Interpretation of arterial blood gases & compensation calculation
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Interpretation of arterial blood gases &
compensation calculation
West China HospitalSichuan University
Nov. 19th, 2003
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Oxygen cascadeDry atmospheric gas: 21 kPa
Humidified tracheal gas: 19.8 kPa Alveolar gas: 14 kPa
Arterial blood: 13.3 kPa
Capillary blood: 6-7 kPa Mitochondria: 1-5 kPa
Venous blood: 5.3 kPaClick for acid base physiology
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+H20 CO2
+HCO3- H+H2CO
3
Acid-base
Normal [H+] = 40 nmol/lpH = - log [H+] = 7.4
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+H20 CO2
+HCO3- H+H2CO
3
ALVEOLAR VENTILATION
Normal PaCO2 = 40 mmHg
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+H20 CO2 +HCO3- H+H2CO3
ALVEOLAR VENTILATION
RENAL HCO3- HANDLING
Normal HCO3- = 22-27
mmol/l
Click here to continue tutorial
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Interpretation of arterialblood gases
• Oxygenation• Ventilation• Acid base status
pH
PaCO2
PaO2
HCO3-
Base excess
Saturation
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Interpretation of arterialblood gases
• Oxygenation• Ventilation• Acid base status
pH
PaCO2
PaO2
HCO3-
Base excessSaturation
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Oxygenation
• What is the PaO2? - Partial O2 pressure
that is physically dissolved in plasma
pH
PaCO2
PaO2
HCO3-
Base excess
Saturation
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Oxygen dissociation curve
50
75
100
Sat u
r atio
n %
PO2 kPa3.5 5.3 13.3
90
8.0
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Oxygenation
• Normal PaO2 breathing air (FiO2 = 21%) is 12-13.3 kPa ; small reduction with age
• Lower values constitute hypoxaemia• PaO2 <6.7 kPa on room air = respiratory failure• PaO2 should go up with increasing FiO2
• PaO2 of 13.3 kPa breathing 60% O2 is not adequate
• You need to know the FiO2 to interpret the ABG
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Oxygenation
Correlate ABG result with the SpO2
- reasons for discrepancy:• problem with the probe
(poor perfusion)• problem with the blood gas
(venous sample)
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Oxygenation
• Is the PO2 is lower than expected?• Calculate the A-a gradient to assess
if the low PO2 is due to:• low alveolar PO2
• Structural lung problems causing failure of oxygen transfer
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Oxygenation
PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25]
The alveolar gas equation:
The alveolar-arterial oxygen difference
(A-a) PO2 = PAO2 - PaO2
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Oxygenation
PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25]
The alveolar gas equation:
The alveolar-arterial oxygen difference
(A-a) PO2 = PAO2 - PaO2
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Oxygenation
normally only a small gradient (1.33kPa). As CO2 accumulates in the alveolus due to HYPOVENTILATION there is less room for oxygen. If the lung is normal this oxygen can pass into blood as normal. If there are problems that limit oxygen diffusion the gradient will get bigger.
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Acid base problems
Is there acidaemia or alkalaemia?
Normal pH = 7.35 – 7.45
Acidaemia < 7.35 Alkalaemia > 7.45
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Acid base problems
Is the primary problem respiratory or metabolic?
Look at PaCO2
Normal PaCO2 = 5.3 kPa
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Acid base problems
Is the primary problem respiratory or metabolic?
