Dusan Hanidziar, MD

Department of Anesthesiology

Tufts Medical Center

O2 transportO2 physical solubilityHbO2 saturationHbO2 dissociation curveP502,3-diphosphoglycerateHb as a buffer

O2 transport

• Oxygen delivery (DO2) is a function of:

1. Cardiac output

2. O2 content in arterial blood

DO2 = Q x CaO2

Q=cardiac output

Q=stroke volume x heart rate

CaO2=oxygen content in arterial blood

CaO2= (1.34 x Hb x SaO2) + (0.003 x PaO2)

Adapted from Marino PL: ICU book, 2007

O2 transport

• O2 is carried in the blood in two forms:

1. bound to hemoglobin within erythrocytes

2. dissolved in plasma

• 1 L of arterial blood contains ~ 200 mL O2

• Arterial and venous blood together contain ~ 800 mL O2

Adopted from Marino PL: ICU book, 2007

• O2 is poorly soluble in plasma (3 mL O2/ 1 L plasma at 100 mm Hg PO2)

Dissolved O2= 0.003 x PaO2

0.003=solubility coefficient

• 100% FiO2 (PaO2 500-600 mmHg) increases amount of dissolved O2 to ~18 mL/L plasma

O2 physical solubility

Adapted from Marino PL: ICU book 2007

• SaO2

SaO2 = HbO2/ (HbO2 + reduced Hb + COHb + MetHb)

obtained by ABG, 97.5% at 100 mmHg PO2

• SpO2

SaO2 = HbO2/ (HbO2 + reduced Hb)

obtained by pulse oximetry

• SvO2

SvO2 = HbO2/ (HbO2 + reduced Hb + COHb + MetHb)

obtained by VBG, 75% at 40 mmHg PO2

Oxygen saturation of hemoglobin

Hb + 4O2 ↔ nH+ Hb(O2) 4

Adapted from Barash PG et al: Clinical Anesthesia, 2006

O2 partial pressures

• Partial pressure of oxygen drops as oxygen moves from inhaled air to tissues

• O2 tension clinically measured by ABG (normal PaO2 >80 mm Hg) or VBG (40-50 mm Hg)

West JB: Respiratory Physiology 2012

HbO2 dissociation curve

• Rightward shift (by ↑CO2, ↑H+, ↑Temp, ↑2,3-DPG) facilitates O2 unloading in the tissues

• Permissive hypercapnia during GA may improve tissue oxygenation and protect from stroke, MI, AKI

Adapted from Miller RD: Basics of Anesthesia, 2011

• End-product of erythrocyte metabolism, located in the central cavity of hemoglobin

• 2,3-DPG promotes release of O2 molecules from Hb

https://iwasa.hms.harvard.edu/project_pages/hemoglobin.html

• High altitude, COPD, CHF increase 2,3-DPG content in erythrocytes

2,3-diphosphoglycerate (2,3-DPG)

Adapted from West JB: Respiratory Physiology 2012

• Amino acid residues (-COO-, -NH2) of Hb bind H+ generated by hydration of CO2 in erythrocytes

• Reduced hemoglobin is a better buffer than oxygenated Hb

Hemoglobin as a buffer

Adapted from Marino PL: ICU book, 2007 and West JB: Respiratory Physiology 2012