Acute Respiratory Failure James Bonnington

Outline Hypoxia Hypercapnia Interpretation of arterial blood gases

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

Hypoxia PAO2 PaO2 PcellO2

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

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

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

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

Hypercapnia PACO2 PaCO2 PcellCO2

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

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

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

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

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

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 } The increase in CO2 shifts the equilibrium between CO2 and HCO3 to result in an acute increase in HCO3. This is a simple physiological event and occurs almost immediately

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} With chronic acidosis, the kidneys respond by retaining HCO3. This takes a few days to reach its maximal value

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 } In practice, rarely results in a HCO3 of less than about 18. Limit to how low low PCO2 can fall as negative values are not possible! A HCO3 of <18 indicates a co-existing metabolic acidosis

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) Takes 2-3 days to reach maximal renal compensation Limit of compensation is a HCO3 of about 12-15

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) Maximal compensation can take 12-24hrs The limit of compensation if a PCO2 of about 10

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) The variation predicted by this is quite large

Biochemistry results: Na+ 127, K+ 5.2, Cl- 79, Urea 50.5, 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 Arterial Blood Gases pH  7.58 pCO2 21 mmHg pO2   154 mmHg HCO3   19 mmol/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

Outline Hypoxia Hypercapnia Interpretation of arterial blood gases