Look at [HCO3-]
Normal [HCO3-] = 24 mmol/l
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Is there Is the PaCO2 Is the HCO3- It is
Acidaemia High( > 6 kPa)
Normal/high( 24 mmol/l)
Respiratory acidosis
Acidaemia Low Low Metabolic acidosis
Alkalaemia Low Normal/low Respiratory alkalosis
Alkalaemia High High Metabolic alkalosis
+H20 CO2
+HCO3- H+H2CO3
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Is there Is the PaCO2 Is the HCO3- It is
Acidaemia High Normal/high Respiratory acidosis
Acidaemia Low( < 4.5 kPa)
Low( 22 mmol/l)
Metabolic acidosis
Alkalaemia Low Normal/low Respiratory alkalosis
Alkalaemia High High Metabolic alkalosis
+H20 CO2 +HCO3- H+H2CO3
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Is there Is the PaCO2 Is the HCO3- It is
Acidaemia High Normal/high Respiratory acidosis
Acidaemia Low Low Metabolic acidosis
Alkalaemia Low( < 4.5 kPa)
Normal/low( 23 mmol/l)
Respiratory alkalosis
Alkalaemia High High Metabolic alkalosis
+H20 CO2
+HCO3- H+H2CO3
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Is there Is the PaCO2 Is the HCO3- It is
Acidaemia High Normal/high Respiratory acidosis
Acidaemia Low Low Metabolic acidosis
Alkalaemia Low Normal/low Respiratory alkalosis
Alkalaemia High( > 6 kPa)
High( 27 mmol/l)
Metabolic alkalosis
+H20 CO2 +HCO3- H+H2CO3
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pH [HCO3- ]
Acute respiratory acidosis
Falls 0.06 Rises 0.8 mmol (up to 30 mmol/ l)
for every 1 kPa rise in PaCO2
Acute respiratory alkalosis
Rises 0.06 Falls 1.5 mmol (down to 18 mmol/ l)
for every 1 kPa fall in PaCO2
Chronic respiratory acidosis
Falls 0.02 Rises 3.0 mmol (up to 36 mmol/ l)
for every 1 kPa rise in PaCO2
Chronic respiratory alkalosis
Rises 0.02 Falls 3.8 mmol (down to 18 mmol/ l)
for every 1 kPa fall in PaCO2
For acute respiratory conditions
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pH [HCO3- ]
Acute respiratory acidosis
Falls 0.06 Rises 0.8 mmol (up to 30 mmol/ l)
for every 1 kPa rise in PaCO2
Acute respiratory alkalosis
Rises 0.06 Falls 1.5 mmol (down to 18 mmol/ l)
for every 1 kPa fall in PaCO2
Chronic respiratory acidosis
Falls 0.02 Rises 3.0 mmol (up to 36 mmol/ l)
for every 1 kPa rise in PaCO2
Chronic respiratory alkalosis
Rises 0.02 Falls 3.8 mmol (down to 18 mmol/ l)
for every 1 kPa fall in PaCO2
Early renal compensation for respiratory conditions
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pH [HCO3- ]
Acute respiratory acidosis
Falls 0.06 Rises 0.8 mmol (up to 30 mmol/ l)
for every 1 kPa rise in PaCO2
Acute respiratory alkalosis
Rises 0.06 Falls 1.5 mmol (down to 18 mmol/ l)
for every 1 kPa fall in PaCO2
Chronic respiratory acidosis
Falls 0.02 Rises 3.0 mmol (up to 36 mmol/ l)
for every 1 kPa rise in PaCO2
Chronic respiratory alkalosis
Rises 0.02 Falls 3.8 mmol (down to 18 mmol/ l)
for every 1 kPa fall in PaCO2
Late renal compensation for respiratory conditions
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Respiratory acidosis• Central nervous system depression
– sedatives, CNS disease, obesity, hypoventilation syndrome
• Pleural Disease– pneumothorax
• Lung Disease – COPD, severe pneumonia, late stage ARDS
• Musculoskelatal disorders– kyphoscoliosis, Guillain-Barre, myasthenia
gravis, polio
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Respiratory alkalosis• Catastrophic CNS event (CNS
hemorrhage) • Infection, fever • Pregnancy (especially the 3rd trimester) • Decreased lung compliance (interstitial
lung disease) • Liver cirrhosis • Anxiety
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Metabolic acidosis
• Anion gap acidosis• Non anion gap acidosis • Any respiratory compensation?
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Anion GapThe anion gap is an artificial
difference between the commonly unmeasured anions and cations.
In reality there is electrochemical neutrality
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Anion GapCations AnionsNa+ HCO3
-
K+ Chloride-
Ca2+ Protein (albumin)Mg2+ Organic acids
PhosphatesSulphates
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Anion Gapnormal anion gap = 12 mmol/l
[Na+] + [unmeasured cations] = [Cl-] + [HCO3-] +
[unmeasured anions]
[unmeasured anions] - [unmeasured cations] = [Na+] - ([Cl-] + [HCO3
-])
144 – ( 108 + 24 ) = 12 [Na+] – ( [Cl-] + [HCO3
-] ) = Anion Gap Na+
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Anion gap acidosisAccumulation of organic acid not
normally present in serum (lactic acid, ketones), replace HCO3
-
Fall in [HCO3-] will widen AG
Link to causes of anion gap acidosis
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Non anion gap acidosisLoss of HCO3
- (GI tract or renal) increase in [chloride], AG not
changeAdministration of exogenous
chloride, [HCO3-] falls without AG
change.
