Formulas Flashcards by Mallory A Prideaux

Ponderal index

PI = [weight(g) x 100] / [crown-heel length(cm)]^3

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Laplace‘s law

P = (2T)/R
P: Pressure
T: Surface tension
R: Alveolar radius

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Anatomic dead space calculation

Vda = [(end tidal CO2 - expired CO2) / end tidal CO2] x tidal volume
Uses Bohr equation

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Physiologic dead space calculation

Vdps = [(PaCO2 - expired CO2) / PaCO2] x tidal volume

Uses Bohr equation

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Ohm’s Law

R = change in P/change in flow
R = P / Q

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Reynolds number

R = (length x viscosity) / (radius)^4 = airway resistance
Low R a/w laminar flow
High R a/w turbulent flow

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Lung compliance

CL = change in volume / change in pressure
CL = V/P

A measure of elasticity or distensibility
Static compliance: Measured during no gas flow, reflects the elastic properties of the lung
Dynamic compliance: Measured during continuous breathing, reflects elastic as well as resistive components

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Lung elastance

E = 1/C

1/compliance

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Time constant

TimeC = Resistance x Compliance
TC = R x CL
Time needed to deflate 63% of volume in lungs

Increased resistance = more Time constants needed
Low in acute RDS
Bigger animals need longer time constants; smaller animals need shorter time constants
3X time constants to equilibrate 95%

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FRC by helium dilution

Initial [He] x V1 = End [He] x (V1 + V2)

V2 = FRC

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Shunt equation

Qp/Qs = (Ao O2 sat - MV O2 sat) / (PV O2 sat - PA O2 sat)

Ao = aorta
MV = mixed venous (SVC)
PV = pulm vein
PA = pulm art

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Oxygenation index

OI = [(MAP x FiO2) / PaO2] x 100

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Alveolar arterial gradient of oxygen

A-a gradient
= [FiO2 (Patm - Ph20)] - (paCO2/0.8) - (paO2)
Patm: atmospheric pressure, usully 760
PH2O: 47 mmHg
0.8: respiratory quotient

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Altitude comparison

(Patm1 - 47) x FiO2(1) = (Patm2 - 47) x FiO2(2)

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Oxygen carrying capacity

CaO2 = [Hb x 1.34 x O2sat] + [0.003xPaO2]
Can leave off the PaO2 because so low

In fetus, hb x 1.37 x O2sat

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Fick’s principal

Measures O2 consumption (VO2)
VO2 = CO (dL/min) x (CaO2 - CvO2)
CO = cardiac output = HR x SV
VO2 = CO x (1.34 x hb x [art sat - ven sat])

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Henderson Hasselbach equation

Hydrogen ion conc = (24 x pCO2) / bicarb conc

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Mean airway pressure

MAP = k(PIP-PEP) x [IT/(IT+ET)] + PEEP

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Cardiac output

CO = HR x SV
CO = systolic BP / total pulm vasc resist (Flow(Q) = P / R)

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Pulmonary vascular resistance

PVR = (mean PA pres - mean LA pres) / Qp
Qp = pulm blood flow

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Systemic vascular resistance

SVR = (mean A pres - mean RA pres) / Qs
Qs = systemic blood flow

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Shunt equation

Qp/Qs = pulm BF/system BF 
Qp/Qs = (Ao O2 sat - MV O2 sat) / (PV O2 sat - PA O2 sat)

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Cerebral blood flow

CBF = CPP/CVR 
(Q = P/R)
CPP = Mean art BP - ICP

OR… CBF = (MAP-JVP)/CVR

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Plasma osmolality

Posm = (2xNa) + (BUN/2.8) + (Gluc/18)

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Sodium deficit

Na def = (Na desired - Na curr) x 0.6 x wt

Anion gap

Na - Cl + HCO3

Fractional excretion of sodium

FeNa = [(Una x PCr) / (UCr x PNa)] x 100 <1% normal 1-2.5% pre-renal >3% intrinsic renal failure

Tubular reabsorption of phosphate

TRP = 1 - [(Uphos x PCr) / (UCr x Pphos)] x100

Estimated GFR

GFR = (0.45 x length(cm)) / plasma Cr | 0.33 for preterm infants instead of 0.45

Renal clearance

Renal CL (mL/min) = (U x V) / P = (Ucr x volume) / Pcr

Exchange equations

Polycythemia Blood vol exchanged = Blood vol x [(Hct obs - Hct desired) / Hct obs] Blood V = wt x 80-100 mL/kg

Volume of distribution

Vd = total amnt drug in body / (plasma drug conc x weight)

