Oxygen Content Equation and Oxygen Transport
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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.
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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
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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
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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)
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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
↓
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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
↓
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HR SV CO
Causes of Hypoxia
X
CaO2= [(1.34 x Hgb x SaO2) + (0.003 x PaO2)
Anemia Reduced Hgb
↓↓O2 Capacity
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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
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Causes of Hypoxia
21Hypothermia. Hypophosphatemia, alkalosis and CO intoxication
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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
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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
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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.
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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
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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.
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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.
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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