Blood Gas Blood Gas Blood Gas QuestionQuestionQuestionGraeme ThomsonGraeme ThomsonGraeme Thomson

80 years old from nursing home with altered 80 years old from nursing home with altered 80 years old from nursing home with altered conscious state. Arterial blood gas results.conscious state. Arterial blood gas results.conscious state. Arterial blood gas results.

FiO2FiO2FiO2 .21.21.21

pHpHpH 7.097.097.09

paCO2paCO2paCO2 72 mmHg72 mmHg72 mmHg

HCO3HCO3HCO3 27 mmol/L27 mmol/L27 mmol/L

paO2paO2paO2 50 mmHg50 mmHg50 mmHg

What is your interpretation?What is your interpretation?What is your interpretation?

Acute pure respiratory acidosis with hypoxaemia but no ventilation/perfusion Acute pure respiratory acidosis with hypoxaemia but no ventilation/perfusion Acute pure respiratory acidosis with hypoxaemia but no ventilation/perfusion mismatch. Pure hypoventilation.mismatch. Pure hypoventilation.mismatch. Pure hypoventilation.

Show 4 calculations to justify your Show 4 calculations to justify your Show 4 calculations to justify your answer.answer.answer.

Show 4 calculations to justify your Show 4 calculations to justify your Show 4 calculations to justify your interpretation.interpretation.interpretation.

Expected pH for pure resp acidosis = 7.35 – 0.08 x (72 – 40)/10 = 7.09Expected pH for pure resp acidosis = 7.35 – 0.08 x (72 – 40)/10 = 7.09Expected pH for pure resp acidosis = 7.35 – 0.08 x (72 – 40)/10 = 7.09

Expected HCO3 for acute resp acidosis = 24 + (72 – 40)/10 = 27.32Expected HCO3 for acute resp acidosis = 24 + (72 – 40)/10 = 27.32Expected HCO3 for acute resp acidosis = 24 + (72 – 40)/10 = 27.32

Alveolar pO2 = (713 x 0.21) – (1.25 x 72) = 60Alveolar pO2 = (713 x 0.21) – (1.25 x 72) = 60Alveolar pO2 = (713 x 0.21) – (1.25 x 72) = 60

A-a gradient = (713 x 0.21) – (1.25 x 72) – 50 = 10A-a gradient = (713 x 0.21) – (1.25 x 72) – 50 = 10A-a gradient = (713 x 0.21) – (1.25 x 72) – 50 = 10

Maximum expected A-a gradient for age = 80/4 + 4 = 24Maximum expected A-a gradient for age = 80/4 + 4 = 24Maximum expected A-a gradient for age = 80/4 + 4 = 24

Expected paO2 for age with this paCO2 = > (60 – 24) = >36Expected paO2 for age with this paCO2 = > (60 – 24) = >36Expected paO2 for age with this paCO2 = > (60 – 24) = >36

Give 2 likely causes and for each an Give 2 likely causes and for each an Give 2 likely causes and for each an intervention to quickly reduce the paCO2.intervention to quickly reduce the paCO2.intervention to quickly reduce the paCO2.

CausesCausesCauses

OpiatesOpiatesOpiates

Benzodiazepines and other sedativesBenzodiazepines and other sedativesBenzodiazepines and other sedatives

Stroke and other acute intracranial eventsStroke and other acute intracranial eventsStroke and other acute intracranial events

Cervical spine injury and other causes of acute respiratory paralysisCervical spine injury and other causes of acute respiratory paralysisCervical spine injury and other causes of acute respiratory paralysis

Recent onset status epilepticusRecent onset status epilepticusRecent onset status epilepticus

HypoglycaemiaHypoglycaemiaHypoglycaemia

Airway obstructionAirway obstructionAirway obstruction

InterventionsInterventionsInterventions

NaloxoneNaloxoneNaloxone

FlumazenilFlumazenilFlumazenil

VentilationVentilationVentilation

AnticonvulsantsAnticonvulsantsAnticonvulsants

GlucoseGlucoseGlucose

Provide an unobstructed airwayProvide an unobstructed airwayProvide an unobstructed airway

What is his O2 saturation What is his O2 saturation What is his O2 saturation approximately?approximately?approximately?

