14. Respiratory Failure And Mechanical Ventilation

Question 1

What is respiratory failure?

Answer 1

Inability to maintain either normal delivery of oxygen to the tissues or the normal removal of CO2 from the tissues

• Failure = PaO2 < 60mmHg, PaCO2 > 50mmHg
• acute respiratory failure pt typically develop both: hypoxaemic and hypercapnic respiratory failure

Question 2

What is hypoxaemic

Answer 2

Low oxygenation

• PaO2 < 60mmHg
• FiO2 > 0.50

Question 3

What is Hypoxaemic Respiratory Failure (Type 1)

Answer 3

Hypoxaemia without CO2 retention
Caused by 
- VQ mismatch (diffusion/perfusion impairment)
- Shunt 
- Alveolar Hypoventilation
- Decreased inspired oxygen

Question 4

What is Hypercapnic Respiratory Failure (Type 2)

Answer 4

Pump/Ventilator Failure resulting in elevated PaCO2 and eventually leading to uncompensated respiratory acidosis

• PaCO2 increase
• alveolar ventilation decreases
• CO2 production increases
• Dead spaces increases

Question 5

Causes of hypoxaemic respiratory failure (PPPLA)

Answer 5

Commonly seen in

• Lung collapse
• Pneumonia
• Asthma
• Pulmonary oedema/embolism

Question 6

Cause of hypercapnic respiratory failure

Answer 6

Causes: Decreased ventilatory drive

• Obesity/OHS (hypoventilation due to difficulty taking deep breath)
• Drug overdose
• Obstructive sleep apnea
• Neurological impairment/Respiratory muscle fatigue (ALS, DMD, GBS)
• Increased WOB, AECOPD, PneumoTx, pleural effusions

Question 7

Indications for Mechanical Ventilation

1. PaO2
2. PaCO2
3. pH, RR
4. PaO2/FiO2
5. Others

Answer 7

1. PaO2 <60mmHg on FiO2 > 50%
2. PaCO2 > 50mmHg
3. pH<7.20, RR>30
4. PaO2/FiO2 <200
5. Sleep apnea, after major surgery, impending Resp. Failure

Question 8

Positive Pressure Ventilation: Benefits

Answer 8

Air is forced in with pathway of least resistance (alveoli) and therefore, will not force alveoli open.

Benefits:

• increased PaO2 and alveolar ventilation
• Decreased shunting by administration of PEEP
• Increased O2 delivery
• Decreased WOB

Question 9

Positive Pressure Ventilation: Complication/Side-effects

Answer 9

Air is forced in with pathway of least resistance (alveoli) and therefore, will not force alveoli open.

• Increased risk of barotrauma
• tracheal lesions
• decreased venous return/CO
• Increased pulmonary vascular resistance
• increased ICP
• decreased renal/portal blood flow
• Increased mean airway pressure (resistance in airway)
• risk of infection
• oxygen hazards (toxicity, retrolental fibroplasia)

Question 10

What is retroletal fibroplasia

Answer 10

• Premature infants who receive 100% O2
• Blood vessels to retina receive excessive blood O2 leading to vasoconstriction and necrosis of blood vessels
• New vessels form in increased numbers resulting in haemorrhage of new vessels leading to scarring behind retina

= retinal detachment and blindness

Question 11

Mechanical Ventilation: FiO2

Answer 11

Fraction of inspired Oxygen

• Concentration of oxygen pt is receiving
• Represented 0.21- 1.0 (i.e. room air - 100% oxygen)
• Indicates severity of pt condition: lower = better (low requirement and better O2 transport)

Question 12

Mechanical Ventilation: PaO2/FiO2

Answer 12

• Ratio indicates severity of lung disease
  Normal +/-400
  Acute Lung Injury <300
  Severe ALI (ARDS) < 200

Question 13

Mechanical Ventilation: PEEP

Answer 13

Maintains open airways and assists with oxygenation

• Recruits collapsed lung and maintains FRC (functional residual capacity) = air in lung at the end of passive expiration
• Prevents collapse of smaller airway at end expiration
• Minimize shunt (high FiO2, low SpO2 = shunt = PEEP required)

Question 14

Mechanical Ventilation: Resistance and Compliance Considerations

Answer 14

Resistance

• ETT and upper airways
• inflammation, bronchospasm, tumor, secretions

Compliance

• secretions
• fibrosis/scaring
• chest wall deformity
• external force (obesity)