Calculations

Question 1

Altitude effect on paO2

Answer 1

=(pB#1 - pH2O) x FiO2#1
=(pB#2 - pH2O) x FiO2#2

B# - barometric pressure in location 1&2
pB at sea level = 760mmHg
pH2O = atmospheric pressure of water = 47mmHg

Question 2

Alveolar-arterial gradient of oxygen

Answer 2

=pAO2 - paO2
=[FiO2x(pB-pH2O)] - (paCO2/R) - paO2
R = 0.8
pB = 760mmHg @ sea level

Question 3

Alveolar gas equation

Answer 3

[FiO2(pB-pH2O)] - (paCO2/R)

Question 4

pH2O

Answer 4

47mmHg

Question 5

R - Respiratory Quotient

Answer 5

0.8 (usually)

Question 6

Alveolar minute ventilation (VA)

Answer 6

(tidal volume - dead space volume) x RR

Question 7

Cardiac Output

Answer 7

=Heart rate x SV
=SBP/Total peripheral vascular resistance

Question 8

Cerebral blood flow

Answer 8

=Cerebral perfusion pressure/
Cerebral vascular resistance

Question 9

Cerebral perfusion pressure

Answer 9

Mean arterial pressure - intracranial pressure

Question 10

Compliance

Answer 10

Change in volume (ml)/
Change in pressure (cmH2O)

Question 11

Dead space, physiologic
(Bohr equation)

Answer 11

=[(arterial CO2 - expired CO2)/
arterial CO2] x tidal volume

Question 12

Elastance (inverse compliance)

Answer 12

Change in pressure (cmH2O)/
Change in volume (ml)

Question 13

Urinary fractional excretion of Na(%)
(FENA)

Answer 13

(Urine [Na]x plasma [Cr]) x100/
(Urine [Cr] x plasma [Na])

Question 14

FENA interpretation

Answer 14

<1% = normal
1-2.5% = pre renal
>3% = intrinsic renal failure

Question 15

GFR

Answer 15

(0.45xheight(cm))/
Plasma Cr (mg/dl)
= ml/min

Preterm: change 0.45 to 0.33

Question 16

Hardy Weinberg Equilibrium

Answer 16

p + q = 1
p^2 + 2pq + q^2 = 1

Question 17

Henderson - Hesselbach equation

Answer 17

[H+] = (24xpCO2)/[HCO3]

Question 18

Laplace’s Law

Answer 18

Distending pressure =
2xsurface tension/radius

Question 19

Mean Airway Pressure

Answer 19

K(PIP - PEEP)x[i-time/(i-time + E-time] + PEEP

K = constant (unspec)

Question 20

Minute ventilation

Answer 20

Tidal volume x RR

Question 21

Plasma osmolality

Answer 21

2x(plasma [Na+] + glucose/18 + BUN/2.8

Glucose mg/dl
BuN mg/dl

Normal 285-295 mOsm/kg

Question 22

O2 content of blood

Answer 22

Bound to Hb + dissolved

Bound = [(1.34mlO2/gHb)xHb(g/dl)xO2 sat
Dissolved = [(0.003mlO2/mmHg/dl)xpaO2 (mmHg)]

Newborn Hb higher affinity - 1.37

Question 23

Oxygen consumption
(Fick’s principle)

Answer 23

COx (arterial O2 content) - (venous O2 content)

CO = dl/min cardiac output
Hb g/dl

Question 24

Oxygen Delivery
(ml/kg/min)

Answer 24

CO x O2 content

Question 25

Oxygenation Index

Answer 25

(MAP x FiO2)x100/post ductal paO2

Question 26

Poiseulle’s law for laminar flow

Answer 26

[Change in pressure x (BIG π) x radius^4] /
[8(length x viscosity)]

Question 27

Ponderal index

Answer 27

Weight(g)x100/[(crown-heel)^3]

Crown and heel in cm
Low PI if asymmetric growth

Question 28

Pulmonary to systemic blood flow ratio
(Qp/Qs)

Answer 28

=PBF/SBF
=[Aorta O2sat - mixed venous O2sat]/[left atrial or pulm venous O2sat - pulm artery O2sat]

Pulse ox = aorta O2sat
No lung disease, Pulm O2sat = 100%

Question 29

Pulmonary vascular resistance

Answer 29

(Mean pulmonary artery pressure - mean left atrial pressure)/PBF

Question 30

Renal clearance (ml/min)

Answer 30

(UxV)/P
U = urinary [solute] (mg/dl)
V = urinary volume/length of time collected (ml/min)
P = plasma [solute] (mg/dl)

Question 31

Resistance

Answer 31

Change in pressure (cmH2O)/
Change in flow (L/sec)

Question 32

Saturation level for cyanosis

Answer 32

= % O2sat
= HbO2/(reduced Hb + Hb O2)

Question 33

Shunt equation, intrapulmonary

Answer 33

=Qs/Qtotal

Qs = shunted blood
Qs = O2 content pulm cap - O2 content systemic arterial

Qtotal = O2 content pulm cap - O2 content mixed venous

O2 content pulm cap: (Hbx1.34xSAlveolar)+(PAlv760x0.003)

Question 34

Systemic vascular resistance

Answer 34

(mean aortic pressure - mean right atrial pressure)/SBF

Question 35

Sodium deficit

Answer 35

[Na+ desired - current (mEq/L)] x 0.6 x weight (kg)

0.6 = volume distribution (L/kg)

Question 36

Time constant

Answer 36

Resistance (cmH2O/ml/sec) x compliance (ml/cmH2O)

Question 37

Free water deficit

Answer 37

4ml x weight x [desired change in serum Na]

Question 38

Tubular reabsorption of phosphate

Answer 38

(1-[(urine Px serum Cr)/(serum Px urine Cr)])x 100

Question 39

Volume transfusion

Answer 39

Desired ∆hct x 1.6 x weight (kg)