Aeromedical transport considerations in acute lung injury
Background
Aeromedical critical care transport is a highly specialized field of transport medicine. Many factors may prove to compromise the patient’s safety, including the hazards of loading and unloading, the physiological effects of acceleration and altitude, and equipment failure or malfunction in a restricted environment. On rare occasions, a non-transport anaesthetist may be asked to transfer a patient in an emergency from a referring unit to a regional centre by air. This case will outline some of the key considerations for aeromedical transport.
Learning outcomes
1 Understand the indications for aeromedical transport
2 Understand the physiological effects of altitude
3 Logistical considerations of aeromedical transfer.
CPD matrix matching
2A02; 2A11
Case history: understanding the indications for aeromedical transport
You are working in a remote, but large, district general hospital which has 24/7 anaesthetic, surgery, and ED cover, which is 120 miles from the regional network’s major trauma centre. You are called to be on standby in the ED one evening for a young male that has come off his motorbike at high speed and hit a small group of trees. Your primary survey reveals he has a compromised airway by way of blood and dental fractures; his respiration is laboured with a flail chest and reduced air entry on the left; his abdomen is soft, and he has a suspected left femoral fracture. His GCS on arrival was E2 M5 V2, and you proceed to perform an uneventful RSI for stabilization of his airway, respiratory support, and anticipating a neurological deterioration. He is taken for a whole body CT which shows he has small intraparenchymal haemorrhages with mild cerebral oedema, fractures of the left ribs 1 to 4, and a small haemopneumothorax. He appears to have a small dissection of his aortic arch, but there is no active bleeding at the moment. He is cardiovascularly stable after a small bolus of volume resuscitation, and his femur has been stabilized with a Kendrick splint. He requires emergency transfer to the regional cardiothoracic unit in the event that thoracic surgery is needed.
What are the advantages and disadvantages of aeromedical vs road transport for this patient?
This is a complex and critically ill patient with a life-threatening injury. Transfer will be challenging, and a progression of his aortic injury may be terminal, irrespective of any interventions. Once cardiorespiratory stability has been achieved, he should be transferred in a time-sensitive manner, but at all time bearing in mind that any severe physiological disturbance may rupture any formed aortic clot and precipitate a catastrophic haemorrhage. There are many factors which must be taken into account to determine the optimal transport modality for this patient:
◆ Road transport: a local road ambulance is usually the most readily available option, though, in remote areas, front-line ambulance resources are often limited to 1–2 crews, serving a large geographical area. Advantages of road transport include a degree of familiarity with the layout for most anaesthetists, reasonable access to the patient, the ability to stop the vehicle if complex interventions need to be performed, and the ability to divert to a hospital en route if there are any equipment or vehicle malfunctions. Disadvantages mainly relate to the speed and smoothness of transfer. As an approximation, 120 miles would take 2.5–3 hours, depending on the road, weather, and traffic conditions. Driving at speeds in excess of the national speed limit is possible in modern vehicles and can be supported with a police escort to enhance safety, but this becomes challenging for staff in the back to manage the patient effectively, due to noise, vibration, and G-forces
◆ Rotary wing: helicopter transport is often perceived as the most rapid form of emergency patient transport, with evidence supporting that emergency patient transport at distances over 45 miles is optimally performed by rotary wing over road ambulance transport. Advantages may include the ability to travel from a referring hospital helicopter landing site (HLS) to a receiving unit HLS, thereby negating the need for any secondary road transfers. Transport is also relatively smooth in the cruise, depending on the weather. A standard civilian air ambulance will cruise at around 120 knots, meaning that a helicopter flight for this patient will take around an hour or so, plus loading and unloading times. Apart from the speed of transfer, there are many disadvantages to helicopter transport; the aircraft is unlikely to be at the referring unit at the time dispatch is required, unless based there, so an additional time delay may be factored in. If neither the referring nor the receiving units have an HLS on site, a secondary road transport will be required (see Figure 14.6), and, even if an HLS is present, there may be weather or lighting limitations to its use, necessitating a diversion to an alternative HLS at short notice. Weather conditions, such as cloud base and temperature, in conjunction with the terrain to be traversed, may limit the aircraft’s capabilities. Notwithstanding the logistical difficulties, many anaesthetists are not familiar with the operating environment in an air ambulance, with respect to the restricted space and limited access to the patient, the negligible scope for performing patient interventions, and the impact of noise and vibration on patient monitoring
