The 15 equations of IRM

Question 1

The respiratory quotient

Answer 1

VCO2/VO2
V = volume
Usually 0.8 with a mixed diet
1 = carbs only
Can be greater than 1 during exercise

Question 2

Gas flow and speed

Answer 2

Flow = speed x area
V(dot) = u x A
As area increases, speed must decrease in order to maintain the same flow rate

Question 3

Dalton’s law of partial pressures

Answer 3

Total Pressure = P1 + P2 + P3 …….

Px = the pressure of an individual gas

Question 4

Dead space

Answer 4

Physiological deadspace = anatomic dead space + alveolar dead space

Question 5

Ventilation

Total and alveolar

Answer 5

VE = VT x f
VA = (VT-VD) x f
Total ventilation = tidal volume x frequency

Question 6

Boyles Law

Answer 6

P1 x V1 = P2 x V2

Question 7

Compliance

Answer 7

compliance = (change in volume/change in pressure)

Question 8

Laplace’s Law

Answer 8

P = 2T/r
Pressure = 2tenstion x radius
Smaller bubbles require a greater internal pressure to keep them from collapsing

Question 9

Pouseille’s Equation

Answer 9

Flow = change in pressure x pi radius^4 / 8Ln
L = length n = viscocity
Airflow resistance = 1/r^4

Question 10

Alveolar Gas Equation

Answer 10

PAO2 = PIO2 -(PaCO2/R)
R = respiratory quotient

Question 11

Henry’s Law

Answer 11

Concentration of gas = partial pressure x solubility coefficient

Question 12

Graham’s law

Answer 12

Diffusibililty of gas = square root of molecular weight

Question 13

Diffusibility (Vdot) of a gas

Answer 13

Vdot = d x (A/T) x (p1-p2)

Question 14

Diffusion constant or Lung transfer factor

Answer 14

DL = V/(p1-P2)

Question 15

Henderson-Hasslebach equation

Answer 15

ph = pK - log(A-/HA)

Question 16

… and for HCO3’ buffer

ph =

Answer 16

pH = pK’ + log(HCO3-/PCO2 x solubility)