- Email: [email protected]
Hyperbaric medicine part II: practical aspects of hyperbaric oxygen therapy
Current Anaesthesia & Critical Care (2001) 12, 166d171 ^ 2001 Harcourt Publishers Ltd doi:10.1054/cacc.2001.0307, available online at http://www.idealibrary.com on
MEDICINE
Hyperbaric medicine part II: practical aspects of hyperbaric oxygen therapy P. I. Shirley and J. A. S. Ross Department of Anaesthesia, Intensive Care and Hyperbaric Medicine, Grampian University Hospitals Trust, Foresterhill, Aberdeen, A.B.25 2ZN, UK
KEYWORDS oxygenation, hyperbaric, decompression sickness, hyperoxia, poisoning, carbon monoxide, pressure
Summary Hyperbaric treatment facilities in the UK are geographically scattered and vary in their size, complexity and staffing levels. The delivery of care to hyperbaric patients should be of a standard expected elsewhere within the hospital system. Hyperbaric chambers form a difficult working environment and serious consideration must be given to the Health and Safety aspects for both staff and patients. Their use is also governed by strict operating standards. The anaesthesia and monitoring equipment may require to be modified and built to a particular specification. The treatment of decompression sickness, arterial gas embolism and carbon monoxide poisoning are clear indications for hyperbaric oxygen therapy. There is less clear evidence as to its efficacy in other conditions. The development of hyperbaric medicine within the National Health Service (NHS) looks set to continue. Further research and long-term follow-up studies are needed to validate current treatments and extend its role in the future. ^ 2001 Harcourt Publishers Ltd
INTRODUCTION
Safety at Work Act 1974 and must be operated by ‘competent’ persons and operate under a formal system of risk appraisal and hazard avoidance. Relevant regulations include:
The standard of care given in hyperbaric medicine should be as good as that given elsewhere.1 With this in mind it is important to focus on the practicalities of the organization and delivery of therapy.
ORGANIZATION OF HYPERBARIC CHAMBERS The pressure chamber is a piece of equipment that is used to expose people to increased ambient pressure. While doctors and nurses can get about their normal business in the chamber it is essential that the unit is operated in a safe and competent fashion.
Applicable safety standards3 There are no specific safety standards in the UK for the operation of a therapeutic pressure chamber. The operation of such a facility is covered by the Health and
Correspondence to: JASR. Dept. of Environmental and Occupational Medicine, Liberty Sac,e Work Research Centre, Forester Hill Road, Aberdeen, AB 25 2ZN, UK. Fax: #44 (0) 1224 662 990. E-mail: [email protected]
i) Diving at Work regulations; ii) Work in Compressed Air special regulations; iii) Pressure Systems and Transportable Gas Container regulations.
British Hyperbaric Association chamber classification Category 1 (multiplace) Comprehensive hyperbaric facilities capable of supporting the treatment of patients who are critically ill from any cause and who may require hyperbaric intensive therapy.
Category 2 (multiplace) Facilities capable of receiving elective or emergency referrals for any accepted application of hyperbaric oxygen therapy but excluding patients who are critically ill at the time of referral or are considered likely to become so.
HYPERBARIC MEDICINE PART II
167
Table 1 Absolute contraindications to work at pressure ENT E Autosclerotic surgery E Chronic or serious otitis media E Meniere’s disease E Inability to equalise middle ear pressure with ambient E Chronic mastoiditis or mastoid fistula E Vestibular lesions Respiratory E Any respiratory infection such as pneumonia, tuberculosis, sarcoidosis or pneumonitis E The presence of lung cysts, emphysematous blebs, poorly aerated areas of lung, pneumothorax, pleural effusion, lung fistula, bronchiectasis, fibrosis or neoplasm E A history of spontaneous pneumothorax. This is because of the significant risk of recurrence. E Chronic bronchitis/emphysema or severe asthma E Functional evidence of generalized or localized air trapping in the lungs. E Any evidence of chronic or recurrent sinusitis, rhinitis nasal congestion or severe allergic conditions of the upper respiratory tract
Category 3 (multiplace) Facilities without some of the capabilities of categories 1 or 2 sited specifically for the treatment of diving emergencies.
Category 4 (monoplace) Facilities operating at relatively low pressure and without an air-lock capability. The expectation is that such chambers provide a treatment service on behalf of the National Health Service (NHS) and would be sited within the boundaries of, or in very close proximity to, a hospital.
