1833 APPENDIX E Formulas Cerebral/Neurologic Formulas Intracranial pressure (ICP) [Normal: <20 cm H 2 O, <15 mmHg] Cerebral perfusion pressure (CPP) = MAP – ICP [Normal: 70–100 mmHg] Cerebral vascular resistance (CVR) [Normal: 1.5–2.1 mmHg/100 g/min/mL] Cerebral blood flow (CBF) = CPP/CVR Hemodynamic Formulas [Normal: 75 mL/100 g gray matter/min] [Normal: 45 mL/100 g white matter/min] Jugular bulb saturation (S jv O 2 ) [Normal: 55–70%] Cerebral metabolic rate (CMRO 2 ) = (CBF)(CaO 2 – C jv O 2 ) [Normal: 3–3.5 mL/100 g/min] Cerebral oxygen extraction = CMO (CBF)(CaO ) CaO CO CaO 2 2 jv 2 2 2 = - Pulse pressure = systolic BP – diastolic BP Mean arterial pressure (MAP) = SBP 2(DBP) 3 + [Normal: 70–105 mmHg] Central venous pressure (CVP) [Normal: 0–8 mmHg] Pulse pressure variation = PPV = (PPmax – PPmin)/PPmean (Over a respiratory cycle or other period of time; Normal: ≤10%) Stroke volume variation = SVV = (SVmax – SVmin)/SVmean (Over a respiratory cycle or other period of time; Normal: ≤10%) Mean pulmonary artery pressure ( ) PA [Normal: 10–20 mmHg] Pulmonary artery occlusion pressure (PAOP) [Normal: 4–12 mmHg] Cardiac output (CO) = Stroke volume (SV) × Heart rate (HR) [Normal: 4–8 L/min] Cardiac index (CI) = CO BSA [Normal: 2.5–4.0 L/min/m 2 ] Pulmonary vascular resistance (PVR) = (PA PAOP)80 CO - [Normal: 150–250 dyne/s/cm −5 ] Pulmonary vascular resistance index (PVRI) = (PA PAOP)80 CI - [Normal: 100–240 dyne/s/cm −5 /m 2 ] Systemic vascular resistance (SVR) = (MAP CVP)80 CO - [Normal: 800–1,200 dyne/s/cm −5 ] Systemic vascular resistance index (SVRI) = (MAP CVP)80 CI - [1,300–2,900 dyne/s/cm −5 /m 2 ] Stroke volume index (SVI) = CI HR [Normal: 40 ± 7 mL/beat/m 2 ] Right ventricular stroke work index (RVSWI) = SVI( PA – CVP)(0.0136) [Normal: 6–10 gm.meter/m 2 per beat] Left ventricular stroke work index (LVSWI) = SVI(MAP – PAOP) (0.0136) [Normal: 43–56 gm.meter/m 2 per beat] Arterial O 2 content (CaO 2 ) = O 2 combined with hemoglobin + O 2 dissolved in the plasma [1 g Hb binds 1.36 mL O 2 ] = (1.36)(Hb)(SaO 2 ) + 0.0031(PaO 2 ) [Normal: 20 mL O 2 /dL] Mixed venous O 2 saturation (S vO 2 ) [Normal: 75%] Mixed venous O 2 content (C vO 2 ) = (1.36)(Hb)(S vO 2 ) + 0.0031(PvO 2 ) [Normal: 15 mL O 2 /dL] O 2 delivery (D . O 2 ) = CO × CaO 2 × 10 [Normal: 600–1,000 mL O 2 /min] Oxygen delivery indexed (DO 2 I) = CI × CaO 2 × 10 [Normal: 500–600 mL/min/m 2 ] O 2 consumption (V . O 2 ) = CI(CaO 2 – C vO 2 ) [Normal: 110–150 mL/min/m 2 ] O 2 extraction ratio = (CaO CvO) CaO 2 2 2 - [Normal: 25%] Respiratory Formulas OXYGENATION Fraction of inspired O 2 (FIO 2 ) [Range: 0.21–1.0] Respiratory quotient (R) = VCO 2 expired/VO 2 inspired [Normal: 0.8] Barometric pressure (PB) [760 mmHg at sea level] LWBK1580-App-E_p1833-1835.indd 1833 29/07/17 7:28 AM
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Formulas - examdev.theaba.orgrequired according to the aminoglycoside clearance: Aminoglycoside clearance = (C cr)(0.6) + 10 where C cr = creatinine clearance in mL/minute. In order
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1833
Appendix
EFormulasCerebral/Neurologic Formulas
Intracranial pressure (ICP)[Normal: <20 cm H2O, <15 mmHg]
[Normal: 3–16 mmHg on room air; 25–65 mmHg on 100% O2]
VEntilatiOnPartial pressure of arterial CO2 (PaCO2)
[Normal: 46 mmHg]Partial pressure of alveolar (expired) CO2 (PetCO2)Dead-space ventilation (VD): Portion of VT that does not par-ticipate in gas exchange
VD = anatomic dead space + physiologic dead space
[Normal: 150 mL]Engelhoff modification of the Bohr formula for dead space
From Halperin Ml, goldstein MB. Fluid, Electrolyte and Acid-Base Emergencies. Philadelphia, Pa: WB Saunders; 1988.
Infectious Diseases Formulas
antibiOtic KinEticsThe volume of distribution (VD) of an antimicrobial is calcu-lated as:
VA
CDp
=
where A = total amount of antibiotic in the body and Cp = antibiotic plasma concentration.
Repetitive dosing of antibiotics depends on the principle of minimal plasma concentrations (Cmin):
CD
(V )(2 1)minD
=−n
where D = dose and n = dosing interval expressed in half-lives.
The plasma concentration at steady state (Css) of an antimi-crobial can be estimated utilizing the following formula:
CDose per half-life
(0.693)(V )ssD
=
antibiOtic adjustmEntsRenal dysfunction in critically ill patients is common. In those patients receiving aminoglycosides, dosage modification is required according to the aminoglycoside clearance:
Aminoglycoside clearance = (Ccr)(0.6) + 10
where Ccr = creatinine clearance in mL/minute.In order to estimate the creatinine clearance, the Cockcrof
and Gault formula is utilized:
Ccr (mL/min) = (140 age) weight
Cr 72− ×
×
where Cr = serum creatinine in mg/dL. Another modification to this formula is the Spyker and Guerrant method:
Ccr (mL/min) = (140 age) (1.03 0.053 Cr)
Cr− × − ×
LWBK1580-App-E_p1833-1835.indd 1835 29/07/17 7:28 AM