A 21-year-old man got into traffic accident and fell from height. He was diagnosed as severe diffuse axonal injury and refractory intracranial hypertension.

• Monitoring was placed and brain metabolic suppression was done by thiopental infusion.
• His condition was complicated by VAP and severe ARDS after 3 days on MV.
• His CXR, ABG results and ventilator waveform were shown above.

Questions:

1.What is the physiologic explanation for non-improving PaCO2 after increase respiratory rate?

2.What should be done to improve PCO2 by ventilator adjustment?

#chest #critcare

โดย น.ท.จรัส ปิตวิวัฒนานนท์ อาจารย์หน่วยโรคระบบการหายใจและเวชบำบัดวิกฤต กองอายุรกรรม รพ.ภูมิพลอดุลยเดช พอ.

ANSWERs

1. Physiologic explanation for non-improving hypercapnia in this patient is increased dead space
   1. CXR showed partial atelectasis of Right lung and posterior segment of left lower lobe: increase opacity of right lung and retrocardiac area silhouette with left hemidiaphragm and bilateral small lung volume
   2. In normal physiology, increasing tidal volume and minute ventilation will result in the decrease of PaCO2
   3. But if alveolar pressure (PA) exceeds pulmonary arterial pressure (Pa) that area of lung will turn to be “dead space”. So, if we increase tidal volume in patients with very high alveolar pressure, the tidal volume increment will worsen lung hyperinflation by turning some lung units into dead space, resulting in no decrement or even worsening of PaCO2
   4. 
2. Treatment of this patient can’t be like other normal situations which increase tidal volume will decrease PaCO2. In fact, the appropriate treatment for this case is to decrease dead space/ lung hyperinflation by decreasing tidal volume which is also proven by ABG in this case.
3. The clues in this case are inverted/ flattened of right hemidiaphragm and very high peak airway pressure, these should raise some suspicions of already hyperinflated lung and follow up ABG after treatment is necessitated.

โดย น.ท.จรัส ปิตวิวัฒนานนท์ อาจารย์หน่วยโรคระบบการหายใจและเวชบำบัดวิกฤต กองอายุรกรรม รพ.ภูมิพลอดุลยเดช พอ.