Interpretation of arterial blood gases & compensation calculation

Interpretation of arterial blood gases & compensation calculation
West China Hospital Sichuan University Nov. 19th, 2003

Oxygen cascade Dry 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 The partial pressure of oxygen falls from dry atmospheric pressure value of of 21 kpa through a series of steps until it reaches only 1-5 kpa in the mitochondria where it used to make energy. As it is humdified in the trachea water vapour makes up more the gas mix and po2 falls. CO2 in the alveolus further reduced po2. There Is only a small gradient between the alveolus and arterial blood which we will discuss again later. As blood moves through the circulation po2 continues to fall until it reaches 5.3 kpa in venous blood. The po2 in mitochondria is very low due to long path from the cappilary to the interior of the cell. Drops in po2 in the blood have deleterious knock on effects for mitochondrial po2 Click for acid base physiology Venous blood: 5.3 kPa

Acid-base + Normal [H+] = 40 nmol/l pH = - log [H+] = 7.4 H20 CO2 H+
The concentration of hydrogen ions is kept very constant due to highly efficient buffering systems. The most important buffer system is the bicarbonate system. Bicarbonate and hydrogen ions are is in equibilrium with carbonic acid which is in turn in equilibrium with carbon dioxide and water. Other buffer systems include phosphate, protein and haemoglobin. Normal [H+] = 40 nmol/l pH = - log [H+] = 7.4

+ + ALVEOLAR VENTILATION Normal PaCO2 = 40 mmHg H20 CO2 H+ H2CO3 HCO3-
Metabolic alterations in the equilibrium of the bicarbonate buffer system result in changes in alveolar ventialtion in an attempt to maintain the hydrogen ion conc. In a metabolic acidosis hydrogen ions accumulate and the equilibrium moves to the left; CO2 is blown off by increased ventilation to compensate for this. In a metabolic alkalosis the hydrogen ion concentration falls and the equilibrium shifts to the right. Co2 is retained to counteract this shift. These changes take place soon after the change in pH occurs

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ALVEOLAR VENTILATION Normal HCO3- = mmol/l + + H20 CO2 H2CO3 HCO3- H+ Respiratory alterations in the equilibrium will affect renal handling of bicarbonate in an attempt to maintain hydrogen ion concentration. In respiratory acidosis hypoventilation causes accumulation of co2 and the equilibrium will shift to the RIGHT. Bicarbonate must be retained by the kidney to buffer the resulting increase in hydrogen ions. In a respiratory alkalosis CO2 is blown off, the equilibrium moves to the LEFT and hydrogen ion concentration falls. Bicarbonate must be lost in order to balance the system. RENAL HCO3- HANDLING Click here to continue tutorial

Interpretation of arterial blood gases
pH PaCO2 PaO2 HCO3- Base excess Saturation Oxygenation Ventilation Acid base status Information on oxygenation is derived from the PaO2 (partial pressure of oxygen in blood) and the saturation. The partial pressure of oxygen is measured directly by the blood gas machine while the saturation is a calculated value. Some ABG machines with an in-built oximeter can give a directly measured value for saturation.

Interpretation of arterial blood gases
pH PaCO2 PaO2 HCO3- Base excess Saturation Oxygenation Ventilation Acid base status Assessment of ventilation and acid base status go hand in hand. pH and PCO2 are directly measured by the ABG machine; bicarbonate and base excess are calculated values.

Oxygenation What is the PaO2?
- Partial O2 pressure that is physically dissolved in plasma pH PaCO2 PaO2 HCO3- Base excess Saturation When we assess oxygenation we start by looking at the Pao2 and decide if this is appropriate for the amount of o2 that the patient is receiving (the FIO2). Also do these results make sense when we look at the patient and the result form the saturation probe.

Oxygen dissociation curve
100 90 75 Saturation % 50 Once saturation falls below approximately 88% (which corresponds to a po2 of around 6.7 kpa) the ODC enters it’s steep part. Only a small fall in po2 at this point will cause a large fall in saturation with a risk of greatly reduced o2 delivery to the tissue. We aim to keep sat above 88 and po2 greater than 6.7 to ensure adequate tissue o2 delivery. 3.5 5.3 8.0 13.3 PO2 kPa

Oxygenation Normal PaO2 breathing air (FiO2 = 21%) is 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 normal pao2 is kP when breathing room air (when FIo2 equals 12%) , any lower is considered to be hypoxaemia , a pao2 lower than 6.7 is considered to be resp failure. Remember that we must know the FIO2 to accurately examine a blood gas result.

