Oxygen Content Equation and Oxygen Transport 1

The Key to Blood Gas Interpretation: Four Equations, Three Physiologic Processes Equation Physiologic Process 1) PaCO2 equation Alveolar ventilation 2) Alveolar gas equationOxygenation 3) Oxygen content equation Oxygenation 4) Henderson-Hasselbalch equation Acid-base balance These four equations, crucial to understanding and interpreting arterial blood gas data.

Severe tissue hypoxia due to capillary microthrombosis in critically ill patient with meningococcal septicaemia 3

Clinical Features of Tissue Hypoxia Dyspnoea Altered mental state Tachypnoea or hypoventilation Arrhythmias Peripheral vasodilatation Systemic hypotension Coma Cyanosis (unreliable) Nausea, vomiting, and gastrointestinal disturbance 4

5

Oxygen Delivery DO 2 = CO X CaO 2

Oxygen Content (CaO 2 ) CaO 2 = [(1.34 x Hgb x SaO 2 ) + (0.003 x PaO 2 ) (units = ml O2/dl) Quantity O2 bound to Hemoglobin Quantity O2 dissolved in plasma

Preload Contractility Determinants of Oxygen Delivery Afterload O 2 Capacity O 2 Dissolved O 2 Binding X CaO 2 = [(1.34 x Hgb x SaO 2 ) + (0.003 x PaO 2 )

Capillary blood to individual cells 9

Effect of oxygen tension gradient and diffusion distance on availability of oxygen to cells 10

Factors affecting extraction ratio of oxygen from capillary blood Rate of oxygen delivery to the capillary Oxygen-haemoglobin dissociation relation Size of the capillary to cellular P O 2 gradient Diffusion distance from the capillary to the cell Rate of use of oxygen by cells 11

Oxygen Dissociation Curve: SaO2 vs. PaO2 Also shown are CaO2 vs. PaO2 for two different hemoglobin contents: 15 gm% and 10 gm%. CaO2 units are ml O2/dl. P50 is the PaO2 at which SaO2 is 50%. CO and metHb do not affect PaO2, but do lower the SaO2 (Mesured vs calculated).

SaO2 – Is it Calculated or Measured? You always need to know this when confronted with blood gas data. SaO2 is measured in a “co-oximeter.” The traditional “blood gas machine“ measures only pH, PaCO2, and PaO2,, whereas the co-oximeter measures SaO2, carboxyhemoglobin, methemoglobin, and hemoglobin content. Newer “blood gas” consoles incorporate a co-oximeter, and so offer the latter group of measurements as well as pH, PaCO2, and PaO2. You should always make sure the SaO2 is measured, not calculated. If SaO2 is calculated from PaO2 and the O2- dissociation curve, it provides no new information and could be inaccurate - especially in states of CO intoxication or excess methemoglobin. CO and metHb do not affect PaO2, but do lower the SaO2.

Carbon Monoxide – An Important Cause of Hypoxemia Normal percentage of COHb in the blood is 1 - 2%, from metabolism and small amount of ambient CO (higher in traffic-congested areas). CO is colorless, odorless gas, a product of combustion; all smokers have excess CO in their blood, typically 5 -10%. CO binds 200x more avidly to hemoglobin than O2, effectively displacing O2 from the heme binding sites. CO is a major cause of poisoning deaths world-wide. CO has a “double-whammy” effect on oxygenation: 1) decreases SaO2 by the percentage of COHb present, and 2) shifts the O2- dissociation curve to the left, retarding unloading of oxygen to the tissues. CO does not affect PaO2, only SaO2. To detect CO poisoning, SaO2 and/or COHb must be measured (requires co-oximeter). In the presence of excess CO, SaO2 (when measured) will be lower than expected from the PaO2.

CO Does Not Affect PaO2 – Be Aware! Review the O2 dissociation curve shown on a previous slide. “X” represents the 2nd set of blood gases for a patient who presented to the ER with headache and dyspnea. His first blood gases showed PaO2 80 mm Hg, PaCO2 38 mm Hg, pH SaO2 on this first set was calculated from the O2- dissociation curve as 97%, and oxygenation was judged normal. He was sent out from the ER and returned a few hours later with mental confusion; this time both SaO2 and COHb were measured (SaO2 shown by “X”): PaO2 79 mm Hg, PaCO2 31 mm Hg, pH 7.36, SaO2 53%, carboxyhemoglobin 46%. CO poisoning was missed on the first set of blood gases because SaO2 was not measured!