Link to causes of non-anion gap acidosis
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Anion gap acidosis• Uremia • Ketoacidosis
– diabetic hyperglycemia, • Alcohol poisons or drug intoxication
– Methanol• Lactic acidosis
– Sepsis, left ventricular failure
Non-anion gap acidosis
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Non-anion gap acidosis• GI loss of HCO3
- (diarrhea) • Renal loss of HCO3
- - Compensation for respiratory alkalosis - Carbonic anhydrase inhibitor - Renal tubular acidosis
• Other causes: HCl or NH4Cl infusion, Cl gas inhalation, hyperalimentation
Return to tutorial
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Metabolic acidosis
• Respiratory compensation?• Occurs rapidly after pH change• Predictable for metabolic acidosis
by Winter’s formula• PaCO2 outside the predicted range
suggest additional respiratory disturbances
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Winter’s formula
Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
Link to examples
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Using Winter’s formula
A patient with a metabolic acidosis has a [HCO3-] of 10
mmol/l. By Winter’s formula expected PaCO2 should be 2.8 – 3.3 kPa
Expected PaCO2 = [ (1.5 x 10) + (8 ± 2) ] x 0.133
A value out side this range suggests an additional respiratory disturbance
Click to continue
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Using Winter’s formula
• If the actual PaCO2 is less than 2.8 kPa there is also RESPIRATORY ALKALOSIS
A patient with a metabolic acidosis has a [HCO3-] of 10 mmol/l.
By Winter’s formula expected PaCO2 should be 2.8 – 3.3 kPa
Click to continue
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Using Winter’s formulaA patient with a metabolic acidosis has a [HCO3
-] of 10 mmol/l.
By Winter’s formula expected PaCO2 should be 2.8 – 3.3 kPa
Click to continue
•If the actual PaCO2 is more than 3.3 kPa there is also RESPIRATORY ACIDOSIS
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Metabolic alkalosis
• Respiratory compensation• Occurs rapidly after pH change• Not complete or easily predictable for
metabolic alkalosis • Rarely achieve PaCO2 > 7 kPa• A suggested formula:
Expected PaCO2 = 0.8 kPa per 10 mmol/l in HCO3-
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Metabolic alkalosis
• Volume contraction (vomiting, overdiuresis, ascites)
• Hypokalemia • Alkali ingestion (bicarbonate)
Return to causes
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Mixed disturbances
Difficult to interpret
Expected corrections
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Respiratory compensation
For metabolic acidosis Winter’s formula:
For metabolic alkalosis:Expected PaCO2 = 0.8 kPa per 10 mmol/l in HCO3-
Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
Return to example
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Expected correctionsPrimary change Compensatory
change
Respiratory acidosis Rise in PaCO2 Rise in [HCO3-] 1. pH change
consistent with PaCO2
2. Calculate expected rise in [HCO3
-]Respiratory alkalosis Fall in PaCO2 Fall in [HCO3
-] 1. pH change consistent with PaCO2
2. Calculate expected fall in [HCO3
-]Metabolic acidosis Fall in [HCO3
-] Fall in PaCO2 1. Winter’s formula for expected PaCO2
2. Corrected [HCO3-]
in anion-gap acidosisMetabolic alkalosis Rise in [HCO3
-] Rise in PaCO2 1. Difficult to predict, use suggested formula
If the correction is NOT as expected there is another disturbance.
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Expected correctionsPrimary change Compensatory
change
Respiratory acidosis Rise in PaCO2 Rise in [HCO3-] 1. pH change
consistent with PaCO2
2. Calculate expected rise in [HCO3
-]Respiratory alkalosis Fall in PaCO2 Fall in [HCO3
-] 1. pH change consistent with PaCO2
2. Calculate expected fall in [HCO3
-]Metabolic acidosis Fall in [HCO3
-] Fall in PaCO2 1. Winter’s formula for expected PaCO2
2. Corrected [HCO3-]
in anion-gap acidosisMetabolic alkalosis Rise in [HCO3
-] Rise in PaCO2 1. Difficult to predict, use suggested formula
If the correction is NOT as expected there is another disturbance.