Oxygenation Correlate ABG result with the SpO2
- reasons for discrepancy: problem with the probe (poor perfusion) problem with the blood gas (venous sample) We must ensure that the abg makes sense. Check the correspondence of the saturation probe and the calculated saturation form the ABG.

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 If the pao2 is lower than we would expect for a given amount of inspired oxygen we can use the A-a gradient to work out why. PAo2 can be low because there is too little oxygen in the alveolus to pass into the blood stream, this is usually due to an increased amount of CO2 in the alveolus. A low Pao2 can also be due to a problem with transfer of the oxygen into the blood stream, even though there is an adequate amount of o2 in the alveolus. This happens if there is mismatching of ventilaiton and perfusion, or a barrier to oxygen diffusion

Oxygenation PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25]
The alveolar gas equation: PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25] The alveolar-arterial oxygen difference (A-a) PO2 = PAO2 - PaO2 To calculate the A-a gradient we need to first to calculate the amount of o2 in the alveolus – The PAo2. The partial pressure of o2 depends on the amount of o2 (fIO2), and the other gases present in the alveolus. Gas in the alveous is fully saturated with water so water vapour comprises some of the gas mixture. Fio2 is multipled by 94.8 to allow for this is derived by subtracting the saturated vapur pressure of water from atmospheric pressure (namely 100kpa – 6.2kpa). CO2 is also present in the alveolus. The amount of co2 depends on the metabolic state of the patient – how much co2 is being produced, and multipling by 1.25 (the reciprocal of the respiratory quotient allows for this. We can use arterial blood gas measurement of CO2 to approximate to alveolar co2 as the transfer of this gas in the lung is extremely quick and rapid calibration between blood and alveolus occurs.

Oxygenation PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25]
The alveolar gas equation: PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25] The alveolar-arterial oxygen difference (A-a) PO2 = PAO2 - PaO2 Once we have the alveolar po2 a and we know the arterial po2 we can calculate the gradient. Normally the gradient is small, about 1.3 kpa, although this does rise with age. If alveolar po is slow due to accumulation of co2 then arterial po2 will also be low but the gardient will be small. If the alveolar po2 is adequate but arterial po2 is low then the gradient is increased due to a problem with oxygen exchange

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. Normally the gradient is small, about 1.3 kpa, although this does rise with age. If alveolar po is low due to accumulation of co2 then arterial po2 will also be low but the gardient will be small. If the alveolar po2 is adequate but arterial po2 is low then the gradient is increased due to a problem with oxygen exchange

Acid base problems Normal pH = 7.35 – 7.45
Is there acidaemia or alkalaemia? Normal pH = 7.35 – 7.45 Acidaemia < Alkalaemia > 7.45 After we have looked at oxygenation next we consider acid base problems, and in doing so we look at ventilation. First we decide if the blood is acidaemic or alkalaemic. Remember that acidaemia and alkalaemia are ph abnormalities in the blood, and an acidosis or alkalosis is a physiological process that causes such abnormalities.

Acid base problems Look at PaCO2 Normal PaCO2 = 5.3 kPa
Is the primary problem respiratory or metabolic? Look at PaCO2 If we detect an abnormal ph we want to know if it is a respiratory or metabolic problem and we do this by looking first at the paco2, is it higher or lower than the normal value of 5.3 kpa? Normal PaCO2 = 5.3 kPa

Acid base problems Look at [HCO3-] Normal [HCO3-] = 24 mmol/l
Is the primary problem respiratory or metabolic? Look at [HCO3-] Next we look at the bicarbonate to see how it deviates from the normal value of 24 mmol/l. Form the CO2 and bicarbonate we can work out what the primary problem is and what compensation has taken place. Normal [HCO3-] = 24 mmol/l

+ Is there Is the PaCO2 Is the HCO3- It is Acidaemia High
( > 6 kPa) Normal/high (  24 mmol/l) Respiratory acidosis Low Metabolic acidosis Alkalaemia Normal/low Respiratory alkalosis Metabolic alkalosis If there is an acidosis and the pco2 is high then the patient is hypoventilating and not clearing co2. As co2 accumulates the equilibrium of the bicarbonate buffer system will be pushed to the right and more hydrogen ions will be generated. The kidney must retain more bicarbonate to buffer these hydrogen ions. This is a slow process and in the acute stages bicarbonate will be normal before slowly rising. This is a primary respiratory acidosis with renal compensation. Click to find out more about respiratory problems. + H20 CO2 HCO3- H+ H2CO3

