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 Hypoxic PIO2 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 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

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 }
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

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] = { (Actual pCO2 - 40) / 10}
With chronic acidosis, the kidneys respond by retaining HCO3. This takes a few days to reach its maximal value

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] = { ( 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

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] = { ( 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

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

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)
The variation predicted by this is quite large

21. 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] = { ( 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