Evaluation of Shunts

Question 1

In the absence of a left-to-right shunt, the oxygen saturation of the ___________ represents the best measurement of _________ venous O2 saturation (SvO2)

In the presence of a shunt, SvO2 is O2 saturation of the chamber just _______________ to the shunt, that is, ___________________

Answer 1

1. PA; mixed
2. Proximal to the shunt; proximal to the O2 step up

Question 2

True or False
O2 saturation of the SVC is normally lower than O2 saturation of the IVC by few percent points (~5%)

Answer 2

True

Normally, SvO2 = O2 saturation of PA ~ O2 saturation of RV ~ O2 saturation of RA ~ average O2 saturation of SVC and IVC (= [3 × SVC + 1 × IVC] / 4)

O2 saturation of the SVC is normally lower than O2 saturation of the IVC by few percent points (~5%); with the renal O2 extraction being relatively low, the renal venous return increases the IVC O2 saturation.

PA being the chamber where the venous blood achieves its best mixing from all sources

Question 3

A significant left-to-right shunt is characterized by an O2 step-up of >/= ___________ between chambers

O2 step up between SVC and PA _____

Answer 3

5 - 7 % between chambers

8 %

Question 4

Give Fick equation for computation of Qp/Qs

Answer 4

Qp = O2 consumption (ml/min) / (PVO2 - PA O2) x 1.36 x Hgb x 10

Qs = O2 consumption (ml/min) / (SA O2 - MVO2) x 1.36 x Hgb x 10

Qp/Qs = (SA O2 - MV O2) / (PVO2 - PAO2)

PV O2 is equal to SA O2 in the absence of a significant right-to-left shunt or bidirectional shunt.

Question 5

Cases need for Vasoreactivity testing with 100% O2 or vasodilators:

Pulmonary pressure that is ________________ the systemic pressure
PVR ______________ or _______________ the SVR
Left-to-right shunt that has become _____________ with significant right-to-left shunting and ______________

Answer 5

1. > 2/3 systemic pressure
2. > 6 WU or >2/3 SVR
3. Bidirectional, hypoxemia

Question 6

Criteria for a significant response after vasoreactivity testing

Answer 6

Reduction of the mean PA pressure by greater than 10 mmHg to less than 40 mmHg
Reduction PVR by greater than 20% to less than 5 Wood units

In case of a significant response, the patient is treated with pulmonary vasodilators then the shunt is closed.

Question 7

Give Qeffective equation

Answer 7

Q effective = O2 consumption (ml/min) / (PVO2 - MVO2) x 1.36 x Hgb x 10

Q effective= Theoretical right- and left-sided flow had there not been any shunt, in which case PVO2 = SA O2 and PA O2 = MV O2

PVO2 = SaO2
PaO2 = MVO2

Question 8

In ASD, the right-to-left shunt is often determined by ________________ rather than by PA pressure, and hypoxemia may be seen without any significant pulmonary hypertension.

Answer 8

RV failure