Rescue oxygenation in small infants

There is a current lack of recommendations about oxygenation strategies during a ‘cannot intubate, cannot oxygenate’ (CICO) scenario in small infants. We evaluated the relationship of different ventilation strategies on oxygenation, intrinsic PEEP (iPEEP) and haemodynamic parameters using a manual jet ventilator (MJV) or ENK oxygen flow modulator (OFM; Cook Medical, Bloomington, IN, USA). Approval for animal experimentation was obtained from the Institutional Committee on the Use of Live Animals in Teaching and Research of University of Hong Kong. Using three rabbits weighing between 2.7 kg and 4 kg to simulate neonatal respiratory system 1, and a bronchial blocker to create reproducible airway obstruction, we provided oxygenation using manual jet ventilation (MJV) first and then the oxygen flowmeter (OFM), or vice versa. One transtracheal cannula was used to deliver oxygen while another was used for measuring iPEEP. Once neuromuscular block was established, oxygen saturation was allowed to fall to 70% before rescue oxygenation to above 90% was initiated. Using the OFM sequentially at 3 l.min−1, 6 l.min−1, 9 l.min−1, 12 l.min−1 or 15 l.min−1 or the MJV at 0.5 or 1 bar, the respiratory rate (RR) was increased sequentially from 15 breaths.min−1, 30 breaths.min−1, 60 breaths.min−1 to 100 breaths.min−1. Each combination was continued for 5 min unless it caused a drop in mean arterial pressure of greater than 40%, at which point the protocol was interrupted, and oxygen saturation restored to above 95% using the lowest flow rate or pressure before continuing. The time for rescue oxygenation in the first rabbit ranged from 39 s to 93 s, using flow rates below 6 l.min−1, with a maximal iPEEP of 18 mmHg. At 9 l.min−1 or above, oxygenation had to be interrupted for all RRs except for 15 breaths.min−1, with a maximum iPEEP being 21 mmHg. Crossing over to MJV at 0.5 bar and a rate of 15 breaths.min−1, the time for rescue oxygenation was 38 s, but all subsequent protocols caused haemodynamic instability. At 1 bar, MJV had to be interrupted at 15 breaths.min−1 and this rabbit died from severe hypotension at 30 breaths.min−1 with a maximum iPEEP of 35 mmHg. Post-mortem examination revealed lung injuries at multiple sites. The second rabbit died during oxygenation at 0.5 bar, 100 breaths.min−1 at 10 s. Post-mortem examination also revealed lung injuries at multiple sites. The maximum iPEEP recorded was 7 mmHg. The third rabbit completed all the protocols with the OFM without interruptions. The time for rescue oxygenation ranged from 29 s to 74 s with maximum iPEEP being 11 mmHg. When MJV was delivered at 0.5 bar, the protocol had to be interrupted at a RR of 100 breaths.min−1. At 1 bar, oxygenation was interrupted for RRs of 30 breaths.min−1 and 60 breaths.min−1. The maximum iPEEP was 33 mmHg. At 100 breaths.min−1, severe hypotension was developed at 10 s from which we were not able to resuscitate the animal. Post-mortem examination also revealed lung injuries at multiple sites. These observations suggest oxygenation with an oxygen flow modulator is likely to be an acceptable strategy, but manual jet ventilation appears to be hazardous in CICO situations. Increasing RR increased iPEEP, and rates as low as 15 breaths.min−1 were effective. Flow rates as low as 3 l.min−1 maybe adequate and are associated with lower iPEEP. Excessive flow rates may cause continuous flow even when the holes are not occluded 2, and therefore lower rates with adequate exhalation time should be attempted first to avoid barotrauma when using the flow modulator.