Formulas Flashcards
Ponderal index
PI = [weight(g) x 100] / [crown-heel length(cm)]^3
Laplace‘s law
P = (2T)/R
P: Pressure
T: Surface tension
R: Alveolar radius
Anatomic dead space calculation
Vda = [(end tidal CO2 - expired CO2) / end tidal CO2] x tidal volume
Uses Bohr equation
Physiologic dead space calculation
Vdps = [(PaCO2 - expired CO2) / PaCO2] x tidal volume
Uses Bohr equation
Ohm’s Law
R = change in P/change in flow R = P / Q
Reynolds number
R = (length x viscosity) / (radius)^4 = airway resistance
Low R a/w laminar flow
High R a/w turbulent flow
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
Lung elastance
E = 1/C
1/compliance
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%
FRC by helium dilution
Initial [He] x V1 = End [He] x (V1 + V2)
V2 = FRC
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
Oxygenation index
OI = [(MAP x FiO2) / PaO2] x 100
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
Altitude comparison
(Patm1 - 47) x FiO2(1) = (Patm2 - 47) x FiO2(2)
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
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])
Henderson Hasselbach equation
Hydrogen ion conc = (24 x pCO2) / bicarb conc
Mean airway pressure
MAP = k(PIP-PEP) x [IT/(IT+ET)] + PEEP
Cardiac output
CO = HR x SV CO = systolic BP / total pulm vasc resist (Flow(Q) = P / R)
Pulmonary vascular resistance
PVR = (mean PA pres - mean LA pres) / Qp Qp = pulm blood flow
Systemic vascular resistance
SVR = (mean A pres - mean RA pres) / Qs Qs = systemic blood flow
Shunt equation
Qp/Qs = pulm BF/system BF Qp/Qs = (Ao O2 sat - MV O2 sat) / (PV O2 sat - PA O2 sat)
Cerebral blood flow
CBF = CPP/CVR (Q = P/R) CPP = Mean art BP - ICP
OR… CBF = (MAP-JVP)/CVR
Plasma osmolality
Posm = (2xNa) + (BUN/2.8) + (Gluc/18)
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