Staff training Chamber operators must hold a qualification as a life support technician. Medical and nursing staff, in addition to their basic training, must be trained and qualified to work in the pressure chamber. At present in the UK there are no accepted training standards for these groups and training consists of what is available in each individual hyperbaric unit. Staff who work in the chamber must be medically fit. Pre-employment screening must be applied to exclude people who are unsuited for the pressure environment (Table 1). Conditions exist which require individual assessment but which may be allowable. These include a history of provoked pneumothorax (respiratory stress or surgery) where the rupture has healed and where detailed respiratory functional investigation shows no evidence of local or generalized air flow obstruction. Relative contraindications include migraine, epilepsy, asthma, diabetes or other endocrine disorder requiring therapy. If active all of these constitute absolute contraindications
but if controlled they need to be assessed individually by an occupational physician.
OPERATIONAL PROCEDURES Most treatment compression schedules last longer than 2 h. Eight and a half h is the longest ‘short’ treatment for decompression illness which might require a ‘saturation’ treatment lasting 5 days. Although not dangerous such compressions are physically stressful and staff must limit the amount of pressure exposure to avoid repetitive dives. Staff may be compressed twice in any 48-h period as long as this avoids a repetitive dive. There must then be a 24-h lay-off. If emergency decompression is anticipated, for instance to defibrillate the patient, the compression schedule must entail no decompression penalty for the attendant staff. In other words, a decompression stop should not be required thus avoiding any risk of decompression sickness. Compression increases gas density and also increases the work of breathing. This factor should be assessed and the patient should not have a degree of pulmonary disease that will be provoked by an increased work of breathing. Patients must be treated in a chamber which allows the appropriate level of care to be given to the patient during treatment. The patient’s life must never be endangered by the pressure environment.
Oxygen administration Oxygen administration may be by tracheal tube and ventilation as described above, but is more usually given
168
to a spontaneously breathing subject. In industry, a mask and demand valve regulator is used. This should be equipped with a system to allow venting of exhaled gas through a regulator to the outside of the chamber (oxygen dump). Masks, however, do not give a good fit and there is no guarantee that 100% oxygen will be delivered. A ventilated hood system does deliver 100% oxygen with a degree of patient comfort. Ventilation through the hood must control carbon dioxide levels in the hood and these must be analyzed. In Aberdeen, the hood carbon dioxide levels are controlled to 1 kPa. This may involve flows through the system as high as 60 1/min and the minimum flow used is 30 1/min. These flow rates cannot be delivered from the usual hospital flowmeters and high capacity flowmeters are required. The oxygen vented from the hoods is discharged from the chamber. Provision must be made to prevent the oxygen supply from becoming disconnected since this would result in a rapid increase in carbon dioxide levels in the hood and this might precipitate a convulsion. Continuous carbon dioxide analysis with alarms acts as a disconnection alarm.
Chamber atmosphere control The chamber system should allow the control of oxygen levels in the chamber and this should never rise above 25% and should be kept below 23%. This is done by carefully venting exhaled gas, ensuring a good fit for oxygen administration equipment and flushing the chamber through if levels are rising. It should also be possible to prevent oxygen levels falling to less than 20% as the chamber is decompressed. Carbon dioxide levels in the chamber should also be controlled by ventilating the chamber, by using an internal carbon dioxide scrubber or by an external gas regeneration system. Humidity should be similarly controlled but is generally less of a problem. Care must be taken that condensation does not occur on electrical components. Chamber humidity, carbon dioxide and oxygen must be continuously analyzed. There are recognized maximum values for contaminants in the hyperbaric environment. The regular monitoring of these is an important health and safety issue for both staff and patients.4
CURRENT ANAESTHESIA & CRITICAL CARE to fire prevention, which have direct applicability to work in the oxygen-enriched chamber environment.5 The risk of fire due to ignition from an electrical source is the single most important factor which limits the use of electrical equipment in pressure chambers. In order to avoid this risk, it is important to keep the level of electrical power used in the chamber to a minimum. Again the BHA has attempted to unify current thinking into a single document.6 Where high power utilization in essential, such as in the use of a defibrillator, extra care and attention must be paid to avoiding the potential fire hazard. Defibrillation is contra-indicated in monoplace chambers compressed with oxygen. Overall it is recommended that large surface area self-adhesive electrodes are used rather than defibrillator paddles to reduce the risk of sparking. In this context brief mention should be made of the power derived from batteries. Although the voltage levels generated by batteries are generally low the power produced can be high and shorting across battery terminals can generate large sparks. Further, the lead acid type batteries which are incorporated into a number of medical electrical devices will generate hydrogen gas during recharging and care must be taken that this does not constitute an explosion hazard. More modern lead acid batteries are sealed and may avoid this risk. An analysis of the cause of chamber fires over 73-years revealed that there were no survivors in any chamber fires where an oxygen rich atmosphere was present (greater than 23.5% oxygen).7
Pre-compression checks Prior to compressing a patient a check must be made to make sure that no inflammables are being put into the chamber. No inflammable substances should be used in the chamber e.g. sugar, talc, alcohol-based solutions. No volatile contaminants should be used.
