Acute Respiratory Failure

Question 1

DEFINE: acute respiratory failure

Answer 1

respiratory system no longer able to meet the metabolic demands of the body, resulting in dropping levels of O2 and rising levels of CO2 for either or both of impaired gas exchange (diffusion/perfusion) or impaired ventilation

Question 2

What are the two major functions of the lung?

Answer 2

oxygenation of blood and elimination of carbon dioxide

Question 3

What are the two types of respiratory failure and how are they defined?

Answer 3

HYPOXEMIC: when arterial partial pressure of O2 (PaO2) = <60mmHg at room air

HYPERCAPNIC: PaCO2 >50mmHg

(acute vs chronic - depending on duration of respiratory failure and nature of compensation)

Question 4

What are the main mechanisms of acute respiratory failure?

Answer 4

1. LOW INSPIRED PARTIAL PRESSURE OF O2
2. HYPOVENTILATION
3. VENTILATION PERFUSION MISMATCH
4. DIFFUSION ABNORMALITY
   - decreased diffusion of O2 across alveolar-capillary membrane → reduced PaO2 → hypoxia

Question 5

How does LOW INSPIRED PARTIAL PRESSURE OF O2 cause acute respiratory failure + give causes

Answer 5

LOW INSPIRED PARTIAL PRESSURE OF O2
- reduces diffusion gradient of O2 across alveolar membrane
(PaO2 = product of inspired O2 FiO2 and barometric pressure)

eg: high altitude

Question 6

How does HYPOVENTILATION cause acute respiratory failure + give causes

Answer 6

HYPOVENTILATION
- complicated by both hypoxia and hypercapnia

CNS: respiratory centre depression, drugs, anaesthetics, head injury, encephalopathy, fatigue etc

Disruption of resp signal during transmission of nerves to respiratory muscles: spinal cord injury, motor neuron disease, Guillain-Barre syndrome

Dysfunction of NMJ (neuromuscular junction): paralytic agents, myasthenia gravis

Dysfunction of muscles of respiration: myopathy, fatigue, malnutrition, muscular dystrophy etc

Chest wall abnormalities: kyphoscoliosis, ankylosing spondylitis, pleural fibrosis etc

Airway obstruction: upper or lower

Decreased Lung compliance:

Question 7

How does VENTILATION PERFUSION MISMATCH cause acute respiratory failure + give causes

Answer 7

VENTILATION PERFUSION MISMATCH
- most common cause of hypoxemic respiratory failure

(1) reduced perfusion to alveolus:
- gas passes in and out of alveolus but doesnt come into contact with capillary bed, effectively increasing the deadspace - Co2 cant be eliminated and O2 cant diffuse across
eg: PE, hypovolaemia, poor cardiac output, high intrathoracic pressures (from positive pressure ventilation), COPD

(2) reduced ventilation to alveolus
eg: alveolus full of fluid or pus ∴ not ventilated → capillary bed is not oxygenated → this deoxygenated blood mixes with oxygenated blood → ↓O2 sats
- hypoxic pulmonary vasoconstriction reduces proportion of blood perfusing non-ventilated lung ∴ minimising shunting
- NOTE: increasing FiO2 in this setting has little effect as well ventilated units are already 100% saturated
eg: pneumonia, ARDS, pulmonary oedema, pulmonary haemorrhage, contusion, atelectasis, pneumothorax etc

(3) anatomical shunting (intracardiac)
eg: Fallot tetralogy, Eisenmenger syndrome

Question 8

How does DIFFUSION ABNORMALITY cause acute respiratory failure + give causes

Answer 8

DIFFUSION ABNORMALITY

• decreased diffusion of O2 across alveolar-capillary membrane → reduced PaO2 → hypoxia
• less common
  eg: severe destructive diseases of the lung - late fibrosing diseases, severe pulmonary oedema, ARDS

Question 9

What is the A-a gradient?

Answer 9

gradient between partial pressure of O2 in alveolus and arterial blood (normal = <20mmHg - but this is altered by age and FiO2)

Normal Aa gradient implies that any hypoxaemia can be explained by the hypercapnia

Increased gradient implies a ventilation/perfusion mismatch (shunt) or diffusion abnormality is causing the hypoxaemia

Question 10

What are common causes of breathlessness based on speed of onset?

Answer 10

MINUTES

• pneumothorax
• PE
• APO

HOURS

• asthma
• pneumonia
• APO
• metabolic acidosis

DAYS/WEEKS

• pleural effusion
• exacerbation of COPD
• pneumonia

Question 11

What are the clinical signs of RESPIRATORY DISTRESS?

Answer 11

RESPIRATORY DISTRESS

1) Increased WOB
- ↑RR
- use of accessory muscles of respiration
- nasal flaring
- intercostal/suprasternal/supraclavicular retraction
- pardoxic/dyskinetic (see-saw) breathing

2) Sweating, ↑HR, ↑BP -> hypotension and bradycardia = late signs
3) Altered mental status eg: agitation to coma/seizures
4) Cyanosis - peripheral and central

Question 12

Why do we use O2 cut off of SpO2 >90%?

Answer 12

S-shaped oxyhaemoglobin dissociation curve describes the relationship btw O2 saturation and PaO2 blood lvls
- a saturation of ~90% is the critical threshold because below this level -> a small fall in arterial O2 produces a sharp fall in O2 saturations

NB: it is O2 saturation not PaO2 that is the main determinant of O2 content of blood and O2 delivery to tissues

Question 13

What are some sources of error is SpO2 interpretation?

Answer 13

poor peripheral perfusion
false nails/varnish
dark skin ( oximeter over-reads slightly)
lipaemia
bright ambient light
 poorly adheretn probe
 excessive motion
carboxyhaemoglobin curve

Question 14

How does capnography work?

Answer 14

CO2 only produced by lungs ∴ presence of CO2 in expired gas confirms that ETT is in correct position
- if pt has relatively normal lungs and cardiac output -> end tidal CO2 tension gives estimate o PaCO2

Question 15

What are the different methods of delivering supplemental O2 and give examples of devices

Answer 15

1) FIXED PERFORMANCE DEVICES = supply a relatively constant O2 concentration to lungs
- air-entrainment masks (venturi masks)
-

2) VARIABLE PERFORMANCE DEVICES = provide variable concentrations of O2 depending on patient’s respiratory rate and inspiratory flow pattern
- - nasal cannula (prongs)/high flow nasal cannula