REFINE YOUR IMPRESSION OF 📕PaO2 Vs 📗SaO2 Vs 📘CaO2

⭕️Oxygen content ( PaO2 )  is the pressure of oxygen molecules dissolved in blood, and is measured by ABG analysis with units of kPa or mmHg 

⭕️Oxygen saturation ( SaO2 )  is a measure of the percentage of haemoglobin sites that have oxygen bound, commonly measured with a pulse oximeter 

⭕️Oxygen content ( CaO2 )   is the real measure of blood oxygen quantity as it accounts for dissolved and haemoglobin bound oxygen. (i.e. CaO2 directly reflects the TOTAL number of oxygen molecules in arterial blood, both bound and unbound to hemoglobin.  It is given as the volume of oxygen carried in each 100 ml blood (mL O 2 /100 mL). Normal CaO2 ranges from 16 to 22 ml O2/dl.

EXPLANATION:

⭕️Oxygen saturation ( SaO2 ) is expressed as the percentage of haemoglobin-binding sites that are occupied by oxygen, thereby forming oxyhaemoglobin. 

⭕️Arterial blood is normally at 97–98% O 2  saturation (i.e. 98% of the available haemoglobin is combined with O 2 ), whereas venous blood is normally at 74% O2  saturation. 

⭕️O2 constitutes 21% of the atmosphere by volume and atmospheric PO2 is 159 mmHg at sea level . At an alveolar pressure of 104 mmHg, alveolar oxygen diffuses into pulmonary venous blood and raises its O2 content from 15 mL/100 mL to 20 mL/100 mL. Of this amount 19.75 mL is combined with haemoglobin and 0.25 mL is ‘free’ or dissolved in simple solution in the plasma. At this pressure of alveolar O2 , haemoglobin in the arterial blood normally becomes 98% saturated and and 2% of the haemoglobin remains reduced, i.e. free of oxygen.

⭕️PaO2 is determined by alveolar PO2 and the state of the alveolar-capillary interface, not by the amount of hemoglobin available to soak them up. PaO2 is not a function of hemoglobin content or of its characteristics. This explains why, for example, patients with severe anemia or carbon monoxide poisoning or methemoglobinemia can (and often do) have a normal PaO2.

⭕️The most common physiologic disturbance of lung architecture, and hence of a reduced PaO2, is ventilation-perfusion (V-Q) imbalance. Less common causes are reduced alveolar ventilation, diffusion block, and anatomic right to left shunting of blood. 

⭕️Think of PaO2 as the driving pressure for oxygen molecules entering the red blood cell and chemically binding to hemoglobin; the higher the PaO2, the higher the SaO2. 

⭕️ In contrast to the other two variables, CaO2 depends on the hemoglobin content and is directly related to it; Since the dissolved oxygen contributes minimally to CaO2 under physiologic conditions, CaO2 is determined almost entirely by hemoglobin content and SaO2, and is related linearly to either variable.

CaO2 = Hb (gm/dl) x 1.34 ml O2/gm Hb x SaO2 + PaO2 x (.003 ml O2/mm Hg/dl).

#oxygen , #spo2 , #respiration , #O2 , #pulmonology , #OxygenCascade , #anaesthesia 

Reference:

(Chapter 5 of Dr. Martin's book All You Really Need to Know to Interpret Arterial Blood Gases, 2nd edition, published February 1999 by Lippincott Williams & Wilkins, Understanding ABGs & Lung Function Tests,Pocket Tutor, Muhunthan  Thillai ,Keith Hattotuwa )