MEDICAL EQUIPMENT Table 2 lists a number of examples of general purpose medical equipment which should be available in a hyperbaric chamber.
Patient monitoring Fire risk The ignition temperature of paper is reduced by approximately 25% by compression to 6 atmospheres absolute in air and the rate at which paper burns is more than doubled. As oxygen concentration increases from 21%, ignition temperature falls and burn rate is further increased. The British Hyperbaric Association (BHA) has attempted to bring together current guidelines relating
Any level of patient monitoring is possible in a pressure chamber. Electrically powered units should be low energy systems and carry a low fire risk as highlighted previously. Electrically powered patient monitors should be compatible with the pressure environment and should have been tested for this purpose. Current standards for the manufacture of medical electronic equipment do not include exposure to increased levels of oxygen or increased ambient pressure.
HYPERBARIC MEDICINE PART II
Table 2 Examples of equipment needed in a hyperbaric chamber Oxygen masks and hoods Ventilators Diagnostic equipment Basic examination tray ECG EEG Blood pressure cuff Neurological assessment equipment Ophtalmoscope Therapeutic equipment Traction equipment for multi-trauma patients Resuscitation equipment Tracheal tubes Suction Intravenous infusions
Historically, measurement of biological electric signals from patients in chambers has been by passing the patient leads through the chamber wall, using a bulkhead connector, to the monitoring equipment outside the chamber. This practice is outdated and can lead to problems with patient electrical isolation and signal noise picked up from the chamber hull. At the National Hyperbaric Centre in Aberdeen, the low voltage (50 volts DC) signal pre-amplification and patient isolation unit is situated inside the chamber with an electrical connector leading through a bulkhead connector to the monitoring computer and VDU outside. Cathode ray tubes should not be used in the chamber as they can implode. Liquid crystal displays can be used although they can get decompression sickness. Some small, portable, battery powered monitoring units with liquid crystal displays may be appropriate for use in the chamber.
Ventilation of the lungs and measurement of gas flow Gas density increases in direct relation to pressure and the volume of a fixed mass of gas is reduced. These effects impair many ventilators and disrupt many gas-flow measurement devices. Tracheal tube cuffs will deflate on compression and expand on decompression. In a multiplace chamber they need not be filled with water but cuff pressure should be monitored and adjusted. Microbial/viral filters should not be affected at pressure since flow through them is laminar and not density dependent. Ventilators must be tested for use in the
169
chamber prior to their use. The following have reported as suitable: E E E
Penlon Oxford ventilator; Siemens 900 B or C ventilator; Bird mark 7 or 8 ventilator.
The calibration of a ball in a tube flowmeter is altered at pressure:
FI"F0 ((o0 /oI) FI"actual flow at pressure; F0"flowmeter reading at recorded pressure; o0"gas density at 1 atmosphere absolute; oI"gas density at recorded pressure.
Patient gas flows The Wright’s Respirometer does not measure accurately at pressure but the more modern electronic equivalent is accurate. This has a very lightweight rotating vane which is not vulnerable to the inertial effects of increasing gas density.
Patient gas analysis Oxygen can be measured in the chamber using a galvanic sensor (fuel cell). Analysers for use in the chamber must be tested and approved for this purpose. More usually gas is taken out of the chamber through a penetrator and analyzed outside the chamber. End tidal analysis may be damped due to the length of sample tube. The partial pressure reading of a monitor outside the chamber will be lower than that inside the chamber by a factor of the pressure of the chamber.
Patient suction Suction is provided by opening a through chamber penetrator. Flow is controlled by a needle valve and pressure may be monitored. As this system depends on pressure in the chamber, an alternative method must be available for use when chamber pressure is close to ambient (e.g. a foot operated suction unit).
PATIENT SELECTION Patients should be screened for pulmonary abnormality (bullae etc) and should be able to equalize pressure in their sinuses and middle ear. There are no absolute contraindications to hyperbaric therapy, the risk to the patient being balanced against the benefit. Patients with significant pulmonary right to left shunt will not benefit from hyperbaric oxygen as much as persons with no pulmonary lesion. The likely increase in arterial oxygen
170
tension in the chamber should be calculated in advance in order to allow a risk benefit assessment to be made. Compression increases gas density and the work of breathing is increased. This factor should be assessed and the patient should not have a degree of pulmonary disease that will be provoked by an increased work of breathing. Patients must be treated in a chamber which allows the appropriate level of care to be given to the patient during treatment. The patient’s life must never be endangered by the pressure environment.
Patient positioning The patient must be able to remain comfortable throughout the treatment. III or immobilized patients may be required to lie down and bunks are suitable for this. Bunks, however, limit access to the patient and head down tilt is unavailable. Ideally high intensity care should be delivered on a tipping trolley with height adjustment.