+ Is there Is the PaCO2 Is the HCO3- It is Acidaemia High Normal/high
Respiratory acidosis Low ( < 4.5 kPa) (  22 mmol/l) Metabolic acidosis Alkalaemia Normal/low Respiratory alkalosis Metabolic alkalosis If there is an acidosis and the pco2 is low the Co2 is being blown off by increased ventilation. The bicarbonate is low and there is an excess of hydrogen ions, which pushes the equilibrium of the bicarbonate buffer system to the left. An increase in CO2 is sensed by chemoreceptors in the brain and ventialtion is increased to clear CO2. This is a rapid compensatory process. THIS is a primary metabolic acidosis with respiratory compensation. Click to find out more about metabolic problems. + H20 CO2 HCO3- H+ H2CO3

Respiratory acidosis Low Metabolic acidosis Alkalaemia ( < 4.5 kPa) Normal/low (  23 mmol/l) Respiratory alkalosis Metabolic alkalosis If there is an alkalosis and the pco2 is low the Co2 is being blown off by increased ventilation. This pulls the equilibrium of the bicarbonate buffer system to the left.. The level of hydrogen ions falls and bicarbonate must be lost from the sytem via the kidneys. This is a slow process and in the acute stages bicarbonate will be normal before slowly falling. This is a primary respiratory alkalosis with renal compensation. Click to find out more about respiratory problems. + H20 CO2 HCO3- H+ H2CO3

Respiratory acidosis Low Metabolic acidosis Alkalaemia Normal/low Respiratory alkalosis ( > 6 kPa) (  27 mmol/l) Metabolic alkalosis If there is an alkalosis and the pco2 is high the Co2 is being retained by reduced ventilation. The bicarbonate is high which over buffers hydrogen ions, and the equilibrium of the bicarbonate buffer system is pulled to the right. It is necessary to keep CO2 high and ventialtion is deccreased to retain CO2. This is a rapid compensatory process This is a primary metabolic alkalosis with respiratory compensation. Click to find out more about metabolic problems. + H20 CO2 HCO3- H+ H2CO3

For acute respiratory conditions
pH changes are more marked for acute conditions compared to chronic ones. ph will fall 0.06 for every one kpa that the Paco2 rises above normal in an acute respiratory acidosis, and rise 0.06 for every one kpa that paco2 falls below normal in an acute respiratory alkalosis.

Early renal compensation for respiratory conditions
For acute conditions bicarbonate will change only a small amount. It will rise 0.8 mmol/l for every one kpa the Paco2 rises above normal in an acute respiratory acidosis, and fall 1.5mmol/l for every one kpa that paco2 falls below normal in an acute respiratory alkalosis.

Late renal compensation for respiratory conditions
In chronic conditions there is more time for renal compensation to occur. Bicarbonate levels vary more from the normal range and ph changes are less marked. Late renal compensation for respiratory conditions

Respiratory acidosis Central nervous system depression Pleural Disease
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

Respiratory alkalosis
Catastrophic CNS event (CNS hemorrhage) Infection, fever Pregnancy (especially the 3rd trimester) Decreased lung compliance (interstitial lung disease) Liver cirrhosis Anxiety

Metabolic acidosis Anion gap acidosis Non anion gap acidosis
Any respiratory compensation? If we have a metabolic acidosis we can get more information by calculating the anion gap and looking at the base deficit/excess. Also we can assess the degree of rrespiratory compensation. Click ton one of the above to continue.

Anion Gap The anion gap is an artificial difference between the commonly unmeasured anions and cations. In reality there is electrochemical neutrality The anion gap is the difference between the sodium conc and the sum of the chloride and bicarbonate. In reality there is no gap as positive charge and negative charge must add up to zero. If there is an increase in unmeasured anions the gap will widen. Changes in the unmeasured cations are rarely great enough to alter the gap, the cations being dominated by sodium

Anion Gap Cations Anions Na+ HCO3- K+ Chloride- Ca2+ Protein (albumin)
Mg2+ Organic acids Phosphates Sulphates Unmeasured cations and anions are shown in green. There are more unmeasured anions than cations.