16

Causes of Hypoxia ↓O 2 Dissolved ↓O 2 Binding X CaO 2 = [(1.34 x Hgb x SaO 2 ) + (0.003 x PaO 2 ) Lung Disease: Shunt or V-Q imbalance Reduced PaO2 ↓

Causes of Hypoxia ↓O 2 Binding X CaO 2 = [(1.34 x Hgb x SaO 2 ) + (0.003 x PaO 2 ) Carbon monoxide poisoning Methemoglobinemia, Rightward shift of the O2- dissociation curve Reduced SaO2 ↓

Causes of Hypoxia X CaO 2 = [(1.34 x Hgb x SaO 2 ) + (0.003 x PaO 2 ) Anemia Reduced Hgb ↓ ↓O 2 Capacity

Causes of Hypoxia X CaO 2 = [(1.34 x Hgb x SaO 2 ) + (0.003 x PaO 2 ) Reduced cardiac output: shock, congestive heart failure ↓DO 2 Delivery Left-to-right systemic shunt: septic shock

Causes of Hypoxia 21 Hypothermia. Hypophosphatemia, alkalosis and CO intoxication

Effect of intercapillary distance on relation between oxygen delivery and consumption when delivery is reduced by hypoxia (a fall in Pa O 2 ), reduced flow (stagnant),and anaemia (fall in haemoglobin concentration) 22 Interstitial Edema

Causes of Hypoxia X CaO 2 = [(1.34 x Hgb x SaO 2 ) + (0.003 x PaO 2 ) ↓Tissue Uptake VO 2 Mitochondrial poisoning (e.g., cyanide poisoning) Left-shifted hemoglobin dissociation curve (e.g., from acute alkalosis, excess CO, or abnormal hemoglobin structure

SaO2 and CaO2: Test Your Understanding which patient, (1) or (2), is more hypoxic 1) Hb 15, PaO 2 100, pH 7.40, COHb 20%: CaO 2 =.78 x 15 x 1.34 = 15.7 ml O 2 /dl 2) Hb 12, PaO 2 100, pH 7.40, COHb 0 CaO2 =.98 x 12 x 1.34 = 15.8 ml O2/dl The oxygen contents are almost identical, and therefore neither patient is more hypoxemic. However, patient (1), with 20% CO, is more hypoxic than patient (2) because of the left-shift of the O 2 -dissociation curve caused by the excess CO.

SaO2 and CaO2: Test Your Understanding which patient, (1) or (2), is more hypoxic 1) Hb 15, PaO 2 90, pH 7.20, COHb 5% CaO 2 =.87 x 15 x 1.34 = 17.5 ml O 2 /dl 2) Hb 15, PaO 2 50, pH 7.40, COHb 0 CaO2 =.85 x 15 x 1.34 = 17.1 ml O 2 /dl A PaO 2 of 90 mm Hg with pH of 7.20 gives an SaO 2 92%; subtracting 5% COHb from this value gives a true SaO 2 of 87%, used in the CaO 2 calculation of patient (1). A PaO 2 of 50 mm Hg with normal pH gives an SaO 2 of 85%. Thus patient (2) is slightly more hypoxemic

SaO2 and CaO2: Test Your Understanding which patient, (1) or (2), is more hypoxemic 1) Hb 5, PaO 2 60, pH 7.40, COHb 0 CaO 2 =.90 x 5 x.1.34 = 6.0 ml O 2 /dl 2) Hb 15, PaO 2 100, pH 7.40, COHb 20% CaO2 =.78 x 15 x 1.34 = 15.7 ml O 2 /dl Patient (1) is more hypoxemic, because of severe anemia.

SaO2 and CaO2: Test Your Understanding which patient, (1) or (2), is more hypoxemic 1) Hb 10, PaO 2 60, pH 7.30, COHb 10% CaO 2 =.87 x 10 x.1.34 = 11.7 ml O 2 /dl 2) Hb 15, PaO 2 100, pH 7.40, COHb 15% CaO2 =.83 x 15 x 1.34 = 16.7 ml O 2 /dl Patient (1) is more hypoxemic.

Factors Affecting Metabolic Rate Increased rate –Temperature—oxygen demand increases 10-15% for every 1£C rise –Sepsis or systemic inflammatory response syndrome –Size of the capillary to cellular P O 2 gradient –Burns, trauma, surgery –Sympathetic activation: pain, agitation, shivering –Interventions: nursing procedures, physiotherapy, visitors –β agonists, amphetamines, and tricyclic antidepressants –Feeding regimens containing excessive glucose Decreased rate –Sedatives, analgesics, and muscle relaxants 28

Clinical goals for anagement of Regional Oxygen Delivery Maintain adequate perfusion pressure and oxygen delivery to ensure regional delivery Maintain Pa O 2 above 7-8 kPa Minimise tissue oedema without causing intravascular depletion (use of colloids) Reduce tissue oxygen demand by reducing metabolic rate 29

30