Case study: UNICAIR on using Noninvasive ventilatory support with High Flow Nasal Cannula Oxygen

Introduction

HFNC utilises high gas flow rates of up 70 l/min with a variable oxygen content between room air (21% oxygen, Fraction of inspired Oxygen (FiO2) 0.21) and 100% (FiO2 1.0). However, uncertainty remains regarding feasibility and safety of HFNC during interhospital transport.

Case Report

We transferred a 62-year-old with respiratory compromise on HFNC (Flow 50 l/min, FiO2 0.65) in our Learjet 45 from Malaga (Spain) to Glasgow (UK). Flight level (FL) was restricted to a maximum of FL 350. HFNC was provided with a Hamilton T1 Transport Ventilator, Hamilton BC4022 Circuit with an In2Flow M nasal cannula and a Hamilton H900 Humidifier. The Hamilton T1 has a battery power supply that enables autonomous operation for up to nine hours. Patient arrived safely at destination.

Transport preparation and planning

Due to the high gas flow rates during HFNC, oxygen consumption needs to be carefully calculated. We planned:

• Ground transportation: Hospital-airport: 30min
• Time at the airport (security, border control, loading, etc): 45min
• Flight time Malaga-Glasgow: 180min
• Time at the airport (security, border control, unloading etc): 30min
• Ground transportation: Airport-hospital: 60min

This results in a total transport time of 345min (5.75h), of which time in flight is 3h. Calculation of oxygen consumption on ground and in-flight: While on ground, gas flow (in l/min) is multiplied with FiO2 minus 0.21, since room air already contains 21% oxygen, which does not need to be added by the ventilator. Multiplication of the result with 60 results in the oxygen consumption per hour on ground.

(FiO2 – 0.21) x Flow rate [l/min] x 60 min (0.65 – 0.21) x 50 l/min x 60 min = 1320 l/h

Cabin pressure decreases during flight, and linear with that the absolute amount of oxygen. FiO2 in flight (FiO2f) can be calculated by dividing the product of FiO2 on ground (FiO2g) and ambient pressure on ground by the cabin pressure at cruise level:

FiO2f = FiO2g x ambient pressure on ground (760mmHg) / cabin pressure in flight

Of note, a high FiO2g may result in impossible (> 1.0) FiO2f. Also, not all patients are at sea-level at origin of transport, which may be considered in the formula above (ambient pressure on ground < 760mmHg). Once FiO2f has been determined, oxygen consumption on cruise level can be calculated using the same formula as for oxygen consumption on ground. Obviously, limiting the decrease in cabin pressure by limiting cruising altitude will limit the increase in oxygen consumption.

Oxygen supplies onboard our Learjet 45 consist of 6800l in the stretcher, plus one portable 8l tank filled with 300 bar, resulting in 2400l oxygen (additional tanks can be accommodated). Two portable 2l tanks filled with 300 bar supply 600l oxygen each. This allows a safety buffer as depicted in the figure below.

Discussion

This case report shows that with meticulous preparation, HFNC can reliably and safely be provided on long-distance, international fixed-wing aeromedical transport. Power and oxygen supply is of critical importance. Escalation to endotracheal intubation and mechanical ventilation needs to be on standby throughout the transport. Back-up for failure of critical equipment due to technical malfunctions should be kept in mind.