paO2 50 equates to O2 sat of approximately 80% (accepted 75-85)paO2 50 equates to O2 sat of approximately 80% (accepted 75-85)paO2 50 equates to O2 sat of approximately 80% (accepted 75-85)

Remember the Hb/O2 saturation curveRemember the Hb/O2 saturation curveRemember the Hb/O2 saturation curve

Useful numbers:Useful numbers:Useful numbers: 90% = pO2 6090% = pO2 6090% = pO2 60

75% = pO2 4075% = pO2 4075% = pO2 40

If a patient with chronic CO2 retention is If a patient with chronic CO2 retention is If a patient with chronic CO2 retention is administered excessive oxygen list 2 administered excessive oxygen list 2 administered excessive oxygen list 2

important mechanisms by which CO2 may important mechanisms by which CO2 may important mechanisms by which CO2 may rise.rise.rise.

Increased shunting due to increased blood flow to poorly ventilated alveoli.Increased shunting due to increased blood flow to poorly ventilated alveoli.Increased shunting due to increased blood flow to poorly ventilated alveoli.

Haldane effect causing offloading of CO2 from red cells.Haldane effect causing offloading of CO2 from red cells.Haldane effect causing offloading of CO2 from red cells.

Not suppression of hypoxic drive that is a mythological concept.Not suppression of hypoxic drive that is a mythological concept.Not suppression of hypoxic drive that is a mythological concept.

ResultsResultsResults

Pass mark set at 11/18Pass mark set at 11/18Pass mark set at 11/18

8/43 candidates passed8/43 candidates passed8/43 candidates passed

Range 0-14Range 0-14Range 0-14

Mean and median scores = 8/18Mean and median scores = 8/18Mean and median scores = 8/18

Most common errorsMost common errorsMost common errors CalculationsCalculationsCalculations

Lack of knowledge of the alveolar gas equationLack of knowledge of the alveolar gas equationLack of knowledge of the alveolar gas equation Using the P/F ratioUsing the P/F ratioUsing the P/F ratio Not determining if this was purely respiratory or mixed respiratory and metabolicNot determining if this was purely respiratory or mixed respiratory and metabolicNot determining if this was purely respiratory or mixed respiratory and metabolic Irrelevant calculations (If primarily metabolic?)Irrelevant calculations (If primarily metabolic?)Irrelevant calculations (If primarily metabolic?)

Causes and interventionsCauses and interventionsCauses and interventions Causes that are not consistent with normal V/Q matching, ie COPD, asthma, sepsisCauses that are not consistent with normal V/Q matching, ie COPD, asthma, sepsisCauses that are not consistent with normal V/Q matching, ie COPD, asthma, sepsis Interventions that increase pO2 not decrease pCO2Interventions that increase pO2 not decrease pCO2Interventions that increase pO2 not decrease pCO2

Interpretation of pulse oximeter resultsInterpretation of pulse oximeter resultsInterpretation of pulse oximeter results Effects of excessive O2 deliveryEffects of excessive O2 deliveryEffects of excessive O2 delivery

How to correct errorsHow to correct errorsHow to correct errors

Have a systematic approach to ABG analysis.Have a systematic approach to ABG analysis.Have a systematic approach to ABG analysis.

When you are asked for differential diagnoses only include conditions that fit When you are asked for differential diagnoses only include conditions that fit When you are asked for differential diagnoses only include conditions that fit the scenario. Read the details of the scenario.the scenario. Read the details of the scenario.the scenario. Read the details of the scenario.

Know how to use and interpret simple monitors like the pulse oximeter.Know how to use and interpret simple monitors like the pulse oximeter.Know how to use and interpret simple monitors like the pulse oximeter.

Be up to date with current concepts in management of critically ill patients.Be up to date with current concepts in management of critically ill patients.Be up to date with current concepts in management of critically ill patients.

Learn the correlation between venous and arterial gases. Learn the correlation between venous and arterial gases. Learn the correlation between venous and arterial gases.

