Acute Respiratory Failure

1. Acute Respiratory Failure James Bonnington

2. Outline • Hypoxia • Hypercapnia • Interpretation of arterial blood gases

3. Case • Call from the ward “57M on 8L of O2, sats are persistently 84%” • What are your thoughts?

4. Hypoxia PAO2 PaO2 PcellO2

5. Hypoxia PIO2 Hypoxic VCO2 PAO2 VA Diffusion PaO2 V/Q mismatch Shunt Hb PcellO2 CO Anaemic Stagnant VO2 Histotoxic

6. Hypoxia PIO2 Hypoventilation -Airway obstruction -Central failure -Muscle weakness Pulmonary fibrosis Interstitial disease Emphysema VCO2 PAO2 VA Diffusion PaO2 V/Q mismatch Shunt Hb PcellO2 CO Consolidation Pulmonary contusion Atelectasis Pulmonary oedema Extrapul shunt VO2

7. Hypoxia PIO2 Hypoventilation -Airway obstruction -Central failure -Muscle weakness Pulmonary fibrosis Interstitial disease Emphysema VCO2 PAO2 VA Diffusion PaO2 V/Q mismatch Shunt Treatment -Adequate VA -Rest -O2 (humidified) -PEEP -Specific Hb PcellO2 CO Consolidation Pulmonary contusion Atelectasis Pulmonary oedema Extrapul shunt VO2

8. PaO2 Hypoxic hypoxia Hypoventilation Bronchospasm Pneumothorax Airway obstruction Misplaced ETT Lung collapse FiO2 Specific treatment Patient assessment Acute reversible cause NO Interstitial disease ALI/ARDS Consolidation COAD PE Heart failure V/Q mismatch

9. Hypercapnia PACO2 PaCO2 PcellCO2

10. Hypercapnia DS TV VA RR PICO2 PACO2 V/Q mismatch PaCO2 Shunt PcellCO2 VCO2

11. Hypercapnia Airway obstruction CO PE ARDS PEEP DS (Ventilation without perfusion) TV VA RR PICO2 PACO2 V/Q mismatch PaCO2 Treatment -Adequate VA -PEEP -Specific Shunt PcellCO2 (perfusion without ventilation) VCO2

12. Interpretation of Arterial Blood Gases • What do you look at first? • The Patient • What do you look at next? • PaO2 • Oxygenation • Treat hypoxia promptly • PaCO2 • Ventilation • pH • Acid-base status

13. Interpretation of Arterial Blood Gases • pH reflects the primary disorder • Is there any compensation or mixed disorder • Boston rules • Anion gap

14. Boston Rules • These assess compensation and are a guide to detecting a second primary acid-base disorder • For example, in a patient with metabolic acidosis, if the measured pCO2 is higher than expected, this points to a coexisting respiratory acidosis

15. Boston Rule 1 • The 1 for 10 Rule for Acute Respiratory Acidosis • The [HCO3] will increase by 1 mmol/l for every 10 mmHg elevation in pCO2 above 40 mmHg. • Expected [HCO3] = 24 + { (Actual pCO2 - 40) / 10 }

16. Boston Rule 2 • The 4 for 10 Rule for Chronic Respiratory Acidosis • The [HCO3] will increase by 4 mmol/l for every 10 mmHg elevation in pCO2 above 40mmHg. • Expected [HCO3] = 24 + 4 { (Actual pCO2 - 40) / 10}

17. Boston Rule 3 • The 2 for 10 Rule for Acute Respiratory Alkalosis • The [HCO3] will decrease by 2 mmol/l for every 10 mmHg decrease in pCO2 below 40 mmHg. • Expected [HCO3] = 24 - 2 { ( 40 - Actual pCO2) / 10 }

18. Boston Rule 4 • The 5 for 10 Rule for a Chronic Respiratory Alkalosis (3-5 days) • The [HCO3] will decrease by 5 mmol/l for every 10 mmHg decrease in pCO2 below 40 mmHg. • Expected [HCO3] = 24 - 5 { ( 40 - Actual pCO2 ) / 10 } ( range: +/- 2)

19. Boston Rule 5 • Rule 5 : The One & a Half plus 8 Rule for a Metabolic Acidosis • The expected pCO2 (in mmHg) is calculated from the following formula: • Expected pCO2 = 1.5 x [HCO3] + 8 (range: +/- 2)

20. Boston Rule 6 • The Point Seven plus Twenty Rule for a Metabolic Alkalosis • The expected pCO2(in mmHg) is calculated from the following formula: • Expected pCO2 = 0.7 [HCO3] + 20 (range: +/- 5)

21. A 70 year old man was admitted with severe congestive cardiac failure. He has been unwell for about a week and has been vomiting for the previous 5 days. He was on no medication. He was hyperventilating and was very distressed. Admission biochemistry is listed below. He was on high concentration oxygen by mask. Biochemistry results: Na+ 127, K+ 5.2, Cl- 79, Urea 50.5, Creatinine 380 & glucose 9.5 mmols/l. Anion gap 33 mmols/l Respiratory alkalosis Boston rule 4 - Expected [HCO3] = 24 - 5 { ( 40 - Actual pCO2 ) / 10 } = 24 – 5 (40-21/10) = 14.5 Mixed respiratory and metabolic alkalosis

22. Outline • Hypoxia • Hypercapnia • Interpretation of arterial blood gases