Calculations Flashcards
PAO2 Calculation
[(Pbaro-47) x FiO2] - (PCO2*1.25)
Pbaro in Calgary is 670mmHg
PA-aO2 Calculation
PAO2- PaO2
Normal in a young adult >4 (5-10)
The A–a gradient will increase with age. For every decade a person has lived, their A–a gradient is expected to increase by 1 mmHg.
A conservative estimate of normal A–a gradient is less than [age in years/4] + 4.
PAO2/ PaO2
PAO2/ PaO2
Normal >0.60
P/F Ratio
PaO2/FiO2
Normal 400-500
CaO2 Vol%
(Hb x 1.34 x (SO2/100)) + (PO2*0.003)
Normal 16-20
CvO2 Vol%
(Hb x 1.34 x (Sv2/100)) + (Pv2*0.003)
Normal 12-15
C(a-v)O2
CaO2-CvO2
Normal = 5
CcO2
(Hb*1.34) + (PAO2* 0.003)
Normal = 20
VO2 ml/min
CO x Ca-vO2 x 10
Normal 200-300
VO2/ml/kg/min
(VO2/ml/min) / Pt body wieght in kg
Normal 3.5-4
O2 Del ml/min
CaO2 Vol% * CO * 10
Normal 1000
O2 Del/kg/min
(O2 Del ml/min) / Pt weight in Kg
Critical is 8-10
O2 ER%
(Ca-vO2/CaO2 vol%) * 100
Normal 20-18
Qs/Qt% Classic
(CcO2-CaO2)/ (CcO2-CvO2 vol%)
Normal <10
Qt (Fick) L/min
250/ (Ca-vO2*10)
Normal 5-8
Normal Hemoglobin Levels
Men 13.8-17.2
Women 12.1- 15.1
Compliance Calculation
Complicance (ml/cmH2O)
= Vt / (Pplat-PEEP)
Normal 60 to 100 ml/cm H2O
Resistance Calculation
R= (PIP- Pplat)/ Flow
Use L/sec for flow
Minute Ventilation Calculation
MV = Rate * Vt
On a ventilator try to aim for 10 x IBW
FiO2 Required
FiO2 required = (PaO2 Desired * FiO2 Present) / PaO2 initial ABG
PaO2 Desired is usually established to give a safe SpO2 of between 90% - 95%
New PaCO2 Desired
New PaCO2 Desired = Present PaCO2 – [ (pH Goal – pH Present) / 0.01}
pH Goal: is usually established with the physicians orders but should be above 7.25
Note: this equation is used usually to correct for accidosis and is only an approximation
MV Required
MV required = (PaCO2 Initial * MV) / PaCO2 desired
I:E
I:E = Ti/Ti : Te/Ti - given has whole numbers ex: 1:4.5
TCT
TCT = 60/Rate
Ti
Ti = Vt/Flow
Use L for Vt, and L/sec for flow
Te
Te = TCT – Ti
Ideal Body Weight
Men: Kg = 50 +2.3(height in inches – 60)
Women: Kg = 45.5 + 2.3(height in inches – 60)
PA-aO2
A measure of the difference between alveolar concentration of oxygen and the arterial concentration of oxygen.
It is used to diagnose the source of hypoxemia. The measurement helps isolate the location of the problem as either intrapulmonary (within the lungs) or extrapulmonary (elsewhere in the body).
Ex. In high altitude arterial oxygen (PaO2) is low due to the fact that alveolar oxygen PAO2 is also low, where with V/Q mismatch (pulmonary embolism or right to left shunt oxygen is not effectively transferred for alveoli
Increased PA-aO2
An increased PA-aO2 suggests a defect in diffusion, V/Q mismatch, or right to left shunt
A high A–a gradient could indicate a patient breathing hard to achieve normal oxygenation, a patient breathing normally and attaining low oxygenation, or a patient breathing hard and still failing to achieve normal oxygenation.
If lack of oxygenation is proportional to low respiratory effort, then the A–a gradient is not increased; a healthy person who hypoventilates would have hypoxia, but a normal A–a gradient.
A–a gradient and Respiratory Effort
CO2 is easily exchanged in the lungs and a low PCO2 directly correlates with high minute ventilation; therefore a low PaCO2 indicates that extra respiratory effort is being used to oxygenate the blood.
A low PaO2 indicates that the patient’s current minute ventilation (whether high or normal) is not enough to allow adequate oxygen diffusion into the blood.
Therefore, the A–a gradient essentially demonstrates a high respiratory effort (low arterial PCO2) relative to the achieved level of oxygenation (arterial PO2).
At an extreme, high CO2 levels from hypoventilation can mask an existing high A-a gradient. This mathematical artifact makes A-a gradient more clinically useful in the setting of hyperventilation.
What RR to Set
Inital MV Goal/ Vt Goal
What Should Be You Inital Goal in MV
IBW x 100