normal anion gap = 12 mmol/l
[Na+] + [unmeasured cations] = [Cl-] + [HCO3-] + [unmeasured anions] [unmeasured anions] - [unmeasured cations] = [Na+] - ([Cl-] + [HCO3-]) [Na+] – ( [Cl-] + [HCO3-] ) = Anion Gap Na+ A normal gap is approxiamtely 12 mm0l/l 144 – ( ) = 12

Anion gap acidosis Accumulation of organic acid not normally present in serum (lactic acid, ketones), replace HCO3- Fall in [HCO3-] will widen AG If organic acids not normally present in serum accumulate excess hydrogen ions must be buffered by bicarbonate and lelevs will fall. As a result the anion gap will widen. Link to causes of an increased anion gap acidosis or to return to the tutorial. Link to causes of anion gap acidosis

Non anion gap acidosis Loss of HCO3- (GI tract or renal)
increase in [chloride], AG not change Administration of exogenous chloride, [HCO3-] falls without AG change. If bicarbonate is lost from the system (as in bicarbonate rich small bowel fluid through an ileostomy or ileal caecal conduit) the additional loss of fluid will concentrate chloride and the elevated chloride concentration will cause the anion gap to remain normal. If extra chloride is added to the system bircarboante will fall but again the anion gap will not change. Link to causes of non-anion gap acidosis

Anion gap acidosis Uremia Ketoacidosis
diabetic hyperglycemia, Alcohol poisons or drug intoxication Methanol Lactic acidosis Sepsis, left ventricular failure Non-anion gap acidosis

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

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 The compensatory change for met acidosis is hyperventilation. PaCO2 should fall as CO2 is “blown off” to compensate for metabolic acidosis; if paco2 is not in the predicted range then an additional resp condition may co-exist. If Paco2 is higher than expected there may be an additional respiratory acidosis. If PaCO2 is lower than expected there may be additional respiratory alkalosis. We can use winter’s formula to calculate the expected range of pco2 for a metabolic acidosis

Winter’s formula Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
Winter’s formula gives a range of expected Pco2, click to go to examples of additional respiratory problems complicating a metabolic acidosis Link to examples

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 NO NARRATION A value out side this range suggests an additional respiratory disturbance Click to continue

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 If the actual PaCO2 is less than 2.8 kPa there is also RESPIRATORY ALKALOSIS NO NARRATION Click to continue

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 If the actual PaCO2 is more than 3.3 kPa there is also RESPIRATORY ACIDOSIS NO NARRATION Click to continue

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: The respiratory compensation for metabolic alkalosis is hypoventilation and retention of CO2 this is usually not complete, as too high co2 causes physiological derangements. Expected co2 is difficult to predict. A suggested formula is ….. Expected PaCO2 =  0.8 kPa per 10 mmol/l  in HCO3-

Metabolic alkalosis Volume contraction (vomiting, overdiuresis, ascites) Hypokalemia Alkali ingestion (bicarbonate) NO NARRATION Return to causes

Mixed disturbances Difficult to interpret Expected corrections
Sometimes more than one disturbance can co-exist. They may act in opposite directions, or in the same directions to exaggerated effect. We can use the expected corrections that we have already discussed, because results outside the expected range suggest other processes at work. We can also use acid base nomograms to plot our results and decide whether they are compatible with a simple disturbance or not. These disturbances can be difficult to interpret. Click to find out more about them or to go to examples. There is no narration. Use the navigation buttons to continue.

Respiratory compensation
For metabolic acidosis Winter’s formula: For metabolic alkalosis: Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133 Expected PaCO2 =  0.8 kPa per 10 mmol/l  in HCO3- Return to example

Expected corrections Primary 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-] 2. Calculate expected fall in [HCO3-] Metabolic acidosis 1. Winter’s formula for expected PaCO2 2. Corrected [HCO3-] in anion-gap acidosis Metabolic alkalosis 1. Difficult to predict, use suggested formula If the correction is NOT as expected there is another disturbance.

