VD/VT; Shunts; V/Q mismatch
QS/QT
(The Shunt Equation) [(A-aDO2)(0.003)]/[(A-aDO2)(0.003) + C(a-v)O2 normal: 3-5% *see notecard for written formula*
A-a Gradient Practice Problem
*see notecard*
Alveolar Air Equation Practice Problem
*see notecard*
Practice the shunt equation
*see notecard*
Unknown VD value practice problem
*see notecard*
VD/VT Practice Problem
*see notecard*
VD/VT Ratio Formula
[(PaCO2-PeCO2)/PaCO2] x 100 *see note card for written formula* 1. The PeCO2 is the average PCO2 of the exhaled air that can be measured by a capnograph. 2. An increase in the VD/VT ratio indicates a deadspace producing disease -i.e. pulmonary edema normal: 20-40% -may be up to 60% for mechanically ventilated patients.
Shunt
perfusion without ventilation perfusion- blood flow ventilation- gas exchange
dead space
ventilation without perfusion ventilation- gas exchange perfusion- blood flow
A-a Gradient Formula
A-aDO2= PAO2-PaO2 PAO2 is obtained using the alveolar air equation PaO2 is obtained from the patient's ABG Normal value varies directly with the Patient's FiO2
VD/VT
Deadspace to tidal volume ratio -measures the percentage of the tidal volume that does not participate in gas exchange (deadspace) *ventilation without perfusion
Alveolar Air Equation formula
PAO2= (PB-PH2O)FiO2-(PaCO2/R) *see notecare for written formula if needed* R is normally 0.8 PH2O is normally 47 torr
Alveolar Air Equation
1. Alveolar PAO2 2. Calculates the partial pressure of oxygen (PO2) in the alveoli 3. Normal value varies directly with the patient's FiO2 and barometric pressure (PB)
Equations needed to complete the shunt equation
1. Alveolar PO2 (PA02) 2. A-a gradient 3. CaO2 (arterial oxygen content) 4. CvO2 (mixed venous oxygen content) 5. C(a-v)O2 (arterial venous oxygen content difference
The Shunt Equation (definition)
1. The portion of the cardiac output (QT) that is shunted (QS) and does not participate in gas exchange 2. When measured correctly, the shunt equation is the gold standard for assessment of hypoxia 3. It's accuracy is derived from it's allowance for the oxygen content of mixed venous blood which contributes to the oxygen content of the arterial blood.
Why doesn't all blood participate in gas exchange; where does it go?
1. Thesbian veins -valveless veins in chambers of heart 2. Bronchial veins- blood nourishing bronchial wall (1% in healthy patients. 10% in patients with COPD or bronchiectasis) 3. Congenital Heart Disease (seen mostly in pediatrics) -R to L shunting- allows R heart to eject into the L circulation, bypassing the lungs 4. Shunt through a useless lung (seen mostly in adults) -blood passing through a diseased pneumonic lung or collapsed lung will exchange to gas, and contribute to shunt
A-a Gradient (A-DO2, PA-aO2)
1. measures the difference (gradient) between alveolar and arterial PO2 2. Therapy to imporve distribution of ventilation can be elvaluated by the A-a gradient -IPPB, IS, etc 3. Best done after the patient has been on 100% O2 for 20 minutes or more.
