Oxygen Content Equation and Oxygen Transport

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The Key to Blood Gas Interpretation:Four Equations, Three Physiologic Processes

Equation Physiologic Process1) PaCO2 equation Alveolar ventilation2) Alveolar gas equation Oxygenation3) Oxygen content equation Oxygenation4) 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

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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

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Oxygen Delivery

DO2= CO X CaO2

Oxygen Content (CaO2)

CaO2= [(1.34 x Hgb x SaO2) + (0.003 x PaO2)

)units = ml O2/dl (

Quantity O2 bound to Hemoglobin

Quantity O2 dissolved in plasma

HR SV CO Preload

Contractility

Determinants of Oxygen Delivery

Afterload

O2 Capacity

O2 Dissolved

O2 Binding

X

CaO2= [(1.34 x Hgb x SaO2) + (0.003 x PaO2)

Capillary blood to individual cells

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

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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 O2 gradient• Diffusion distance from the capillary to the cell• Rate of use of oxygen by cells

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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).

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HR SV CO

Causes of Hypoxia

↓O2 Dissolved

↓O2 Binding

X

CaO2= [(1.34 x Hgb x SaO2) + (0.003 x PaO2)

Lung Disease :Shunt or V-Q imbalance

Reduced PaO2

↓

HR SV CO

Causes of Hypoxia

↓O2 Binding

X

CaO2= [(1.34 x Hgb x SaO2) + (0.003 x PaO2)

Carbon monoxide poisoning Methemoglobinemia,Rightward shift of the O2-dissociation curve

Reduced SaO2

↓

HR SV CO

Causes of Hypoxia

X

CaO2= [(1.34 x Hgb x SaO2) + (0.003 x PaO2)

Anemia Reduced Hgb

↓↓O2 Capacity

HR SV CO

Causes of Hypoxia

X

CaO2= [(1.34 x Hgb x SaO2) + (0.003 x PaO2)

Reduced cardiac output:shock, congestive heart failure

↓DO2 Delivery

Left-to-right systemic shunt: septic shock

Causes of Hypoxia

21Hypothermia. 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

HR SV CO

Causes of Hypoxia

X

CaO2= [(1.34 x Hgb x SaO2) + (0.003 x PaO2)

↓Tissue Uptake VO2 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, PaO2 100, pH 7.40, COHb 20%:

CaO2 = .78 x 15 x 1.34 = 15.7 ml O2/dl

2) Hb 12, PaO2 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 O2-dissociation curve caused by the excess CO.

SaO2 and CaO2: Test Your Understanding

which patient, (1) or (2), is more hypoxic

1) Hb 15, PaO2 90, pH 7.20, COHb 5%

CaO2 = .87 x 15 x 1.34 = 17.5 ml O2/dl

2) Hb 15, PaO2 50, pH 7.40, COHb 0

CaO2 = .85 x 15 x 1.34 = 17.1 ml O2/dl

A PaO2 of 90 mm Hg with pH of 7.20 gives an SaO2 of @ 92%; subtracting 5% COHb from this value gives a true SaO2 of 87%, used in the CaO2 calculation of patient (1). A PaO2 of 50 mm Hg with normal pH gives an SaO2 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, PaO2 60, pH 7.40, COHb 0

CaO2 = .90 x 5 x .1.34 = 6.0 ml O2/dl

2) Hb 15, PaO2 100, pH 7.40, COHb 20%

CaO2 = .78 x 15 x 1.34 = 15.7 ml O2/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, PaO2 60, pH 7.30, COHb 10%

CaO2 = .87 x 10 x .1.34 = 11.7 ml O2/dl

2) Hb 15, PaO2 100, pH 7.40, COHb 15%

CaO2 = .83 x 15 x 1.34 = 16.7 ml O2/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 O2 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

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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

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30 Thank You