Calculations

Alveolar Air Equation: Calculates partial pressure of O2 in Alveoli PAO2 = (([blank_start]Pb[blank_end] - 47)[blank_start]FiO2[blank_end]) - (PaCO2/[blank_start].8[blank_end]) Shortcut: PAO2 = (7 x FiO2) - (PaCO2 +10)

A-a Gradient: Measures difference between aveolar and arterial O2 A-aDO2 = PAO2 - PaO2 25-65 on 100% O2 is [blank_start]Normal[blank_end] 66 - 300 is [blank_start]V/Q[blank_end] mismatch >300 = [blank_start]Shunting[blank_end]

Arterial O2 Content (CaO2) Best measurement of O2 delivered to tissues CaO2 = ([blank_start]Hb[blank_end] x [blank_start]1.34[blank_end] x SaO2) + (PaO2 x [blank_start].003[blank_end]) O2 in red blood cells plus O2 in plasma Normal value is [blank_start]17[blank_end]-[blank_start]20[blank_end] vol% (mL/dL)

Mixed Venous O2 Content (CvO2) Total O2 in venous blood CvO2 = (Hb x [blank_start]1.34[blank_end] x [blank_start]SvO2[blank_end]) + [blank_start]PvO2[blank_end] x .003) Normal [blank_start]12[blank_end] - [blank_start]16[blank_end] vol% CvO2 will [blank_start]decrease[blank_end] when cardiac output decreases SvO2 will decrease also.

Arterial - Venous O2 Content Difference Ca-vO2 - Provides estimate of Tissue O2 consumption Ca-vO2 = CaO2 - CvO2 Normal is [blank_start]4[blank_end] - [blank_start]5[blank_end] vol % Used in Fick equation to calculate [blank_start]Cardiac[blank_end] Output

Cardiac Output Qt Modified [blank_start]Fick[blank_end] Used to calculate Cardiac output when O2 consumption and O2 content differences are known Qt = ([blank_start]VO2[blank_end] / (Ca-vO2([blank_start]10[blank_end])) Normal value is 4-8 L/min VO2 = Qt x Ca-vO2 x 10

Shunt Equation Qs / Qt Portion of cardiac output that is shunted and does not participate in gas exchange Qs/Qt = ((A-aDO2) x .003) / (A-aDO2) x .003) + Ca-vO2 Normal is [blank_start]3[blank_end]-[blank_start]5[blank_end] % Can be estimated: Assuming 5% shunt increase for each 100 torr of A-aDO2

SaO2 Arterial O2 Saturation % of Hb that is bound by O2 calculated by ABG analyzer Measured by hemoximeter or co-ox Large differences in calculated and measured could be because of increased carboxyhemoglobin levels PaO2 is estimated by subtracting [blank_start]30[blank_end] from [blank_start]PaO2[blank_end]

HB / Hct HB is normally [blank_start]12[blank_end] - [blank_start]16[blank_end] g/dl HCT is usually [blank_start]3[blank_end] times HB in percent

Deadspace to Vt Ratio VD/Vt % of gas that doesn't participate in gas exchange Normal is [blank_start]20[blank_end]-[blank_start]40[blank_end]% ((PaCO2 - PeCo2) / [blank_start]PaCO2[blank_end]) x 100 Increased VD/VT indicates deadspace disease like a [blank_start]PE[blank_end]

Normal Values PCO2 Arterial [blank_start]40[blank_end] Range [blank_start]35[blank_end]-[blank_start]45[blank_end] venous [blank_start]46[blank_end] PO2 Arterial [blank_start]97[blank_end] Range [blank_start]80[blank_end] - [blank_start]100[blank_end] Venous [blank_start]40[blank_end] pH Arterial [blank_start]7.4[blank_end] Range [blank_start]7.35[blank_end] - [blank_start]7.45[blank_end] Venous [blank_start]7.35[blank_end] SO2 Arterial [blank_start]98[blank_end]% Range [blank_start]95[blank_end] - [blank_start]100[blank_end]% Venous [blank_start]70[blank_end] - [blank_start]75[blank_end]% HCO3- Arterial [blank_start]24[blank_end] Range [blank_start]22[blank_end]-[blank_start]26[blank_end] Venous [blank_start]24[blank_end] Normal BE [blank_start]0[blank_end] +- [blank_start]2[blank_end] Hb Arteral/Venous [blank_start]15[blank_end] Range [blank_start]12[blank_end] - [blank_start]16[blank_end] CaO2 Arterial [blank_start]20[blank_end] vol% Range [blank_start]17[blank_end]-[blank_start]20[blank_end] vol% CvO2 Venous [blank_start]15[blank_end] vol% Range [blank_start]12[blank_end] - [blank_start]16[blank_end] vol %