◆ Fixed wing: this is unequivocally the fastest and smoothest way to transport patients over longer distances, with a recommendation that distances over 150 miles be performed preferentially by fixed wing. Purpose-built air ambulance aeroplanes, such as the Beechcraft King Air (see Figure 14.7), which is in use globally for this purpose, have large cargo doors, with mechanical stretcher lifts, 240 V AC power, extensive oxygen supplies, and a cruising speed of around 240 knots. This would give a flying time of around 30 min for this patient’s journey. However, every fixed wing journey will require a secondary road transfer before and after each flight, and, as well as adding considerable delay to mobilization, this increases the risk of tube, line, or monitor displacement every time the patient is transferred in and out of a vehicle. Fixed wing aircraft are much less subject to weather limitations than helicopters, but nonetheless extremely cold temperatures or fog may prevent landing at the desired destination airport. As with helicopters, the aircraft will often have to attend from a base airport which may add further delay into the process.
Fig. 14.6 Most helicopter retrievals currently require a secondary road transport at the receiving unit.
Fig. 14.7 The Scottish Ambulance Service Beechcraft KingAir 200 aircraft.
For this patient, arguments could be made for and against each of the three transport platforms. Road ambulances will be the most readily available and present the fewest challenges for the non-transport specialist, but they will be the slowest option. If a helicopter is available and the weather is favourable, this may be the quickest option but is the most difficult working environment for the anaesthetist. Fixed wing transfer has the drawbacks of secondary road transport but is likely to be faster and smoother, and it may be the preferred option for this patient.
Case update: understanding the physiological effects of altitude and forces of flight
There is a helicopter presently on your hospital HLS which is available for immediate dispatch, and the receiving cardiothoracic unit also has an HLS on site. You choose to perform this transport by helicopter. The weather is favourable, and your route on a direct track will take you to 5000 ft (1500 m) to clear the nearest mountains. Your estimated total transfer time will be 80 min, including loading and unloading at either end.
What physical factors during the flight may influence the patient’s physiology, and what can be done to minimize disturbances to the patient?
There are many issues relating to the working environment within an air ambulance helicopter, but external forces may have a severe impact on the patient’s cardiorespiratory status and must be anticipated in advance of a flight.
Barometric pressure
◆ Oxygenation: as a general rule, the barometric pressure decreases by 10% per 1000 m ascent from sea level, this being subject to local weather conditions, and reducing less so at altitudes >5000 m. For a proposed flight at 1500 m, one would anticipate a 15% fall in the barometric pressure from around 101 kPa at sea level to 85 kPa, with a subsequent reduction in the atmospheric oxygen pressure from around 21 kPa to 18 kPa. This will have negligible effects on either the crew’s or the patient’s oxygenation. The key implication for this fall in oxygen pressure would be, if a patient was spontaneously ventilating on high-flow oxygen by a trauma mask at sea level and only just maintaining an adequate oxygenation, the 15% reduction in partial pressure will worsen hypoxia. Managing this situation would include requesting a low-level flight or using a pressurized aircraft
◆ Pressure: the patient in this case has a pneumothorax, the gas volume of which will expand in direct proportion with the fall in atmospheric pressure at altitude. If the pneumothorax is of considerable size, this may cause tension effects, and an elective intercostal drainage should be performed prior to air transport in any patient with a pneumothorax. Other air spaces which may distend include the middle ear, sinuses, and bowel, or any pathological air, e.g. a pneumocranium or pneumoperitoneum. Unlike commercial aircrafts which have a ‘cabin altitude’ of 1800–2000 m, air ambulance aeroplanes can pressurize the cabin to sea level, i.e. 101 kPa, negating the effects of oxygen reduction and gas space expansion; however, a rapid decompression may occur at any stage, and an intrathoracic drainage should be performed prior to any flight. Also, sea level cabin pressurization considerably increases fuel consumption and causes repeated stresses on the air frame, so it should only be requested when clinically necessary
