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Mechanical ventilation in hypobaric environment: aeromedical transport of critically ill patients


Mechanical ventilation is used in the most of the aeroevacuations of critically ill patients. Patients and mechanical ventilators suffer from variations in pressure, partial pressure oxygen, acceleration and vibration.

Study objective

We tested the changes that decreased environmental pressure produced in a mechanical ventilator and also in the individuals that were connected to this device receiving mechanical ventilation, with the main task being to evaluate the changes in tidal volume.

Material and methods

We applied invasive mechanical ventilation (endotracheal tube) in assisted/controlled modality with two transport mechanical ventilators (Dräger Oxylog 2000), with a fraction inspiration of oxygen (FiO2) of 100%, respiratory frequency of 12–16 breaths/min and positive end expiratory pressure (PEEP) of 5–6 cmH2O to four and 10 beagle dogs with weights from 8 to 17 kg under intravenous sedation. The animals with the mechanical ventilator and the oxygen tank were introduced into the Hypobaric Chamber for Physiological Studies of the Spanish Unit of Aviation and Space Medicine of the Air Force. The pressure conditions of a profile of high altitude flight (from 2000 to 35,000 feet) were applied inside the chamber.

The dogs were ventilated for 45 min before the experimental flight and along the whole test. Parameters from the mechanical ventilator were measured and also vital signs (monitoring) from the animals. The controls of the tidal volume were measured at 14 altitudes in the climbing phase and six in the descent phase. All the measure were taken 1 min after to arise the altitudes and to stabilize the pressures.


The tidal volume was always bigger with the decrease in the environmental pressure and even in the descenct phase we found bigger tidal volumes that in the same altitudes in the climbing phase. The increase in the tidal volume with the decreasing atmospheric pressure was checked statistically with a bilateral significance of P < 0.01 applying one Pearson test. Other variables such as minute volume, respiratory frequency, cardiac frequency, SpO2, Paw peak, atmospheric pressure, tidal volume measured, PEEP, Paw medium, inspiratory time, perimeter and theoric tidal volume were studied also.


The tidal volume increases or decreases with changes in the environmental pressure when mechanical ventilation is used. This is very important in the transport of critically ill patients. These changes can produce damage to the patients that are under mechanical ventilation in the aeromedical evacuations.

Correction rules can be calculated for this phenomenon using the compliance, the saturation of oxygen, the different airway pressures and the expiratory tension of CO2. This is a new investigation area in which the monitoring of different parameters (especially the compliance) plays the main role to control the mechanical ventilation.

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de Barbara, A.H.A., López López, J. Mechanical ventilation in hypobaric environment: aeromedical transport of critically ill patients. Crit Care 8 (Suppl 1), P19 (2004).

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