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Metabolic acidosis
Corrected [HCO3-] = measured [HCO3
-] + (anion gap – 12)
Correcting bicarbonate
Anion Gap = [Na+] – [Cl-] - [HCO3-]
Anion Gap
Return to example
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Corrected bicarbonate
• Anion gap acidosis may co-exist non-anion gap acidosis or metabolic alkalosis
• Simple anion gap acidosis: widened gap is due to absent bicarbonate
Click to continue
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Corrected bicarbonate
• An patient with metabolic acidosis • AG 26 mmol/l [HCO3
-] 10 mmol/l
• Corrected [HCO3-] = 24 mmol/l
Corrected [HCO3-] = 10 + (26 – 12)
• No other metabolic disturbance exists
Corrected [HCO3-] = measured [HCO3
-] + (anion gap – 12)
Click to continue
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Corrected bicarbonate• An patient with metabolic acidosis AG 26 mmol/l HCO3
- 15 mmol/l
• The corrected [HCO3-] = 29 mmol/l
Corrected [HCO3-] = 15 + (26 – 12)
• There is extra bicarbonate in the system and a metabolic alkalosis co-exists
Return to expected corrections
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Example 1.A 33 male patient with SARS has a saturation of 91% on Fi02 0.4
1. Is he hypoxic?2. Is there an acid base
or ventilation problem?
pH 7.43PaCO2 4.76PaO2 8.1HCO3
- 23Base excess
-0.6Saturation
90%
Click to continue
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Example 1.1. Is he hypoxic?
YES.The SpO2 and calculated saturation agree
pH 7.43PaCO2 4.76PaO2 8.1HCO3
- 23Base excess
-0.6Saturation
90%
Click to continue
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Example 1.1. Is he hypoxic?YES.(A-a) PO2 = 23.9 kPa There is major problem with oxygen transfer into the lung
pH 7.43PaCO2 4.76PaO2 8.1HCO3
- 23Base excess
-0.6Saturation
90%
To calculate (A-a) PO2 Click to continue
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Example 1.2. Is there an acid base or ventilation problem?NO.pH, PaCO2 and PaCO2 are normalThis is pure respiratory failure
pH 7.43PaCO2 4.76PaO2 8.1PaCO2 23Base excess
-0.6Saturation
90%
Return to examples
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Example 2.A patient with in the recovery room has been found to be cyanosed, with shallow breathing. This is the ABG result on room air.
pH 7.08PaCO2 10.6PaO2 4.9HCO3
- 26Base excess
+2Saturation
86%
Click to continue
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Example 2.1. Is the patient hypoxic? 2. due simply to
hypoventilation as a result of residual anaesthetic agents
3. or have they also aspirated and developed lung parenchymal problems?
pH 7.08PaCO2 10.6PaO2 4.9HCO3
- 26Base excess
+3Saturation
86%
Click to continue
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Example 2.Calculate the A-a gradient:
PAO2 = [94.8 x 0.21] – [10.6 x 1.25]= 6.65 kPa
(A-a) PO2 = 6.65 – 4.9= 1.75 kPa
This is a near normal A-a gradient, and hypoventilation alone can explain the hypoxaemia. Increased ventilation will improve hypercapnia and oxygenation too.
pH 7.08PaCO2 10.6PaO2 4.9HCO3
- 26Base excess
+3Saturation
86%
Click to continue
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Example 2.4. Is there an acid base
or ventilation problem?
YES.
pH 7.08PaCO2 10.6PaO2 4.9HCO3
- 26Base excess
+2Saturation
86%
Click to continue
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Example 2.There is:• Acidosis• PaCO2 is elevated
RESPIRATORY ACIDOSIS
pH 7.08PaCO2 10.6PaO2 4.9HCO3
- 28Base excess
+2Saturation
86%
Click to continue
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Example 2.There is:• HCO3
- = 28• Expected HCO3
- • = 24 + [(10.6 – 5.3) x 0.8] =
28.2This is the expected [HCO3
- ] if there has only been a small amount of renal compensation ACUTE RESPIRATORY ACIDOSIS
pH 7.08PaCO2 10.6PaO2 4.9HCO3
- 28Base excess
+2Saturation
86%
Click to continue
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Example 2.There is:• pH change:
[10.6 – 5.3] x 0.06 = 0.32pH = [7.4 – 0.32] = 7.08
CONSISTENT WITH SIMPLE ACUTE RESPIRATORY ACIDOSIS;
NO ADDITIONAL DISTURBANCE
pH 7.08PaCO2 10.6PaO2 4.9HCO3
- 28Base excess
+2Saturation
86%
Return to examplesRenal compensation
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Example 3.A patient has been brought to ER after a head injury; he is deeply unconscious. This is the ABG on room air.
