Mechanical ventilation with heliox decreases respiratory system resistance and facilitates CO₂ removal in obstructive airway disease

Sir: Helium has a density that is one-seventh that of air, resulting in a decreased resistance to gas flow allowing for increased bulk and oxygen flow. Additionally, CO₂ diffuses through helium much easier than through air [1, 2]. The beneficial effect of heliox on lowering the respiratory system resistance (R_rs) has been demonstrated in a pediatric porcine model of obstructive airway disease (OAD) [3]. However, the effect of heliox on R_rs and CO₂ diffusion in a mechanically ventilated infant with OAD and refractory hypercapnia has not reported before.

A 2.5-month-old infant was admitted to our pediatric intensive care unit with OAD. Chest radiography demonstrated hyperinflation below the 6th anterior rib. Pressure controlled mechanical ventilation (MV) was initiated with prolonged expiratory time (75% of the breathing cycle) and low respiratory rate (30 breaths/min). In addition, incremental positive end-expiratory pressure (PEEP) was applied, aimed at stenting the airway to reduce functional residual capacity and intrinsic PEEP in order to facilitate CO₂ removal [4]. A central venous line was inserted into the left subclavian vein. A pneumothorax was found upon radiological verification of the position of the central venous line. The pneumothorax developed into a tension pneumothorax with a strong rise in PaCO₂ (83–119 mmHg), for which a drainage tube was inserted. However, despite these measures severe respiratory acidosis (pH < 7.20 and PaCO₂ > 75 mmHg) and hyperinflation on chest radiography persisted. Subsequently we decided to replace the conventional gas mixture with heliox with similar ventilator settings (AVEA, Viasys Healthcare, Yorba Linda, Calif., USA). The AVEA ventilator is uniquely equipped with an integrated heliox delivery system and pulmonary function testing module. The pressure at the distal end of the endotracheal tube and esophageal pressure as a representative of the pleural pressure were continuously measured. The R_rs was determined according to the manufacturer's manual. Briefly, R_rs is the ratio of the airway pressure differential peak (peak minus plateau) to the inspiratory flow 12 ms prior to the end of inspiration [R_rs = (P _peak – P _plateau)/flow]. Figure 1 summarizes the time course of the R_rs and PaCO₂. With heliox 50/50 (50% helium and 50% oxygen) the R_rs decreased from 39 to 35.5 cmH₂O l⁻¹ s⁻¹. PaCO₂ decreased from 78 to 64 mmHg. After reintroduction of the conventional gas mixture the R_rs rose to 49.5 cmH₂O l⁻¹ s⁻¹ and PaCO₂ to 87 mmHg. Heliox was reintroduced, now with 65% helium. This resulted in a strong decrease in R_rs from 49.5 to 36 cmH₂O l⁻¹ s⁻¹ and subsequently of PaCO₂ from 87 to 64 mmHg. The following day heliox, and after 10 days, MV could be discontinued. Oxygenation was not impaired. Throughout the heliox trial the ventilator settings remained unaltered. Viral cultures remained negative.

Fig. 1

Time course of PaCO₂ and respiratory system resistance (R_rs) after start of mechanical ventilation with nitrox and heliox

We observed that lowering R_rs using heliox facilitates CO₂ removal. This effect disappeared when the heliox was discontinued but returned after its reintroduction. Our observation opposes a previously reported randomized controlled trial of ten infants with viral bronchiolitis [5]. This study found that gas exchange was not improved by MV with heliox. However, data of this study cannot be easily extrapolated to our patient. First, no data on R_rs of the included infants were available. Secondly, the included infants seemed relatively less ill as they were normocapnic to mildly hypercapnic. Our patient had an increased R_rs and refractory respiratory acidosis, which was overcome by MV with heliox.

Based on our observation we advocate a randomized controlled trial in infants with severe refractory respiratory acidosis caused by (viral) OAD aimed at a reduction in R_rs and subsequent improved CO₂ elimination.