Blood gas analysis rules page 1/3Blood gas analysis rules page 1/3Blood gas analysis rules page 1/3 Acute respiratory acidosis expected HCO3 Acute respiratory acidosis expected HCO3 Acute respiratory acidosis expected HCO3 = 24 + (CO2 – 40)/10= 24 + (CO2 – 40)/10= 24 + (CO2 – 40)/10

Chronic respiratory acidosis expected HCO3 Chronic respiratory acidosis expected HCO3 Chronic respiratory acidosis expected HCO3 = 24 + 4(CO2 – 40)/10= 24 + 4(CO2 – 40)/10= 24 + 4(CO2 – 40)/10

Acute respiratory alkalosis expected HCO3 Acute respiratory alkalosis expected HCO3 Acute respiratory alkalosis expected HCO3 = 24 – 2(40 – CO2)/10= 24 – 2(40 – CO2)/10= 24 – 2(40 – CO2)/10

Chronic respiratory alkalosis expected HCO3Chronic respiratory alkalosis expected HCO3Chronic respiratory alkalosis expected HCO3 = 24 – 5(40 – CO2)/10= 24 – 5(40 – CO2)/10= 24 – 5(40 – CO2)/10

Acute respiratory acidosis expected pHAcute respiratory acidosis expected pHAcute respiratory acidosis expected pH = 7.35 – 0.08(CO2 – 40)/10= 7.35 – 0.08(CO2 – 40)/10= 7.35 – 0.08(CO2 – 40)/10 Reverse for acute respiratory alkalosisReverse for acute respiratory alkalosisReverse for acute respiratory alkalosis

Chronic respiratory acidosis expected pHChronic respiratory acidosis expected pHChronic respiratory acidosis expected pH = 7.35 – 0.03(CO2 – 40)/10= 7.35 – 0.03(CO2 – 40)/10= 7.35 – 0.03(CO2 – 40)/10 Reverse for chronic respiratory alkalosisReverse for chronic respiratory alkalosisReverse for chronic respiratory alkalosis

Blood gas analysis rules page 2/3Blood gas analysis rules page 2/3Blood gas analysis rules page 2/3

Metabolic acidosis expected pCO2 Metabolic acidosis expected pCO2 Metabolic acidosis expected pCO2 = 1.5 x HCO3 + 8= 1.5 x HCO3 + 8= 1.5 x HCO3 + 8

Metabolic alkalosis expected pCO2Metabolic alkalosis expected pCO2Metabolic alkalosis expected pCO2 = 0.7 x HCO3 + 20= 0.7 x HCO3 + 20= 0.7 x HCO3 + 20

Anion gap (normal = 12 or less)Anion gap (normal = 12 or less)Anion gap (normal = 12 or less) = Na – (Cl + HCO3)= Na – (Cl + HCO3)= Na – (Cl + HCO3)

Osmolar gap (Osmolality measured in lab – Osmolarity calculated from Osmolar gap (Osmolality measured in lab – Osmolarity calculated from Osmolar gap (Osmolality measured in lab – Osmolarity calculated from U+Es)U+Es)U+Es) Osmolarity = (1.86 x Na) + glucose + urea + 9Osmolarity = (1.86 x Na) + glucose + urea + 9Osmolarity = (1.86 x Na) + glucose + urea + 9

Delta RatioDelta RatioDelta Ratio = (AG – 12) / (24 – HCO3)= (AG – 12) / (24 – HCO3)= (AG – 12) / (24 – HCO3) <0.4 = NAGMA;<0.4 = NAGMA;<0.4 = NAGMA; 1-2 = HAGMA1-2 = HAGMA1-2 = HAGMA

Blood gas analysis rules page 3/3Blood gas analysis rules page 3/3Blood gas analysis rules page 3/3

A-a gradient A-a gradient A-a gradient = (713 x FiO2) – (1.25 x CO2) – paO2= (713 x FiO2) – (1.25 x CO2) – paO2= (713 x FiO2) – (1.25 x CO2) – paO2

Expected A-a for age Expected A-a for age Expected A-a for age = Age/4 + 4= Age/4 + 4= Age/4 + 4

paO2/FiO2paO2/FiO2paO2/FiO2 = respiratory failure if <300= respiratory failure if <300= respiratory failure if <300

Correlation between Arterial and Venous gasesCorrelation between Arterial and Venous gasesCorrelation between Arterial and Venous gases pH and BE correlate wellpH and BE correlate wellpH and BE correlate well pCO2 and HCO3 correlate poorly (CO2 +/- 10; HCO3 +/- 2)pCO2 and HCO3 correlate poorly (CO2 +/- 10; HCO3 +/- 2)pCO2 and HCO3 correlate poorly (CO2 +/- 10; HCO3 +/- 2)