Cardio Pulmonary Equations

Pulmonary to Systemic Blood Flow Ratio (Qp:Qs Equation):

Qp:Qs = Sat (aorta) - Sat(SVC)

Sat (pulm venous) - Sat (PA)

This equation can be used to determine the relative blood flow between the body and the

lungs. The goal in any patient is for this ratio to approach 1, so there is equal blood flow to the lungs and body. In patients who have all of their pulmonary blood flow supplied by a shunt from the aorta (i.e. Norwood after stage 1 repair), you can use this equation to help determine the ideal oxygen saturation for a patient. For these patients:

Sat (aorta) = arterial oxygen saturation (as measured from ABG)

Sat(SVC) = mixed venous sat, which in this case should be measured

before the right atrium as mixing occurs from pulmonary venous return, artificially elevating the MVS.

Sat(pulm venous) = as we usually cannot measure this we assume

that, with healthy lungs the blood will be fully oxygenated, i.e. =100%

Sat (PA) = pulmonary artery saturation, which in a patient whose entire

pulmonary blood flow comes from a shunt from the aorta, should equal the Sat(aorta).

Example:

A patient POD#3 s/p Norwood, relatively stable has oxygen saturations around 80% (by pulse ox, correlating with gases). MVS from a jugular line is 60%. What is this patient's Qp:Qs?

Qp 80%-60% = 20 = 1

Qs 100-%-80% 20

Oxygen Content (CxO₂) (for any sample of blood)

CxO₂ = 1.34x [Hgb] x (O₂ sat) + 0.003PxO₂

Where:

[Hgb] is the concentration of hemoglobin in gm/dl

O₂Sat is the oxygen saturation of the specimen of blood

PxO₂ is the partial pressure of oxygen (in mmHg) of the sample of blood

Fick Principle

VO₂ = (CaO₂ - CvO₂) * Q

Where:

VO₂ is the oxygen consumption

CaO₂ is arterial oxygen content

CvO₂ is venous oxygen content

Q is cardiac output

The following equations can be used in solving the Fick Principle, but give you values based on patients body size

Oxygen availability (DO₂) (620+/-50 ml/min/m2)

DO₂= CaO₂*CI*10

CI = cardiac index = CO/Body surface area

Oxygen Consumption (VO₂) (120-200ml/min/m2)

VO₂ = CI*avDO₂*10

avDO₂ = DO₂ (arterial blood) - DO₂ (venous blood)

Oxygen Extraction (26 +/- 2 %)

O₂ ext = (avDO₂/CaO₂)*100

Oxygenation Index

OI = MAPx(FiO₂x100)/PaO₂

Where:

MAP is the mean airway pressure

FiO₂ is the set fraction of inspired oxygen the patient is receiving

PaO₂ is the partial pressure of oxygen (in mmHg) from an arterial blood gas

Hospital Disclaimer

THE INFORMATION CONTAINED IN THIS SITE IS NOT INTENDED NOR IMPLIED TO BE A SUBSTITUTE FOR PROFESSIONAL MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF YOUR PHYSICIAN OR OTHER QUALIFIED HEALTH PROVIDER PRIOR TO STARTING ANY NEW TREATMENT OR WITH ANY QUESTIONS YOU MAY HAVE REGARDING A MEDICAL CONDITION. NOTHING CONTAINED IN THE SITE IS INTENDED FOR MEDICAL DIAGNOSIS OR TREATMENT.

This page was created by Laura M. Ibsen, M.D. for the use of Pediatric Residents in training. Comments or suggestions should be forwarded to Dr. Ibsen at ibsenl@ohsu.edu