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Operating Principles IĆ37
The system determines the physiologic shunt using the following equation:
32
Qst/Qt(T) =
ctO
2
(c)-ctO
2
(a)
ctO
2
(c)-ctO
2
(v)
where ctO
2
(c) = [1.39 x ctHb x (1 - FCOHb - FMetHb)] + (0.00314 x A);
A = [(F
I
O
2
/100) x (pAtm - pH2O)] - {pCO
2
x [1.25 - (0.25 x F
I
O
2
/100)]};
ctO
2
(v) is for a mixed venous sample;
1.39 is the system default value for the oxygen binding factor.
Estimated Shunt
Pulmonary artery blood gases are not always readily available, but there may still
be a need to determine changes in the physiologic shunt. The best alternative
method for reflecting changes in the physiologic shunt is the estimated shunt
[Qsp/Qt(est,T)] value, which is applicable to most hypoxemic patients with
cardiovascular stability.
51
The system determines the estimated shunt using the following equation:
Qst/Qt(est,T)=
ctO
2
(c)–ctO
2
(a)
[ctO
2
(a-v) entered]+ctO
2
(c)–ctO
2
(a)
where ctO
2
(c) = [1.39 x ctHb x (1 - FCOHb - FMetHb)] + (0.00314 x A);
A = [(F
I
O
2
/100) x (pAtm - pH2O)] - {pCO
2
x [1.25 - (0.25 x F
I
O
2
/100)]};
1.39 is the system default value for the oxygen binding factor;
[ctO
2
(a-v) entered] uses 3.5 mL/dL, which is the system default value for the
arterial-venous oxygen content difference.