Usmle Step 1 Equations 4m1z2y

USMLE Equations Pharmacology Question

Answer

V<J

Amount of drug In body / (Plasma drug concent rail on)

CL

(Rate of elimination of drug) / (Plasma drug concentration)

Half Life

(0.7 * Vd) / CL

Loading dose

( x Vd) IF — Plasma drug concentration * Vd / bioavailability

Maintenance dose

* CL / F

pH

pKa + Log (HC03/[0 03 Pco2)

Km = [S] at ?

[S] at SVmax

Non-competitive inhibitor

Decreases efficacy (Vmax) -> inc 1/Vmax

Competitive inhibitor

Decreases potency (increased Km)—> dec 1/Km

Respiratory Question

Answer

Dead space

Tidat volume " ([PaC02 - Pexp air C02]/PaC02|

Alveolar ventilation

(Tidal volume - Dead space)" Breaths/mln

PA 0 2 =

PA02 = ( Fi02 ' (760 - 47)) - (PaC02 / 0.8) — at room air (21% 02). PA02 = .21 ( 7 1 3 i - P a C 0 2 / 0 . 8 = 150 - PaC02 / 0.8 [1 instead of 0 8 if pt on 100% 02] = 150 - 1 25 - P a C 0 2

A-a gradient

Alveolar 02 - arterial 02 = (at room air) 150mmHg - Pa02 (given)

no mi a I A-a gradient

10mmHa or pt's age / 4

diffusion

D - pressure x surface area x solubility / [membfane thickness x sqrt molec wtj

Winter's formula (HC03PaC02)

PaC02 = 1 5 x [HC03-] + 8 +/- 2

dec Pa02 with normal A-a gradient

hypoventilation (inc PC02) . dec Fi02 (normal PC02)

dec Pa02 with inc A-a gradient

shunt (does not improve •:;ith inc 02) ; — V'Q mismatch or diffusion impairment (improve with inc 021

difference btwn V/Q mismatch and diffusion Impairment

V/Q mismatch •• normal DLCO (diffusion): diffusion impairment dec DLCO (diffusion)

causes of shunting (inc A-a gradient w/ no improvement on 100% 02)

Atelectasis, pulmonary edema. Intra cardiac shunt (RightLeft) due to VSD; ASD: Vascular shunt within the lungs

Cardiology Question

Answer

CO

rate of 02 consumption/{Arterial 02-Venous 02)

CO

SV ' M R

MAP

CO * TPR = 2/3 diastolic + 1/3 systolic

sv

EDV - ESV

EF

SWEDV

R

8n 11 (pi) r ft 4

R in series

R = R1 + R2

R in parallel

1/R = 1/R1 + 1/R2

factors that inc viscosity

polycythemia, high protein (MM), spherocytosis

pulse pressure

systolic - diastolic

Biostats Question

Answer

Sensitivity

TP / (TP+FN) = 1 - FN = TP / all real Ps

Specificity

TNI / (TN+FP) = 1 - FP = TN / all real Ns

IPPV

TP / (TP+FP) = TP / all apparent Ps

INPV

TN / (TN+FN) = TN / all apparent Ns

Odds ratio

OR = (a/b)/ (c/d) = ad / be — where a (risk factor + ds); b (risk factor - ds)r c (- risk factor + ds), d (- risk factor - ds)

Relative risk

|[a/(a+b)J / [c/(c+d)] — where a (risk factor + ds), lb (risk factor - ds), c (risk factor + ds) d (- risk factor - ds) = Incidence of exposed / incidence of unexposed

Attributable risk

|[a/(a+b)] - [c/(c+d)] — where a (risk factor + ds). lb (risk factor - ds). c (risk factor + ds). d (- risk factor - ds) = Incidence of exposed - incidence of unexposed

Positive likelihood ratio

Sens / [1 - Spec] = Sens / FP

Negative likelihood ratio

[1 - Sens) / Spec

accuracy = ?

accuracy = (TP+TN) / (TP+TN+FP+FN) = all accurates / total

IH a rdy-Weinberg equil

p*2 + 2pq + q*2 = 1 p + q = 1

confidence interval

CI = mean +/- 2'slgma »> 95% CI

type I error = ?

b / (b+d)

type II error = ?

c / (a+e)

s

FN / Spec

Renal GFR

Uinulin / PinuJin * V= Cinulin

GFR

Kf[Pgc-Pbs) - (ONCgc-ONCbs)]

RPF

Upah / Ppah * V = ah

RBF

RPF/(1-Hct)

FF

GFFWRPF