Clearly he is very hypoxic
pH 7.23PaCO2 8.1PaO2 4.9HCO3
- 26Base excess
+3Saturation
86%
Click to continue
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Example 3.1. Is the patient hypoxic
due simply because of hypoventilation as a result of CNS depression
1. or has he also aspirated and developed lung parenchymal problems?
pH 7.23PaCO2 8.1PaO2 4.9HCO3
- 26Base excess
+3Saturation
86%
Click to continue
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Example 3.Calculate the A-a gradient:
PAO2 = [94.8 x 0.21] – [8.1 x 1.25]= 10.1 kPa
(A-a) PO2 = 10.1 – 4.9= 5.2 kPaThe A-a gradient is increased suggesting that less of the O2 available in the alveolus is able to get into the arterial blood. There is a lung problem; possibly aspiration
pH 7.23PaCO2 8.1PaO2 4.9HCO3
- 26Base excess
+3Saturation
86%
Click to continue
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Example 3.
pH 7.23PaCO2 8.1PaO2 4.9HCO3
- 26Base excess
+3Saturation
86%
3. Is there an acid base or ventilation problem?
YES.
Click to continue
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Example 3.
pH 7.23PaCO2 8.1PaO2 4.9HCO3
- 26Base excess
+3Saturation
86%
There is• Acidosis • PaCO2 is elevated
RESPIRATORY ACIDOSIS
Click to continue
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Example 3.
pH 7.23PaCO2 8.1PaO2 4.9HCO3
- 26Base excess
+3Saturation
86%
There is:• HCO3
- = 26• Expected HCO3
- • = 24 + [(8.1 – 5.3) x 0.8] =
26.2This is the expected [HCO3
- ] if there has only been a small amount of renal compensation
ACUTE RESPIRATORY ACIDOSIS
Click to continue
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Example 3.
pH 7.23PaCO2 8.1PaO2 4.9HCO3
- 26Base excess
+3Saturation
86%
There is:• pH change:
[8.1 – 5.3] x 0.06 = 0.17pH = [7.4 – 0.17] = 7.23
CONSISTENT WITH SIMPLE ACUTE RESPIRATORY ACIDOSIS; NO ADDITIONAL DISTURBANCE
Return to examples
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Example 4.A patient with COPD has a ABG taken in out-patient clinic to assess his need for home oxygen. He is breathing room air.
pH 7.34PaCO2 8.0PaO2 7.5HCO3
- 32.1Base excess
+8Saturation
86%
Click to continueClick to continue
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Example 4.
pH 7.34PaCO2 8.0PaO2 7.5HCO3
- 32.1Base excess
+8Saturation
86%
1. Is he hypoxic?YES.
The (A-a) PO2 = 2.4 kPa The (A-a) gradient is
increased, and home oxygen might be appropriate
Click to continue
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Example 4.
pH 7.34PaCO2 8.0PaO2 7.5HCO3
- 32.1Base excess
+8Saturation
86%
2. Is there an acid base or ventilation problem?
YES.
Click to continue
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Example 4.There is:• Mild acidosis• PaCO2 is elevated
RESPIRATORY ACIDOSIS
pH 7.34PaCO2 8.0PaO2 7.5HCO3
- 32.1Base excess
+8Saturation
86%
Diagnose disturbance Click to continue
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Example 4.There is:• HCO3
- = 32.1• Expected HCO3
- • = 24 + [(8.0 – 5.3) x 3.0] = 32.1
This is the expected [HCO3- ] if
there has been significant renal compensation over a long period; in addition the base excess has increased.
CHRONIC RESPIRATORY ACIDOSIS
pH 7.34PaCO2 8.0PaO2 7.5HCO3
- 32.1Base excess
+8Saturation
86%
Click to continue
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Example 4.There is:• pH change:
[8.0 – 5.3] x 0.02 = 0.054pH = [7.4 – 0.054] =
7.346
CONSISTENT WITH SIMPLE CHRONIC RESPIRATORY ACIDOSIS; NO ADDITIONAL DISTURBANCE
pH 7.34PaCO2 8.0PaO2 7.5HCO3
- 32.1Base excess
+8Saturation
86%
Return to examples
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Example 5.A 35 year old woman with a history of anxiety attacks presents to ER with alpitations.
pH 7.54PaCO2 2.9PaO2 12.1HCO3
- 22Base excess
+2Saturation
100%
Click to continue
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Example 5.1. Is she hypoxic?