PATIENT TRANSPORT TO HYPERBARIC FACILITIES AND PATIENT RECEPTION It is important that the accepting doctor is satisfied that the medical cover is adequate for the patient whilst being transported to the hyperbaric chamber. Unpressurized aircraft flying at low altitude are used to transport divers from incident scenes to hyperbaric facilities. The time advantage outweighs the small risk of bubble enlargement at these low altitudes (less than 3000 feet). Of more concern is the more prolonged flight in a pressurized aircraft, typically with a cabin altitude pressure of 6000}8000 feet. If necessary aircraft can be pressurized to sea level (i.e. zero altitude). However flights tend to take longer and be more costly in terms of fuel. Of more importance is that all patients with decompression sickness, air embolism or carbon monoxide poisoning are transported breathing as close to 100% oxygen as possible. It may be appropriate to bypass a local hospital to attend a centre with hyperbaric facilities but such decisions should be made in conjunction with attendants on-scene and the various hospitals involved. It has been recognized for some time that the secondary transport of the seriously injured has been a neglected area. It is recommended that monitoring of such patients should not be compromised.8 The Australian and New Zealand College of Anaesthetists have produced guidelines and the UK Intensive Care Society have also published good practice guidelines. These have attempted to bring together advice from different sources and encourage an improvement in standards in the transport of the critically ill.9,10
CURRENT ANAESTHESIA & CRITICAL CARE
FUTURE DEVELOPMENTS IN HYPERBARIC MEDICINE Historically, hyperbaric medicine has been an offshoot of diving medicine. Diving medicine has assumed importance owing to the developments in military applications and more recently with sports diving and offshore deep-sea drilling operations. Currently in the UK, physicians with an interest in hyperbaric medicine come from a variety of parent specialities (general medicine, anaesthesia, emergency medicine, occupational medicine). With an increasing tendency to subspecialization in the UK, it seems likely that more formalized fellowship training will become the norm. As most of the current indications for HBO are based on evidence from uncontrolled clinical trials and with the move towards evidence-based medicine with the formation of bodies such as the National Institute for Clinical Excellence (NICE), further research will be necessary to validate current treatment regimes. Hyperbaric medicine has become a recognized treatment for a number of disorders. Its indications for primary therapy are for decompression sickness, air embolism and carbon monoxide poisoning. There is a rational basis for HBO therapy in conditions where hypoxia forms the basis of the pathophysiology but controlled studies are lacking. It seems likely that as the levels of evidence improve then HBO therapy will impact on more disease states. This will require more facilities, more multidisciplinary co-operation and may have major funding implications (Table 3). A system of clinical governance has been instituted in Scotland to ensure minimum standards in the hyperbaric chambers utilised to treat NHS patients.
Table 3 Examples of areas for future research in hyperbaric medicine10,11 Basic sciences Animal experiments Human volunteer studies
Human patient studies
Long-term follow up studies Controlled clinical trials
effects of HBO on mitochondrial oxygen tension toxic encephalopathies the effects of physical exercise under HBO acceleration induced loss of consciousness and HBO AIDS encephalopathy altitude sickness toxic epidermal necrolysis HBO and multiple sclerosis acute severe head injury acute stroke
HYPERBARIC MEDICINE PART II
REFERENCES 1. Shirley P J, Ross J A S. Hyperbaric medicine part I: theory and practice. Curr Anaesth Crit Care 2001; 12: 114}120. 2. Wattel F, Mathieu D. (eds) Reports and recommendations of 1st European consensus conference on hyperbaric medicine, 1994. 3. Faculty of Occupational Medicine. A code of good working practice for the operation and staffing of hyperbaric chambers for therapeutic purposes. London: Royal College of Physicians, 1994. 4. Hamilton R W, Sheffield P J. Hyperbaric Chamber Safety. In: Davis J C, Hunt T R (eds). Hyperbaric Oxygen Therapy Bethesda: Undersea and Hyperbaric Medical Society, 1977. 5. British Hyperbaric Association Technical Working Party Guide to fire safety standards for hyperbaric treatment centres. BHA, 1996. 6. British Hyperbaric Association Technical Working Party Guide to electrical safety standards for hyperbaric treatment centres. BHA, 1996.
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7. Sheffield P J, Desautels D A, Hyperbaric and hypobaric chambers fires: a 73-year analysis. Undersea Hyperb Med 1997; 24: 153}164. 8. Oakley P A. The need for standards in inter-hospital transfer. [Editorial] Anaesthesia 1994; 49: 565}566. 9. Faculty of Intensive Care of the Australian and New Zealand College of Anaesthetists and Australasian College for Emergency Medicine. Minimum standards for transport of the critically ill, 1996. 10. Intensive Care Society. Guidelines for the transport of the critically ill. UK: ICS, 1997. 11. Gail D B, Hyperbaric oxygen therapy: NHLB workshop summary. Ann Rev Resp Dis 1991; 144: 1414}1421. 12. Cramer F S. Oxygen and acceleration. Military Med 1991; 156: 608}611.