Correcting bicarbonate
Metabolic acidosis Anion Gap = [Na+] – [Cl-] - [HCO3-] Anion Gap Corrected [HCO3-] = measured [HCO3-] + (anion gap – 12) Correcting bicarbonate Return to example

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

Corrected [HCO3-] = measured [HCO3-] + (anion gap – 12) 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 Click to continue

An patient with metabolic acidosis AG 26 mmol/l HCO 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

Example 1. A 33 male patient with SARS has a saturation of 91% on Fi02 0.4 Is he hypoxic? Is there an acid base or ventilation problem? pH 7.43 PaCO2 4.76 PaO2 8.1 HCO3- 23 Base excess -0.6 Saturation 90% Click to continue

Example 1. 1. Is he hypoxic? YES.
The SpO2 and calculated saturation agree pH 7.43 PaCO2 4.76 PaO2 8.1 HCO3- 23 Base excess -0.6 Saturation 90% Click to continue

Example 1. 1. Is he hypoxic? (A-a) PO2 = 23.9 kPa YES.
There is major problem with oxygen transfer into the lung pH 7.43 PaCO2 4.76 PaO2 8.1 HCO3- 23 Base excess -0.6 Saturation 90% To calculate (A-a) PO2 Click to continue

Example 1. 2. Is there an acid base or ventilation problem? NO.
pH, PaCO2 and PaCO2 are normal This is pure respiratory failure pH 7.43 PaCO2 4.76 PaO2 8.1 23 Base excess -0.6 Saturation 90% Return to examples

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 7.08 PaCO2 10.6 PaO2 4.9 HCO3- 26 Base excess +2 Saturation 86% Click to continue

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 7.08 PaCO2 10.6 PaO2 4.9 HCO3- 26 Base excess +3 Saturation 86% Click to continue

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 7.08 PaCO2 10.6 PaO2 4.9 HCO3- 26 Base excess +3 Saturation 86% Click to continue

Example 2. 4. Is there an acid base or ventilation problem? YES.
pH 7.08 PaCO2 10.6 PaO2 4.9 HCO3- 26 Base excess +2 Saturation 86% Click to continue

Example 2. There is: Acidosis PaCO2 is elevated  RESPIRATORY ACIDOSIS

Example 2. There is: HCO3- = 28 Expected HCO3-
This is the expected [HCO3- ] if there has only been a small amount of renal compensation  ACUTE RESPIRATORY ACIDOSIS pH 7.08 PaCO2 10.6 PaO2 4.9 HCO3- 28 Base excess +2 Saturation 86% Click to continue

Example 2. There is: pH change: [10.6 – 5.3] x 0.06 = 0.32
CONSISTENT WITH SIMPLE ACUTE RESPIRATORY ACIDOSIS; NO ADDITIONAL DISTURBANCE pH 7.08 PaCO2 10.6 PaO2 4.9 HCO3- 28 Base excess +2 Saturation 86% Renal compensation Return to examples

Example 3. Clearly he is very hypoxic
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 7.23 PaCO2 8.1 PaO2 4.9 HCO3- 26 Base excess +3 Saturation 86% Click to continue

Example 3. Is the patient hypoxic due simply because of hypoventilation as a result of CNS depression or has he also aspirated and developed lung parenchymal problems? pH 7.23 PaCO2 8.1 PaO2 4.9 HCO3- 26 Base excess +3 Saturation 86% Click to continue

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 kPa The 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 7.23 PaCO2 8.1 PaO2 4.9 HCO3- 26 Base excess +3 Saturation 86% Click to continue

Example 3. YES. 3. Is there an acid base or ventilation problem?

Example 3. There is Acidosis PaCO2 is elevated  RESPIRATORY ACIDOSIS

This is the expected [HCO3- ] if there has only been a small amount of renal compensation  ACUTE RESPIRATORY ACIDOSIS pH 7.23 PaCO2 8.1 PaO2 4.9 HCO3- 26 Base excess +3 Saturation 86% Click to continue

 CONSISTENT WITH SIMPLE ACUTE RESPIRATORY ACIDOSIS; NO ADDITIONAL DISTURBANCE pH 7.23 PaCO2 8.1 PaO2 4.9 HCO3- 26 Base excess +3 Saturation 86% Return to examples

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 7.34 PaCO2 8.0 PaO2 7.5 HCO3- 32.1 Base excess +8 Saturation 86% Click to continue Click to continue

Example 4. 1. Is he hypoxic? YES. The (A-a) PO2 = 2.4 kPa
The (A-a) gradient is increased, and home oxygen might be appropriate pH 7.34 PaCO2 8.0 PaO2 7.5 HCO3- 32.1 Base excess +8 Saturation 86% Click to continue

pH 7.34 PaCO2 8.0 PaO2 7.5 HCO3- 32.1 Base excess +8 Saturation 86% Click to continue