NO.
This is a normal PaO2 for room air
pH 7.54PaCO2 2.9PaO2 12.1HCO3
- 22Base excess
+2Saturation
100%
Click to continue
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Example 5.2. Is there an acid
base or ventilation problem?
YES.
pH 7.54PaCO2 2.9PaO2 12.1HCO3
- 22Base excess
+2Saturation
100%
Click to continue
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Example 5.There is:
• Alkalosis• PaCO2 is decreased
RESPIRATORY ALKALOSIS
pH 7.54PaCO2 2.9PaO2 12.1HCO3
- 22Base excess
+2Saturation
100%
Diagnose disturbance Click to continue
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Example 5.
pH 7.54PaCO2 2.9PaO2 12.1HCO3
- 20Base excess
+2Saturation
100%
There is:• HCO3
- = 20• Expected HCO3
- • = 24 - [(5.3 – 2.9) x 1.5] =
20.4This is the expected [HCO3
- ] if there has only been a small amount of renal compensation
ACUTE RESPIRATORY ALKALOSIS
Click to continue
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Example 5.
pH 7.54PaCO2 2.9PaO2 12.1HCO3
- 22Base excess
+2Saturation
100%
There is:• pH change:
[5.3-2.9] x 0.06 = 0.144pH = [7.4 + 0.144] =
7.54
CONSISTENT WITH SIMPLE ACUTE RESPIRATORY ALKALOSIS; NO ADDITIONAL DISTURBANCE
Return to examples
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Example 6.
pH 7.23PaCO2 3.3PaO2 29.9HCO3
- 12Base excess
-10Saturation
100%
A 42 year old diabetic woman present with UTI symptoms; she has deep sighing respiration. This is the ABG on FiO2 0.4
Click to continue
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Example 6.
pH 7.23PaCO2 3.3PaO2 29.9HCO3
- 12Base excess
-10Saturation
100%
1. Is she hypoxic?
NO.
This PaO2 is adequate for an FiO2 of 0.4
Click to continue
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Example 6.
pH 7.23PaCO2 3.3PaO2 29.9HCO3
- 12Base excess
-10Saturation
100%
2. Is there an acid base or ventilation problem?
YES.
Click to continue
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Example 6.
pH 7.23PaCO2 3.3PaO2 29.9HCO3
- 12Base excess
-10Saturation
100%
There is:• Acidosis• PaCO2 is decreased NOT respiratory acidosis• Look at [HCO3
-]• [HCO3
-] is reduced• Base excess is negative
METABOLIC ACIDOSISClick to continue
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Example 6.
pH 7.23PaCO2 3.3PaO2 29.9HCO3
- 12Base excess
-10Saturation
100%
Using Winter’s formula:Expected PaCO2 = [ (1.5 x 12) + (8 ± 2) ] x
0.133= 3.2 – 3.7 kPaThe PaCO2 falls within this
range SIMPLE METABOLIC
ACIDOSISClick to continue
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Example 6.
pH 7.23PaCO2 3.3PaO2 29.9HCO3
- 12Base excess
-10Na+ 135Cl- 99
What is the anion gap?= [Na+] – ( [Cl-] + [HCO3
-] )
= [135] – ( 99 + 12 ) Na= 24 mmol/l
• There is an anion gap acidosis due to accumulation of organic acids caused by diabetic ketoacidosis
Click to continue
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Example 6.
pH 7.23PaCO2 3.3PaO2 29.9HCO3
- 12Base excess
-10Na+ 135Cl- 99
Corrected bicarbonate = 24 mmol/l
The PaCO2 falls within the expected range
SIMPLE METABOLIC ACIDOSIS; NO OTHER DISTURBANCE
Return to examples
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Example 7.A 70 year old man presents with a 3 day history of severe vomiting.Here is his ABG on room air.
pH 7.5PaCO2 6.2PaO2 10.6HCO3
- 38Base excess
+8Saturation
96%
Click to continue
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Example 7.1.Is he hypoxic?
NO.
This is a normal PaO2 for a patient this age breathing room air
pH 7.5PaCO2 6.2PaO2 10.6HCO3
- 38Base excess
+8Saturation
96%
Click to continue
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Example 7.2. Is there an acid base
or ventilation problem?
