Pocket Medicine

CONSULTS

FORMULAE AND QUICK REFERENCE

CARDIOLOGY

Fick cardiac output

Oxygen consumption (L/min) = CO (L/min) × arteriovenous (AV) oxygen difference

CO = oxygen consumption/AV oxygen difference

Oxygen consumption must be measured (can estimate w/ 125 mL/min/m², but inaccurate)

AV oxygen difference = Hb (g/dL) × 10 (dL/L) × 1.36 (mL O₂/g of Hb) × (S_aO₂—S_vO₂)

S_aO₂ is measured in any arterial sample (usually 93—98%)

S_vO₂ (mixed venous O₂) is measured in RA, RV or PA (assuming no shunt) (nl ~75%)

PULMONARY

CXR in heart failure

• ↑ cardiac silhouette (in systolic dysfxn, not in diastolic)

• Pulmonary venous hypertension: cephalization of vessels (vessels size > bronchi in upper lobes), peribronchial cuffing (fluid around bronchi seen on end → small circles), Kerley B lines (horizontal 1—2-cm lines at bases), ↑ vascular pedicle width, loss of sharp vascular margins, pleural effusions (~75% bilateral)

• Pulmonary edema: ranges from ground glass to consolidation; often dependent and central, sparing outer third (“bat wing” appearance)

Dead space = lung units that are ventilated but not perfused

Intrapulmonary shunt = lung units that are perfused but not ventilated

NEPHROLOGY

Anion gap (AG) = Na − (Cl + HCO₃) (normal = [alb] × 2.5; typically 12 ± 2 mEq)

Delta-delta (ΔΔ) = [Δ AG (ie, calc. AG - expected) / Δ HCO₃ (ie, 24 - measured HCO₃)]

Urine anion gap (UAG) = (U_Na + U_K) − U_Cl

HEMATOLOGY

✓ PTT q6h after every Δ (t₁⁄₂ of heparin ~90 min) and then qd or bid once PTT is therapeutic

✓ CBC qd (to ensure Hct and plt counts are stable)

Warfarin-heparin overlap therapy

• Indications: when failure to anticoagulate carries ↑ risk of morbidity or mortality

(eg, DVT/PE, intracardiac thrombus)

• Rationale: (1) Half-life of factor VII (3—6 h) is shorter than half-life of factor II (60—72 h);

∴warfarin can elevate PT before achieving a true antithrombotic state

(2) Protein C also has half-life less than that of factor II;

∴theoretical concern of hypercoagulable state before antithrombotic state

• Method: (1) Therapeutic PTT is achieved using heparin

(2) Warfarin therapy is initiated

(3) Heparin continued until INR therapeutic for ≥2 d and ≥4—5 d of warfarin

(roughly corresponds to ~2 half-lives of factor II or a reduction to ~25%)

OTHER