Clinical Scenario:
A term male newborn is delivered following prolonged perinatal asphyxia and requires resuscitation at birth. At 10 minutes, he remains hypotonic with poor respiratory effort and is intubated. He develops moderate-to-severe hypoxic-ischaemic encephalopathy (HIE) and is started on therapeutic hypothermia.
Six hours later, he remains mechanically ventilated. His current settings are:
- Mode: Pressure control
- PIP: 20 cmH₂O
- PEEP: 5 cmH₂O
- Rate: 30/min
- FiO₂: 0.30
He has good bilateral air entry and adequate chest movement. His SpO₂ is 94% and blood pressure is stable.
An arterial blood gas shows:
pH 7.18 | PaCO₂ 72 mmHg | PaO₂ 68 mmHg | HCO₃⁻ 26 mmol/L
Which single ventilator setting should be changed first?
A. Increase respiratory rate
B. Increase PEEP
C. Increase FiO₂
D. Increase PIP
E. Decrease respiratory rate
Correct answer & Explanation:
Correct answer: A. Increase respiratory rate
The ABG shows marked hypercapnia with respiratory acidosis:
- PaCO₂ = 72 mmHg
- pH = 7.18
- PaO₂ = 68 mmHg
This indicates inadequate alveolar ventilation.
Because the infant is receiving pressure-control ventilation, increasing the respiratory rate is an appropriate first adjustment to increase minute ventilation and improve CO₂ elimination. The PEEP and FiO₂ should not be increased simply to correct hypercapnia.
In an infant with hypoxic-ischaemic encephalopathy (HIE), careful control of ventilation is particularly important. Both significant hypercapnia and hypocapnia should be avoided. Hypercapnia may increase cerebral blood flow and contribute to cerebral injury, whereas excessive ventilation causing hypocapnia can reduce cerebral blood flow.
The aim is therefore to correct significant hypercapnia gradually while avoiding aggressive hyperventilation and hypocapnia.
Why the other options are incorrect:
- Increase PEEP: PEEP primarily affects end-expiratory lung volume and oxygenation; it is not the appropriate first response to isolated hypercapnia.
- Increase FiO₂: FiO₂ improves oxygenation but does not correct CO₂ retention.
- Increase PIP: This may increase delivered tidal volume in pressure-control ventilation, but should not be the first adjustment when chest movement and ventilation are already adequate.
- Decrease respiratory rate: This would further reduce minute ventilation and worsen hypercapnia.
Exam pearl:
Hypercapnia + respiratory acidosis → increase minute ventilation. In pressure-control ventilation, increasing the respiratory rate is a simple first adjustment when delivered tidal volume and chest movement are adequate.
References
- Turkish Neonatal Society. Guideline on neonatal encephalopathy. The guideline emphasizes avoiding hyperoxia and excessive ventilation/hypocarbia in infants with HIE.
- Meconium Aspiration Syndrome, Hypoxic-Ischemic Encephalopathy and Therapeutic Hypothermia—A Recipe for Severe Pulmonary Hypertension? Discusses avoidance of hypercarbia, hypoxia, acidosis and hypocapnia in HIE.
- NICE. Specialist neonatal respiratory care for babies born preterm. Provides recommendations for neonatal oxygen and CO₂ monitoring and targets