YES.
pH 7.5PaCO2 6.2PaO2 10.6HCO3
- 38Base excess
+8Saturation
96%
Click to continue
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Example 7.There is:• Alkalosis• PaCO2 is elevated NOT respiratory
alkalosis
Look at [HCO3-]
• [HCO3-] is increased
• Base excess is positive METABOLIC ALKALOSIS
pH 7.5PaCO2 6.2PaO2 10.6HCO3
- 38Base excess
+8Saturation
96%
Click to continue
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Example 7.3. Is there respiratory
compensation?
Expected PaCO2
= 0.8 kPa per 10 mmol/l in HCO3
-
= 5.3 + (0.8 x ([ 38 – 24 ]/10))
= 6.4 CONSISTENT WITH SIMPLE
METABOLIC ALKALOSIS
pH 7.5PaCO2 6.3PaO2 10.6HCO3
- 38Base excess
+8Saturation
96%
Return to examples
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Example 8.A 54 year old woman has multiple organ failure due to intra-abdominal sepsis. She has ARDS, renal failure. This is her ABG on FiO2 1.0
pH 7.07PaCO2 8.63PaO2 11.8HCO3
- 17.9Base excess
-6.5Saturation
95%
Click to continue
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Example 8.1.Is she hypoxic?
YES. This PaO2 is very
low for an FiO2 of 1.0
pH 7.07PaCO2 8.63PaO2 11.8HCO3
- 17.9Base excess
-6.5Saturation
95%
Click to continue
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Example 8.
2. Is there an acid base or ventilation problem?
YES.
pH 7.07PaCO2 8.63PaO2 11.8HCO3
- 17.9Base excess
-6.5Saturation
95%
Click to continue
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Example 8.There is• Acidosis • PaCO2 is elevated
RESPIRATORY ACIDOSIS
pH 7.07PaCO2 8.63PaO2 11.8HCO3
- 17.9Base excess
-6.5Saturation
95%
Click to continueDiagnose disturbance
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Example 8.Expected pH= 7.4 – ([8.63-5.3] x 0.06)= 7.2 Observed pH is lowerExpected bicarbonate = 24 + ([8.63-5.3] x 0.8)= 26.7 mmol/l Observed bicarbonate is
too low
pH 7.07PaCO2 8.63PaO2 11.8HCO3
- 17.9Base excess
-6.5Saturation
95%
Click to continue
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Example 8.Lower pHLower bicarbonateBase deficit negative ADDITIONAL METABOLIC
ACIDOSIS
pH 7.07PaCO2 8.63PaO2 11.8HCO3
- 17.9Base excess
-6.5Saturation
95%Severe ARDS leads to hypoxia & hypercapnia with respiratory acidosis; renal failure and poor perfusion lead to metabolic acidosis
Return to examples
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Example 9.
A 43 year old man presents with an overdose of aspirin. This is his ABG on air.
pH 7.37PaCO2 2.3PaO2 12HCO3
- 10Base excess
-7.4Saturation
97%
Click to continue
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Example 9.1. Is he hypoxic?
NO.
This is a normal PaO2 for a patient this age breathing room air
pH 7.37PaCO2 2.3PaO2 12HCO3
- 10Base excess
-7.4Saturation
97%
Click to continue
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Example 9.
pH 7.37PaCO2 2.3PaO2 12HCO3
- 10Base excess
-7.4Saturation
97%
2. Is there an acid base or ventilation problem?
NO? Or is there?
Click to continue
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Example 9.
pH 7.37PaCO2 2.3PaO2 12HCO3
- 10Base excess
-7.4Saturation
97%
PaCO2 is low• Respiratory alkalosis?• Metabolic acidosis?
HCO3- is lowNegative base deficit• Metabolic acidosis?
Click to continueDiagnose disturbance
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Example 9.
pH 7.37PaCO2 2.3PaO2 12HCO3
- 10Base excess
-7.4Saturation
97%
Expected PaCO2 by Winter’s formula=2.8 – 3.3 kPa
Observed PaCO2 is out of this range
MIXED DISTURBANCE: RESPIRATORY ALKALOSIS
AND METABOLIC ACIDOSIS
Click to continueRespiratory compensation
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Example 9.
pH 7.37PaCO2 2.3PaO2 12HCO3
- 10Base excess
-7.4Saturation
97%
Aspirin overdose characteristically causes a metabolic acidosis due the effect of salicylic acidand a respiratory alkalosis due to hyperventilation
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Questions?
Comments?