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R26 Respirology
Respiratory Failure
Toronto Notes 2019
Respiratory Failure
Definition
• failureofrespiratorysystemtomaintainnormalbloodgases • hypoxemic (PaO2 <60 mmHg)
• hypercapnic (PaCO2 >50 mmHg)
• acutevs.chronic(compensatorymechanismsactivated)
Signs and Symptoms
• signsofunderlyingdisease
• hypoxemia:restlessness,confusion,cyanosis,coma,corpulmonale
• hypercapnia:headache,dyspnea,drowsiness,asterixis,warmperiphery,plethora,increasedICP
(secondary to vasodilatation)
Investigations
• serialABGs
• CXRand/orCT,bronchoscopytocharacterizeunderlyingcauseifunclear
Hypoxemic Respiratory Failure
Definition
• PaO2 decreased, PaCO2 normal or decreased
Treatment
• reversetheunderlyingpathology
• oxygentherapy:maintainoxygenation(ifshuntpresent,supplementalO2islesseffective;
see Anesthesia and Perioperative Medicine, A10, for oxygen delivery systems)
• ventilation,BiPAP,andPEEP/CPAP(seeAnesthesiaandPerioperativeMedicine,A100):positive
pressure can recruit alveoli and redistribute lung fluid
• improvecardiacoutput:±hemodynamicsupport(fluids,vasopressors,inotropes),reductionofO2
requirements
Table 27. Approach to Hypoxemia
Type of Hypoxemia
1. Low FiO2
2. Hypoventilation
3a. Shunt (Intrapulmonary)
3b. Shunt (Right to Left)
4. Low Mixed Venous O2 Content
5. V/Q Mismatch
6. Diffusion Impairment
Settings
Postop, high altitude Drug overdose
ARDS, pneumonia
Pulmonary HTN Shock
COPD
ILD, emphysema
PaCO2
N or  
N or  N or 

N or  N
A-aDO2 Oxygen Therapy
N Improves N Improves
 No change  No change
 Improves or no change
 Improves (small amounts)
 Improves
Ventilation, BiPAP, and PEEP
No change
Improves with ventilation
Improves (except if one-sided)
Worsens Worsens
Often improves
Improves with positive pressure
Change in Cardiac Output
No change No change
Improves
Worsens Improves
Improves
No change or worsens
Dead Space
• Ventilation without perfusion • The opposite of shunt
Causes of Hypercapnia
• High inspired CO2
• Low total ventilation
• High deadspace ventilation • High CO2 production
Hypercapnic Respiratory Failure
Definition
• PaCO2increased,PaO2decreased
Pathophysiology
Reprinted with permission from Dr. Ian Fraser
• increasedCO2production:fever,sepsis,seizure,acidosis,carbohydrateload
• alveolarhypoventilation:COPD,asthma,CF,chestwalldisorder,deadspaceventilation(rapidshallow
breathing)
■ inefficient gas exchange results in inadequate CO2 removal in spite of normal or increased minute
volume
• hypoventilation
■ central: brainstem stroke, hypothyroidism, severe metabolic alkalosis, drugs (opiates, benzodiazepines)
■ neuromuscular: myasthenia gravis, Guillain-Barré, phrenic nerve injury, muscular dystrophy, polymyositis, kyphoscoliosis
■ muscle fatigue