MEDICINE
Hyperbaric medicine part II: practical aspects of hyperbaric oxygen therapy P. I. Shirley and J. A. S. Ross Department of Anaesthesia, Intensive Care and Hyperbaric Medicine, Grampian University Hospitals Trust, Foresterhill, Aberdeen, A.B.25 2ZN, UK
KEYWORDS oxygenation, hyperbaric, decompression sickness, hyperoxia, poisoning, carbon monoxide, pressure
Summary Hyperbaric treatment facilities in the UK are geographically scattered and vary in their size, complexity and staffing levels. The delivery of care to hyperbaric patients should be of a standard expected elsewhere within the hospital system. Hyperbaric chambers form a difficult working environment and serious consideration must be given to the Health and Safety aspects for both staff and patients. Their use is also governed by strict operating standards. The anaesthesia and monitoring equipment may require to be modified and built to a particular specification. The treatment of decompression sickness, arterial gas embolism and carbon monoxide poisoning are clear indications for hyperbaric oxygen therapy. There is less clear evidence as to its efficacy in other conditions. The development of hyperbaric medicine within the National Health Service (NHS) looks set to continue. Further research and long-term follow-up studies are needed to validate current treatments and extend its role in the future. ^ 2001 Harcourt Publishers Ltd
INTRODUCTION
Safety at Work Act 1974 and must be operated by ‘competent’ persons and operate under a formal system of risk appraisal and hazard avoidance. Relevant regulations include:
The standard of care given in hyperbaric medicine should be as good as that given elsewhere.1 With this in mind it is important to focus on the practicalities of the organization and delivery of therapy.
ORGANIZATION OF HYPERBARIC CHAMBERS The pressure chamber is a piece of equipment that is used to expose people to increased ambient pressure. While doctors and nurses can get about their normal business in the chamber it is essential that the unit is operated in a safe and competent fashion.
Applicable safety standards3 There are no specific safety standards in the UK for the operation of a therapeutic pressure chamber. The operation of such a facility is covered by the Health and
Correspondence to: JASR. Dept. of Environmental and Occupational Medicine, Liberty Sac,e Work Research Centre, Forester Hill Road, Aberdeen, AB 25 2ZN, UK. Fax: #44 (0) 1224 662 990. E-mail: [email protected]
i) Diving at Work regulations; ii) Work in Compressed Air special regulations; iii) Pressure Systems and Transportable Gas Container regulations.
British Hyperbaric Association chamber classification Category 1 (multiplace) Comprehensive hyperbaric facilities capable of supporting the treatment of patients who are critically ill from any cause and who may require hyperbaric intensive therapy.
Category 2 (multiplace) Facilities capable of receiving elective or emergency referrals for any accepted application of hyperbaric oxygen therapy but excluding patients who are critically ill at the time of referral or are considered likely to become so.
HYPERBARIC MEDICINE PART II
167
Table 1 Absolute contraindications to work at pressure ENT E Autosclerotic surgery E Chronic or serious otitis media E Meniere’s disease E Inability to equalise middle ear pressure with ambient E Chronic mastoiditis or mastoid fistula E Vestibular lesions Respiratory E Any respiratory infection such as pneumonia, tuberculosis, sarcoidosis or pneumonitis E The presence of lung cysts, emphysematous blebs, poorly aerated areas of lung, pneumothorax, pleural effusion, lung fistula, bronchiectasis, fibrosis or neoplasm E A history of spontaneous pneumothorax. This is because of the significant risk of recurrence. E Chronic bronchitis/emphysema or severe asthma E Functional evidence of generalized or localized air trapping in the lungs. E Any evidence of chronic or recurrent sinusitis, rhinitis nasal congestion or severe allergic conditions of the upper respiratory tract
Category 3 (multiplace) Facilities without some of the capabilities of categories 1 or 2 sited specifically for the treatment of diving emergencies.
Category 4 (monoplace) Facilities operating at relatively low pressure and without an air-lock capability. The expectation is that such chambers provide a treatment service on behalf of the National Health Service (NHS) and would be sited within the boundaries of, or in very close proximity to, a hospital.
Staff training Chamber operators must hold a qualification as a life support technician. Medical and nursing staff, in addition to their basic training, must be trained and qualified to work in the pressure chamber. At present in the UK there are no accepted training standards for these groups and training consists of what is available in each individual hyperbaric unit. Staff who work in the chamber must be medically fit. Pre-employment screening must be applied to exclude people who are unsuited for the pressure environment (Table 1). Conditions exist which require individual assessment but which may be allowable. These include a history of provoked pneumothorax (respiratory stress or surgery) where the rupture has healed and where detailed respiratory functional investigation shows no evidence of local or generalized air flow obstruction. Relative contraindications include migraine, epilepsy, asthma, diabetes or other endocrine disorder requiring therapy. If active all of these constitute absolute contraindications
but if controlled they need to be assessed individually by an occupational physician.
