Respiratory PFTs & Equations

Question 1

FVC Male

Answer 1

4.8L

Question 2

FVC Female

Answer 2

3.7L

Question 3

FEV1

Answer 3

Forced expiratory volume in 1 second
Volume air forcefully exhaled
Normal 4L TLC 5L

Question 4

FVC

Answer 4

Forced vital capacity

Volume air forcefully exhaled after deep inhalation

Question 5

FEV1/FVC

Answer 5

75-80%

4L/5L

Question 6

FEF 25-75

Answer 6

Measures airflow at midpoint forced expiration

Question 7

DLCO

Answer 7

17-25mL/min/mmHg
Normal >75% predicted
Mild 60-75%
Moderate 40-60%
Severe <40%

Question 8

Static Compliance

Answer 8

Cst = Vt / (Pplat - PEEP)

Normal 60-100mL/cmH2O

Question 9

Dynamic Compliance

Answer 9

Cdyn = Vt / (PIP - PEEP)
Normal 50-100mL/cmH2O
PIP - peak airway pressure

Question 10

Reynold’s Number

Answer 10

Re = pvd/η 
P - density
V - velocity
D - diameter
η  - Viscosity 
Laminar <2,000
Transitional 2,000-4,000
Turbulent >4,000

Question 11

Closing Capacity

Answer 11

CV + RV
30% TLC
↑55% TLC 70yo

Question 12

Fick’s Law

Answer 12

Vgas = (D x SA x ΔP) / T

Diffusion coefficient = gas solubility / MW
Surface area
Partial pressure difference
Membrane thickness

Question 13

PAO2

Answer 13

(PB - PH2O) x FiO2 - (PaCO2/RQ)
RQ = 0.8
Normal 80-100

Question 14

Alveolar-Arterial Oxygen Gradient

Answer 14

PAO2 - PaO2
Normal 5-15mmHg

*Increases w/ age (20-30mmHg)

Directly proportional to shunt
Inversely proportional to PvO2

Question 15

a/A Ratio

Answer 15

PaO2/PAO2

Normal >75%

Question 16

Arterial Oxygen Content

CaO2

Answer 16

(Hgb x 1.34 x SaO2) + (PaO2 x 0.003)
Normal 20.4mL O2 per 100mL blood
CvO2 = 15mL normal extraction rate

Question 17

Oxygen Delivery

Answer 17

DO2 = QT x CaO2 = 1,000mL/min
QT= CO
CaO2 200mL/min/L (20.4 x 10)

Question 18

Oxygen Consumption

Answer 18

VO2 = QT x (CaO2 - CvO2)
= 5L/min x (200mL - 150mL)
= 250mL/min

Question 19

Alveolar CO2 Tension

PACO2

Answer 19

PACO2 = VCO2/VA
PACO2 ≈ PaCO2

VCO2 - total CO2 production
VA - alveolar ventilation

PaCO2 35-45 adequate ventilation
<45 inadequate
<35 excessive

Question 20

P:F Ratio

Answer 20

PaO2/FiO2
Normal 400-500
Mild ARDS <300
Moderate <200 
Severe <100

Question 21

Dalton’s Law

Answer 21

Total pressure = P1 + P2 + P3 + PN

Question 22

Henry’s Law

Answer 22

Amount gas dissolved in liquid = directly proportional to the pressure applied to the gas as it overlies that liquid

Question 23

Ptranspulmonary

Answer 23

Ptp = Palv - Pip

Palveolar
Pintrapleural

Question 24

LaPlace

Answer 24

Pressure = (2 x tension) / radius

*Surfactant impact on surface tension

Question 25

PACO2 vs. PaCO2 vs. ETCO2

Answer 25

PACO2 virtually identical to PaCO2
PACO2 or PaCO2 to ETCO2 gradient normally <5mmHg (2-5mmHg)

Normal PaCO2 35-45mmHg
Normal ETCO2 30-42mmHg

Question 26

Minute Ventilation VE

Answer 26

Sum all Exhaled gas volume in 1min
VE = frequency x Vt ≈ 5L/min
RR x tidal volume

Question 27

Alveolar Ventilation VA

Answer 27

VA = Frequency x (Vt - VD)
RR x (tidal volume - dead space) ≈ 4L/min

Not all Vt participates in gas exchange (conducting zone or dead space VD)

Question 28

Anatomic Dead Space

Answer 28

Conducting zone

Adult 150mL
1/3 or 30% Vt
1mL/lb
2mL/kg

Question 29

Alveolar Dead Space

Answer 29

Non-perfused alveoli

Question 30

Physiologic Dead Space

Answer 30

Anatomic + alveolar dead space

Question 31

Bohr’s Equation

Answer 31

Dead space

VD = Vt x [(PaCO2 - PeCO2) / PaCO2]

Question 32

↑ Dead Space

Answer 32

Upright
Neck extension
Age
PPV
Drugs (anticholinergic)
PE
Hypotension
Emphysema

Question 33

↓ Dead Space

Answer 33

Supine position
Neck flexion
Artificial airway