to validate this formula.