Example 4. There is: Mild acidosis PaCO2 is elevated
 RESPIRATORY ACIDOSIS pH 7.34 PaCO2 8.0 PaO2 7.5 HCO3- 32.1 Base excess +8 Saturation 86% Diagnose disturbance Click to continue

Example 4. There is: HCO3- = 32.1 Expected HCO3-
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 7.34 PaCO2 8.0 PaO2 7.5 HCO3- 32.1 Base excess +8 Saturation 86% Click to continue

 CONSISTENT WITH SIMPLE CHRONIC RESPIRATORY ACIDOSIS; NO ADDITIONAL DISTURBANCE pH 7.34 PaCO2 8.0 PaO2 7.5 HCO3- 32.1 Base excess +8 Saturation 86% Return to examples

Example 5. A 35 year old woman with a history of anxiety attacks presents to ER with alpitations. pH 7.54 PaCO2 2.9 PaO2 12.1 HCO3- 22 Base excess +2 Saturation 100% Click to continue

Example 5. 1. Is she hypoxic? NO. This is a normal PaO2 for room air

Example 5. There is: Alkalosis PaCO2 is decreased
 RESPIRATORY ALKALOSIS pH 7.54 PaCO2 2.9 PaO2 12.1 HCO3- 22 Base excess +2 Saturation 100% Diagnose disturbance Click to continue

This is the expected [HCO3- ] if there has only been a small amount of renal compensation  ACUTE RESPIRATORY ALKALOSIS pH 7.54 PaCO2 2.9 PaO2 12.1 HCO3- 20 Base excess +2 Saturation 100% Click to continue

Example 5. There is: pH change: [5.3-2.9] x 0.06 = 0.144
 CONSISTENT WITH SIMPLE ACUTE RESPIRATORY ALKALOSIS; NO ADDITIONAL DISTURBANCE pH 7.54 PaCO2 2.9 PaO2 12.1 HCO3- 22 Base excess +2 Saturation 100% Return to examples

Example 6. A 42 year old diabetic woman present with UTI symptoms; she has deep sighing respiration. This is the ABG on FiO2 0.4 pH 7.23 PaCO2 3.3 PaO2 29.9 HCO3- 12 Base excess -10 Saturation 100% Click to continue

Example 6. 1. Is she hypoxic? NO.
This PaO2 is adequate for an FiO2 of 0.4 pH 7.23 PaCO2 3.3 PaO2 29.9 HCO3- 12 Base excess -10 Saturation 100% Click to continue

pH 7.23 PaCO2 3.3 PaO2 29.9 HCO3- 12 Base excess -10 Saturation 100% Click to continue

Example 6. There is: Acidosis PaCO2 is decreased
 NOT respiratory acidosis Look at [HCO3-] [HCO3-] is reduced Base excess is negative  METABOLIC ACIDOSIS pH 7.23 PaCO2 3.3 PaO2 29.9 HCO3- 12 Base excess -10 Saturation 100% Click to continue

Example 6. Using Winter’s formula: Expected PaCO2
= [ (1.5 x 12) + (8 ± 2) ] x 0.133 = 3.2 – 3.7 kPa The PaCO2 falls within this range  SIMPLE METABOLIC ACIDOSIS pH 7.23 PaCO2 3.3 PaO2 29.9 HCO3- 12 Base excess -10 Saturation 100% Click to continue

Example 6. = [Na+] – ( [Cl-] + [HCO3-] ) = [135] – ( 99 + 12 ) Na
What is the anion gap? = [Na+] – ( [Cl-] + [HCO3-] ) = [135] – ( ) Na = 24 mmol/l There is an anion gap acidosis due to accumulation of organic acids caused by diabetic ketoacidosis pH 7.23 PaCO2 3.3 PaO2 29.9 HCO3- 12 Base excess -10 Na+ 135 Cl- 99 Click to continue

Example 6. = 24 mmol/l The PaCO2 falls within the expected range
Corrected bicarbonate = 24 mmol/l The PaCO2 falls within the expected range SIMPLE METABOLIC ACIDOSIS; NO OTHER DISTURBANCE pH 7.23 PaCO2 3.3 PaO2 29.9 HCO3- 12 Base excess -10 Na+ 135 Cl- 99 Return to examples

