Calculations

Question 1

P_AO₂ Calculation

Answer 1

[(P_baro-47) x FiO₂] - (PCO₂*1.25)

P_baro in Calgary is 670mmHg

Question 2

P_A-aO₂ Calculation

Answer 2

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.

Question 3

PAO2/ PaO2

Answer 3

PAO2/ PaO2

Normal >0.60

Question 4

P/F Ratio

Answer 4

PaO2/FiO2

Normal 400-500

Question 5

CaO2 Vol%

Answer 5

(Hb x 1.34 x (SO2/100)) + (PO2*0.003)

Normal 16-20

Question 6

CvO2 Vol%

Answer 6

(Hb x 1.34 x (Sv2/100)) + (Pv2*0.003)

Normal 12-15

Question 7

C(a-v)O2

Answer 7

CaO2-CvO2

Normal = 5

Question 8

CcO2

Answer 8

(Hb*1.34) + (P_AO2* 0.003)

Normal = 20

Question 9

VO2 ml/min

Answer 9

CO x Ca-vO2 x 10

Normal 200-300

Question 10

VO2/ml/kg/min

Answer 10

(VO2/ml/min) / Pt body wieght in kg

Normal 3.5-4

Question 11

O2 Del ml/min

Answer 11

CaO2 Vol% * CO * 10

Normal 1000

Question 12

O2 Del/kg/min

Answer 12

(O2 Del ml/min) / Pt weight in Kg

Critical is 8-10

Question 13

O2 ER%

Answer 13

(Ca-vO2/CaO2 vol%) * 100

Normal 20-18

Question 14

Qs/Qt% Classic

Answer 14

(CcO2-CaO2)/ (CcO2-CvO2 vol%)

Normal <10

Question 15

Qt (Fick) L/min

Answer 15

250/ (Ca-vO2*10)

Normal 5-8

Question 16

Normal Hemoglobin Levels

Answer 16

Men 13.8-17.2

Women 12.1- 15.1

Question 17

Compliance Calculation

Answer 17

Complicance (ml/cmH2O)

= Vt / (Pplat-PEEP)

Normal 60 to 100 ml/cm H2O

Question 18

Resistance Calculation

Answer 18

R= (PIP- Pplat)/ Flow

Use L/sec for flow

Question 19

Minute Ventilation Calculation

Answer 19

MV = Rate * Vt

On a ventilator try to aim for 10 x IBW

Question 20

FiO2 Required

Answer 20

FiO2 required = (PaO2 Desired * FiO2 Present) / PaO2 initial ABG

PaO2 Desired is usually established to give a safe SpO2 of between 90% - 95%

Question 21

New PaCO2 Desired

Answer 21

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

Question 22

MV Required

Answer 22

MV required = (PaCO2 Initial * MV) / PaCO2 desired

Question 23

I:E

Answer 23

I:E = Ti/Ti : Te/Ti - given has whole numbers ex: 1:4.5

Question 24

TCT

Answer 24

TCT = 60/Rate

Question 25

Ti

Answer 25

Ti = Vt/Flow

Use L for Vt, and L/sec for flow

Question 26

Te

Answer 26

Te = TCT – Ti

Question 27

Ideal Body Weight​

Answer 27

Men: Kg = 50 +2.3(height in inches – 60)

Women: Kg = 45.5 + 2.3(height in inches – 60)

Question 28

PA-aO2

Answer 28

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

Question 29

Increased P_A-aO₂

Answer 29

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.

Question 30

A–a gradient and Respiratory Effort

Answer 30

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.

Question 31

What RR to Set

Answer 31

Inital MV Goal/ Vt Goal

Question 32

What Should Be You Inital Goal in MV

Answer 32

IBW x 100