Calculations Flashcards
Altitude effect on paO2
=(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
Alveolar-arterial gradient of oxygen
=pAO2 - paO2
=[FiO2x(pB-pH2O)] - (paCO2/R) - paO2
R = 0.8
pB = 760mmHg @ sea level
Alveolar gas equation
[FiO2(pB-pH2O)] - (paCO2/R)
pH2O
47mmHg
R - Respiratory Quotient
0.8 (usually)
Alveolar minute ventilation (VA)
(tidal volume - dead space volume) x RR
Cardiac Output
=Heart rate x SV
=SBP/Total peripheral vascular resistance
Cerebral blood flow
=Cerebral perfusion pressure/
Cerebral vascular resistance
Cerebral perfusion pressure
Mean arterial pressure - intracranial pressure
Compliance
Change in volume (ml)/
Change in pressure (cmH2O)
Dead space, physiologic
(Bohr equation)
=[(arterial CO2 - expired CO2)/
arterial CO2] x tidal volume
Elastance (inverse compliance)
Change in pressure (cmH2O)/
Change in volume (ml)
Urinary fractional excretion of Na(%)
(FENA)
(Urine [Na]x plasma [Cr]) x100/
(Urine [Cr] x plasma [Na])
FENA interpretation
<1% = normal
1-2.5% = pre renal
>3% = intrinsic renal failure
GFR
(0.45xheight(cm))/
Plasma Cr (mg/dl)
= ml/min
Preterm: change 0.45 to 0.33
Hardy Weinberg Equilibrium
p + q = 1
p^2 + 2pq + q^2 = 1
Henderson - Hesselbach equation
[H+] = (24xpCO2)/[HCO3]
Laplace’s Law
Distending pressure =
2xsurface tension/radius
Mean Airway Pressure
K(PIP - PEEP)x[i-time/(i-time + E-time] + PEEP
K = constant (unspec)
Minute ventilation
Tidal volume x RR
Plasma osmolality
2x(plasma [Na+] + glucose/18 + BUN/2.8
Glucose mg/dl
BuN mg/dl
Normal 285-295 mOsm/kg
O2 content of blood
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
Oxygen consumption
(Fick’s principle)
COx (arterial O2 content) - (venous O2 content)
CO = dl/min cardiac output
Hb g/dl
Oxygen Delivery
(ml/kg/min)
CO x O2 content
Oxygenation Index
(MAP x FiO2)x100/post ductal paO2
Poiseulle’s law for laminar flow
[Change in pressure x (BIG π) x radius^4] /
[8(length x viscosity)]
Ponderal index
Weight(g)x100/[(crown-heel)^3]
Crown and heel in cm
Low PI if asymmetric growth
Pulmonary to systemic blood flow ratio
(Qp/Qs)
=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%
Pulmonary vascular resistance
(Mean pulmonary artery pressure - mean left atrial pressure)/PBF
Renal clearance (ml/min)
(UxV)/P
U = urinary [solute] (mg/dl)
V = urinary volume/length of time collected (ml/min)
P = plasma [solute] (mg/dl)
Resistance
Change in pressure (cmH2O)/
Change in flow (L/sec)
Saturation level for cyanosis
= % O2sat
= HbO2/(reduced Hb + Hb O2)
Shunt equation, intrapulmonary
=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)
Systemic vascular resistance
(mean aortic pressure - mean right atrial pressure)/SBF
Sodium deficit
[Na+ desired - current (mEq/L)] x 0.6 x weight (kg)
0.6 = volume distribution (L/kg)
Time constant
Resistance (cmH2O/ml/sec) x compliance (ml/cmH2O)
Free water deficit
4ml x weight x [desired change in serum Na]
Tubular reabsorption of phosphate
(1-[(urine Px serum Cr)/(serum Px urine Cr)])x 100
Volume transfusion
Desired ∆hct x 1.6 x weight (kg)