Example 7. A 70 year old man presents with a 3 day history of severe vomiting. Here is his ABG on room air. pH 7.5 PaCO2 6.2 PaO2 10.6 HCO3- 38 Base excess +8 Saturation 96% Click to continue

Example 7. 1. Is he hypoxic? NO.
This is a normal PaO2 for a patient this age breathing room air pH 7.5 PaCO2 6.2 PaO2 10.6 HCO3- 38 Base excess +8 Saturation 96% Click to continue

Example 7. There is: Alkalosis PaCO2 is elevated
 NOT respiratory alkalosis Look at [HCO3-] [HCO3-] is increased Base excess is positive  METABOLIC ALKALOSIS pH 7.5 PaCO2 6.2 PaO2 10.6 HCO3- 38 Base excess +8 Saturation 96% Click to continue

Example 7. 3. Is there respiratory compensation? Expected PaCO2
=  0.8 kPa per 10 mmol/l  in HCO3- = (0.8 x ([ 38 – 24 ]/10)) = 6.4  CONSISTENT WITH SIMPLE METABOLIC ALKALOSIS pH 7.5 PaCO2 6.3 PaO2 10.6 HCO3- 38 Base excess +8 Saturation 96% Return to examples

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 7.07 PaCO2 8.63 PaO2 11.8 HCO3- 17.9 Base excess -6.5 Saturation 95% Click to continue

Example 8. 1. Is she hypoxic? YES.
This PaO2 is very low for an FiO2 of 1.0 pH 7.07 PaCO2 8.63 PaO2 11.8 HCO3- 17.9 Base excess -6.5 Saturation 95% Click to continue

pH 7.07 PaCO2 8.63 PaO2 11.8 HCO3- 17.9 Base excess -6.5 Saturation 95% Click to continue

Example 8. There is Acidosis PaCO2 is elevated  RESPIRATORY ACIDOSIS
pH 7.07 PaCO2 8.63 PaO2 11.8 HCO3- 17.9 Base excess -6.5 Saturation 95% Diagnose disturbance Click to continue

Example 8. Expected pH = 7.4 – ([8.63-5.3] x 0.06) = 7.2
 Observed pH is lower Expected bicarbonate = 24 + ([ ] x 0.8) = 26.7 mmol/l  Observed bicarbonate is too low pH 7.07 PaCO2 8.63 PaO2 11.8 HCO3- 17.9 Base excess -6.5 Saturation 95% Click to continue

Example 8. Lower pH Lower bicarbonate Base deficit negative
ADDITIONAL METABOLIC ACIDOSIS pH 7.07 PaCO2 8.63 PaO2 11.8 HCO3- 17.9 Base excess -6.5 Saturation 95% Severe ARDS leads to hypoxia & hypercapnia with respiratory acidosis; renal failure and poor perfusion lead to metabolic acidosis Return to examples

Example 9. A 43 year old man presents with an overdose of aspirin. This is his ABG on air. pH 7.37 PaCO2 2.3 PaO2 12 HCO3- 10 Base excess -7.4 Saturation 97% Click to continue

Example 9. 1. Is he hypoxic? NO.
This is a normal PaO2 for a patient this age breathing room air pH 7.37 PaCO2 2.3 PaO2 12 HCO3- 10 Base excess -7.4 Saturation 97% Click to continue

Example 9. 2. Is there an acid base or ventilation problem? NO?
Or is there? pH 7.37 PaCO2 2.3 PaO2 12 HCO3- 10 Base excess -7.4 Saturation 97% Click to continue

Example 9. PaCO2 is low Respiratory alkalosis? Metabolic acidosis?
HCO3- is low Negative base deficit pH 7.37 PaCO2 2.3 PaO2 12 HCO3- 10 Base excess -7.4 Saturation 97% Diagnose disturbance Click to continue

Respiratory compensation
Example 9. 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 pH 7.37 PaCO2 2.3 PaO2 12 HCO3- 10 Base excess -7.4 Saturation 97% Respiratory compensation Click to continue

Example 9. Aspirin overdose characteristically causes a metabolic acidosis due the effect of salicylic acid and a respiratory alkalosis due to hyperventilation pH 7.37 PaCO2 2.3 PaO2 12 HCO3- 10 Base excess -7.4 Saturation 97% Return to examples

THANK YOU! Questions? Comments?

Interpretation of arterial blood gases & compensation calculation

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Interpretation of arterial blood gases & compensation calculation

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