OPERATIONAL PROCEDURES Most treatment compression schedules last longer than 2 h. Eight and a half h is the longest ‘short’ treatment for decompression illness which might require a ‘saturation’ treatment lasting 5 days. Although not dangerous such compressions are physically stressful and staff must limit the amount of pressure exposure to avoid repetitive dives. Staff may be compressed twice in any 48-h period as long as this avoids a repetitive dive. There must then be a 24-h lay-off. If emergency decompression is anticipated, for instance to defibrillate the patient, the compression schedule must entail no decompression penalty for the attendant staff. In other words, a decompression stop should not be required thus avoiding any risk of decompression sickness. Compression increases gas density and also increases the work of breathing. This factor should be assessed and the patient should not have a degree of pulmonary disease that will be provoked by an increased work of breathing. Patients must be treated in a chamber which allows the appropriate level of care to be given to the patient during treatment. The patient’s life must never be endangered by the pressure environment.
Oxygen administration Oxygen administration may be by tracheal tube and ventilation as described above, but is more usually given
168
to a spontaneously breathing subject. In industry, a mask and demand valve regulator is used. This should be equipped with a system to allow venting of exhaled gas through a regulator to the outside of the chamber (oxygen dump). Masks, however, do not give a good fit and there is no guarantee that 100% oxygen will be delivered. A ventilated hood system does deliver 100% oxygen with a degree of patient comfort. Ventilation through the hood must control carbon dioxide levels in the hood and these must be analyzed. In Aberdeen, the hood carbon dioxide levels are controlled to 1 kPa. This may involve flows through the system as high as 60 1/min and the minimum flow used is 30 1/min. These flow rates cannot be delivered from the usual hospital flowmeters and high capacity flowmeters are required. The oxygen vented from the hoods is discharged from the chamber. Provision must be made to prevent the oxygen supply from becoming disconnected since this would result in a rapid increase in carbon dioxide levels in the hood and this might precipitate a convulsion. Continuous carbon dioxide analysis with alarms acts as a disconnection alarm.
Chamber atmosphere control The chamber system should allow the control of oxygen levels in the chamber and this should never rise above 25% and should be kept below 23%. This is done by carefully venting exhaled gas, ensuring a good fit for oxygen administration equipment and flushing the chamber through if levels are rising. It should also be possible to prevent oxygen levels falling to less than 20% as the chamber is decompressed. Carbon dioxide levels in the chamber should also be controlled by ventilating the chamber, by using an internal carbon dioxide scrubber or by an external gas regeneration system. Humidity should be similarly controlled but is generally less of a problem. Care must be taken that condensation does not occur on electrical components. Chamber humidity, carbon dioxide and oxygen must be continuously analyzed. There are recognized maximum values for contaminants in the hyperbaric environment. The regular monitoring of these is an important health and safety issue for both staff and patients.4
CURRENT ANAESTHESIA & CRITICAL CARE to fire prevention, which have direct applicability to work in the oxygen-enriched chamber environment.5 The risk of fire due to ignition from an electrical source is the single most important factor which limits the use of electrical equipment in pressure chambers. In order to avoid this risk, it is important to keep the level of electrical power used in the chamber to a minimum. Again the BHA has attempted to unify current thinking into a single document.6 Where high power utilization in essential, such as in the use of a defibrillator, extra care and attention must be paid to avoiding the potential fire hazard. Defibrillation is contra-indicated in monoplace chambers compressed with oxygen. Overall it is recommended that large surface area self-adhesive electrodes are used rather than defibrillator paddles to reduce the risk of sparking. In this context brief mention should be made of the power derived from batteries. Although the voltage levels generated by batteries are generally low the power produced can be high and shorting across battery terminals can generate large sparks. Further, the lead acid type batteries which are incorporated into a number of medical electrical devices will generate hydrogen gas during recharging and care must be taken that this does not constitute an explosion hazard. More modern lead acid batteries are sealed and may avoid this risk. An analysis of the cause of chamber fires over 73-years revealed that there were no survivors in any chamber fires where an oxygen rich atmosphere was present (greater than 23.5% oxygen).7
Pre-compression checks Prior to compressing a patient a check must be made to make sure that no inflammables are being put into the chamber. No inflammable substances should be used in the chamber e.g. sugar, talc, alcohol-based solutions. No volatile contaminants should be used.
MEDICAL EQUIPMENT Table 2 lists a number of examples of general purpose medical equipment which should be available in a hyperbaric chamber.