◆ G-forces and positioning: the acceleration and deceleration forces in a helicopter are typically gentle and should not cause excessive redistribution of venous blood. Some manoeuvres after take-off and on approaching landing require sharp turns, and the pilot should be briefed to avoid such manoeuvres wherever possible. Low-level flying will result in an unsatisfactory transport experience for the patient, due to the multiple severe directional changes. A helicopter flies with a typical ‘nose-down’ pitch which, over the period of the hour or so of the flight, may be undesired in view of the intracranial injury this patient has sustained. This is unavoidable, due to the physics of a helicopter flight, but it may be possible to package the patient in the transport vacuum mattress with a slightly head-up attitude to counter the effects during flight. This is in contrast to a fixed wing aircraft which flies with a ‘nose-up’ pitch and tends to manoeuvre less aggressively than a helicopter. However, the linear G-forces on take-off and landing are considerable and unavoidable. These should be anticipated at these stages of flight, with extreme attention to detail to the BP and treatment, as necessary
Case update: logistical considerations of aeromedical transfer
You have prepared the patient for helicopter transport. He is intubated and ventilated on a transport ventilator, with standard and invasive pressure monitoring on a transport monitor. He is sedated with a low-dose propofol infusion and has had opioid analgesia and a bolus of muscle relaxant. He has been packaged on a vacuum mattress and has a cervical collar applied. A left intercostal chest drainage has drained around 100 mL of blood and is swinging with ventilation. The aircraft is ready on the helipad outside your ED.
What are the logistical considerations to transferring this patient by helicopter?
This critically ill patient will be challenging to manage in a well-staffed operating theatre or ICU, so this will be a considerable order of magnitude more difficult to manage in a helicopter.
Clinical environment
Once loaded into an air ambulance helicopter, access to the patient is usually very limited. Access to an injection port should be readily available, without having to unstrap either the patient or staff member from their safety harnesses. The aim during preparation for transfer should be to have the patient as stable as possible and packaged in such a manner that minimal interventions are anticipated in flight. Any major interventions that require the team to come out of their harnesses must involve the pilot, who may elect instead to divert to the nearest safe landing site. Defibrillation can be safely performed in flight, but only after approval from the pilot, as there may be transient interference on the aircraft’s electrical systems. The monitor and ventilator should be secured to aviation-approved air frame mounting brackets, and all carry-on items must be stowed securely in the appropriate cargo area. Adequate oxygen supplies within the aircraft must be ensured prior to departure.
Communication
As with road transfer, the receiving unit must be briefed of the patient’s condition and an estimated arrival time prior to departure. Once in flight, all communications out of the aircraft are limited to being relayed via the pilot and air traffic control or via the flight paramedic’s VHF radio through ambulance control. Direct communication with a receiving unit would only be possible via satellite phones which are not regularly carried on most UK air ambulance helicopters. Standard mobile phones do interfere significantly with the VHF radio system in the aircraft, and they lose signal once over 500–1000 m altitude, so they cannot be used.
Weather
The route may have to be diverted or the destination altered, depending on the weather. In general, helicopters can fly through cloud on instrument flight rules (IFR), provided the temperature is above 5°C. At temperatures lower than this, a helicopter cannot fly through cloud, as there is a risk of icing on the rotor blades which significantly compromises performance and may necessitate an emergency landing.
Summary
Aeromedical transfer of critically ill patients requires careful consideration of the risk–benefit ratio. Advantages, including speed and a relatively smooth transport platform, may be offset against drawbacks, including a restricted clinical environment, limited communications, and the effects of a fall in the barometric pressure on both internal air-filled spaces and on oxygenation.