Loading dose

Ld = (Vdxpeak concentration)/(SxF) If S (drug active or salt form) and F (drug bioavailable) are not given assume them to be one

Drug clearance

Clearance = Kd x Vd = (dose/interval) / avg steady state conc ``` Kd = elim rate constant Kd = clearance / Vd ```

Steady state of drug

Steady state = infusion rate / (Vd x Kd) Or = infusion rate / Drug clearance

Boyles law

P1 x V1 = P2 x V2 The pressure of a gas decreases as the volume that it is contained in increases Pressure is inversely proportional to volume

Dead space calculation

Vd / Vt = (PaCO2 - PeCO2) / PaCO2

Ventilation/perfusion equation

V/Q = 0 Mucus plug, trapped air = underventilated alveoli Perfusion but no ventilation V/Q = 1 Normal V/Q = infinity No blood flow Ventilation but no perfusion

Poiseuille Equation for laminar flow

R = (8nL)/[pi(r^4)]

Fick’s equation of diffusion

dQ/dt = k x A x dC/dl ``` dQ/dt is rate of diffusion (mL/min) k is diffusion coefficient A is area available for diffusion dC is molecule concentration difference across membrane dl Is length of the diffusion pathway ```

Stroke volume

SV = EDV - ESV

How many kcal/g in carbohydrates?

3.4 kcal/g

How many kcal/g in protein?

4 kcal/g

How many kcal/g in lipids?

9 kcal/g

Amount of RhoGAM to give mother

# vials = volume fetal blood (mL) / 30 mL Round to closest whole number and add one vial

Amount of fetal/maternal hemorrhage

Fetal blood (mL) = % fetal blood cells x 5000 mL

Saturation level for cyanosis (calculation)

O2 sat = HgbO2/(HgbO2+reduced Hgb) | Reduced Hgb is HgbO2 - 3 to 5

GIR

(% dextrose x rate IVF mL/kg/day) / 144

Hardy Weinberg Equilibrium

P^2 + 2pq + q^2 = 1 P - Population of dominant allele Q - population of recessive allele

Minute ventilation

TV x RR

Oxygen delivery

= CO x (1.34 x Hgb x O2 sat)

Elimination half-life

T1/2 = (0.693 x Vd) / clearance

Attributable risk

[a/(a+b)] - [c/(c+d)]

Attributable risk %

(Attributable risk (AR) / [a/(a+b)] ) x 100

False negative (FN)

1 - Sensitivity

False positive (FP)

1 - Specificity

Cumulative Incidence

# New cases in a given time period / Total population at risk

Incidence rate

New cases in a given time period / Person-time years of observation

Likelihood ratios (LR)

LR + test = sensitivity/(1-specificity) LR - test = (1-sensitivity)/specificity

Fetal mortality rate

Fetal deaths/ (Live births+fetal deaths) x 1000

Infant mortality rate

Infant deaths / live births x 1000

Neonatal mortality rate

Neonatal deaths <28 days / live births | X1000

Perinatal mortality rate

(Fetal deaths >28 wks + neonatal deaths <7 days) / (Live births + fetal deaths >28 weeks gestation) X1000

Negative predictive value (NPV)

D/(c+d) | TN / TN + FN

Number needed to treat (NNT)

1 / [(c/c+d) - (a/a+b)] C/C+D is events in control group A/A+B is events in treated group 1/ARR

Odds ratio (OR)

AxD/BxC

Positive predictive value (PPV)

TP / TP + FP

Power

1 - type 2 error

Prevalence

``` # New cases at a point in time / total population at risk X1000 ```

Sensitivity

TP/TP+FN | A/A+C

Specificity

TN/TN+FP | D/B+D

Relative risk (RR)

(A/A+B) / (C/C+D)

Relative risk reduction (RRR)

1- RR

Absolute risk reduction (ARR)

(C/C+D) - (A/A+B)

Intervention Odds

A/B | Odds of adverse outcome in the intervention group

Control odds

C/D | Odds of adverse outcome in control group

Absolute neutrophil count (ANC)

[(%PMN + %bands) x total WBC] / 100

I/T ratio

(bands+metamyelo+myelo) / (PMNs+bands+metamyelo+myelo) | All %

Sample size calculation

N = [2(Za+Z1-b)^2x(SD)^2] / d^2 ``` Za = constant based on alpha error and 1vs2-sided; for 5% 2-sided it is 1.96 Z1-b = constant based on power; for 80% power it is 0.84 SD= estimated standard deviation d = delta or the difference in effect size ```