Patient monitoring Fire risk The ignition temperature of paper is reduced by approximately 25% by compression to 6 atmospheres absolute in air and the rate at which paper burns is more than doubled. As oxygen concentration increases from 21%, ignition temperature falls and burn rate is further increased. The British Hyperbaric Association (BHA) has attempted to bring together current guidelines relating
Any level of patient monitoring is possible in a pressure chamber. Electrically powered units should be low energy systems and carry a low fire risk as highlighted previously. Electrically powered patient monitors should be compatible with the pressure environment and should have been tested for this purpose. Current standards for the manufacture of medical electronic equipment do not include exposure to increased levels of oxygen or increased ambient pressure.
HYPERBARIC MEDICINE PART II
Table 2 Examples of equipment needed in a hyperbaric chamber Oxygen masks and hoods Ventilators Diagnostic equipment Basic examination tray ECG EEG Blood pressure cuff Neurological assessment equipment Ophtalmoscope Therapeutic equipment Traction equipment for multi-trauma patients Resuscitation equipment Tracheal tubes Suction Intravenous infusions
Historically, measurement of biological electric signals from patients in chambers has been by passing the patient leads through the chamber wall, using a bulkhead connector, to the monitoring equipment outside the chamber. This practice is outdated and can lead to problems with patient electrical isolation and signal noise picked up from the chamber hull. At the National Hyperbaric Centre in Aberdeen, the low voltage (50 volts DC) signal pre-amplification and patient isolation unit is situated inside the chamber with an electrical connector leading through a bulkhead connector to the monitoring computer and VDU outside. Cathode ray tubes should not be used in the chamber as they can implode. Liquid crystal displays can be used although they can get decompression sickness. Some small, portable, battery powered monitoring units with liquid crystal displays may be appropriate for use in the chamber.
Ventilation of the lungs and measurement of gas flow Gas density increases in direct relation to pressure and the volume of a fixed mass of gas is reduced. These effects impair many ventilators and disrupt many gas-flow measurement devices. Tracheal tube cuffs will deflate on compression and expand on decompression. In a multiplace chamber they need not be filled with water but cuff pressure should be monitored and adjusted. Microbial/viral filters should not be affected at pressure since flow through them is laminar and not density dependent. Ventilators must be tested for use in the
169
chamber prior to their use. The following have reported as suitable: E E E
Penlon Oxford ventilator; Siemens 900 B or C ventilator; Bird mark 7 or 8 ventilator.
The calibration of a ball in a tube flowmeter is altered at pressure:
FI"F0 ((o0 /oI) FI"actual flow at pressure; F0"flowmeter reading at recorded pressure; o0"gas density at 1 atmosphere absolute; oI"gas density at recorded pressure.
Patient gas flows The Wright’s Respirometer does not measure accurately at pressure but the more modern electronic equivalent is accurate. This has a very lightweight rotating vane which is not vulnerable to the inertial effects of increasing gas density.
Patient gas analysis Oxygen can be measured in the chamber using a galvanic sensor (fuel cell). Analysers for use in the chamber must be tested and approved for this purpose. More usually gas is taken out of the chamber through a penetrator and analyzed outside the chamber. End tidal analysis may be damped due to the length of sample tube. The partial pressure reading of a monitor outside the chamber will be lower than that inside the chamber by a factor of the pressure of the chamber.
Patient suction Suction is provided by opening a through chamber penetrator. Flow is controlled by a needle valve and pressure may be monitored. As this system depends on pressure in the chamber, an alternative method must be available for use when chamber pressure is close to ambient (e.g. a foot operated suction unit).
PATIENT SELECTION Patients should be screened for pulmonary abnormality (bullae etc) and should be able to equalize pressure in their sinuses and middle ear. There are no absolute contraindications to hyperbaric therapy, the risk to the patient being balanced against the benefit. Patients with significant pulmonary right to left shunt will not benefit from hyperbaric oxygen as much as persons with no pulmonary lesion. The likely increase in arterial oxygen
170
tension in the chamber should be calculated in advance in order to allow a risk benefit assessment to be made. Compression increases gas density and the work of breathing is increased. This factor should be assessed and the patient should not have a degree of pulmonary disease that will be provoked by an increased work of breathing. Patients must be treated in a chamber which allows the appropriate level of care to be given to the patient during treatment. The patient’s life must never be endangered by the pressure environment.
Patient positioning The patient must be able to remain comfortable throughout the treatment. III or immobilized patients may be required to lie down and bunks are suitable for this. Bunks, however, limit access to the patient and head down tilt is unavailable. Ideally high intensity care should be delivered on a tipping trolley with height adjustment.
PATIENT TRANSPORT TO HYPERBARIC FACILITIES AND PATIENT RECEPTION It is important that the accepting doctor is satisfied that the medical cover is adequate for the patient whilst being transported to the hyperbaric chamber. Unpressurized aircraft flying at low altitude are used to transport divers from incident scenes to hyperbaric facilities. The time advantage outweighs the small risk of bubble enlargement at these low altitudes (less than 3000 feet). Of more concern is the more prolonged flight in a pressurized aircraft, typically with a cabin altitude pressure of 6000}8000 feet. If necessary aircraft can be pressurized to sea level (i.e. zero altitude). However flights tend to take longer and be more costly in terms of fuel. Of more importance is that all patients with decompression sickness, air embolism or carbon monoxide poisoning are transported breathing as close to 100% oxygen as possible. It may be appropriate to bypass a local hospital to attend a centre with hyperbaric facilities but such decisions should be made in conjunction with attendants on-scene and the various hospitals involved. It has been recognized for some time that the secondary transport of the seriously injured has been a neglected area. It is recommended that monitoring of such patients should not be compromised.8 The Australian and New Zealand College of Anaesthetists have produced guidelines and the UK Intensive Care Society have also published good practice guidelines. These have attempted to bring together advice from different sources and encourage an improvement in standards in the transport of the critically ill.9,10
CURRENT ANAESTHESIA & CRITICAL CARE
FUTURE DEVELOPMENTS IN HYPERBARIC MEDICINE Historically, hyperbaric medicine has been an offshoot of diving medicine. Diving medicine has assumed importance owing to the developments in military applications and more recently with sports diving and offshore deep-sea drilling operations. Currently in the UK, physicians with an interest in hyperbaric medicine come from a variety of parent specialities (general medicine, anaesthesia, emergency medicine, occupational medicine). With an increasing tendency to subspecialization in the UK, it seems likely that more formalized fellowship training will become the norm. As most of the current indications for HBO are based on evidence from uncontrolled clinical trials and with the move towards evidence-based medicine with the formation of bodies such as the National Institute for Clinical Excellence (NICE), further research will be necessary to validate current treatment regimes. Hyperbaric medicine has become a recognized treatment for a number of disorders. Its indications for primary therapy are for decompression sickness, air embolism and carbon monoxide poisoning. There is a rational basis for HBO therapy in conditions where hypoxia forms the basis of the pathophysiology but controlled studies are lacking. It seems likely that as the levels of evidence improve then HBO therapy will impact on more disease states. This will require more facilities, more multidisciplinary co-operation and may have major funding implications (Table 3). A system of clinical governance has been instituted in Scotland to ensure minimum standards in the hyperbaric chambers utilised to treat NHS patients.
Table 3 Examples of areas for future research in hyperbaric medicine10,11 Basic sciences Animal experiments Human volunteer studies
Human patient studies
Long-term follow up studies Controlled clinical trials
effects of HBO on mitochondrial oxygen tension toxic encephalopathies the effects of physical exercise under HBO acceleration induced loss of consciousness and HBO AIDS encephalopathy altitude sickness toxic epidermal necrolysis HBO and multiple sclerosis acute severe head injury acute stroke
HYPERBARIC MEDICINE PART II
REFERENCES 1. Shirley P J, Ross J A S. Hyperbaric medicine part I: theory and practice. Curr Anaesth Crit Care 2001; 12: 114}120. 2. Wattel F, Mathieu D. (eds) Reports and recommendations of 1st European consensus conference on hyperbaric medicine, 1994. 3. Faculty of Occupational Medicine. A code of good working practice for the operation and staffing of hyperbaric chambers for therapeutic purposes. London: Royal College of Physicians, 1994. 4. Hamilton R W, Sheffield P J. Hyperbaric Chamber Safety. In: Davis J C, Hunt T R (eds). Hyperbaric Oxygen Therapy Bethesda: Undersea and Hyperbaric Medical Society, 1977. 5. British Hyperbaric Association Technical Working Party Guide to fire safety standards for hyperbaric treatment centres. BHA, 1996. 6. British Hyperbaric Association Technical Working Party Guide to electrical safety standards for hyperbaric treatment centres. BHA, 1996.
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7. Sheffield P J, Desautels D A, Hyperbaric and hypobaric chambers fires: a 73-year analysis. Undersea Hyperb Med 1997; 24: 153}164. 8. Oakley P A. The need for standards in inter-hospital transfer. [Editorial] Anaesthesia 1994; 49: 565}566. 9. Faculty of Intensive Care of the Australian and New Zealand College of Anaesthetists and Australasian College for Emergency Medicine. Minimum standards for transport of the critically ill, 1996. 10. Intensive Care Society. Guidelines for the transport of the critically ill. UK: ICS, 1997. 11. Gail D B, Hyperbaric oxygen therapy: NHLB workshop summary. Ann Rev Resp Dis 1991; 144: 1414}1421. 12. Cramer F S. Oxygen and acceleration. Military Med 1991; 156: 608}611.