Concept: Boyle's law
Managing patients with stable respiratory disease planning air travel: a primary care summary of the British Thoracic Society recommendations
- Primary care respiratory journal : journal of the General Practice Airways Group
- Published over 7 years ago
Air travel poses medical challenges to passengers with respiratory disease, principally because of exposure to a hypobaric environment. In 2002 the British Thoracic Society published recommendations for adults and children with respiratory disease planning air travel, with a web update in 2004. New full recommendations and a summary were published in 2011, containing key recommendations for the assessment of high-risk patients and identification of those likely to require in-flight supplemental oxygen. This paper highlights the aspects of particular relevance to primary care practitioners with the following key points: (1) At cabin altitudes of 8000 feet (the usual upper limit of in-flight cabin pressure, equivalent to 0.75 atmospheres) the partial pressure of oxygen falls to the equivalent of breathing 15.1% oxygen at sea level. Arterial oxygen tension falls in all passengers; in patients with respiratory disease, altitude may worsen preexisting hypoxaemia. (2) Altitude exposure also influences the volume of any air in cavities, where pressure x volume remain constant (Boyle’s law), so that a pneumothorax or closed lung bulla will expand and may cause respiratory distress. Similarly, barotrauma may affect the middle ear or sinuses if these cavities fail to equilibrate. (3) Patients with respiratory disease require clinical assessment and advice before air travel to: (a) optimise usual care; (b) consider contraindications to travel and possible need for in-flight oxygen; © consider the need for secondary care referral for further assessment; (d) discuss the risk of venous thromboembolism; and (e) discuss forward planning for the journey.
Baro-otalgia is a common complaint among passengers in an aircraft, in particular those who had a recent upper respiratory tract infection. The underlying pathophysiology is secondary to unequal aeration of the middle ear cleft with the surrounding atmosphere and it can be explained using Boyle’s Law. We describe an unusual presentation of baro-otalgia in a pilot secondary to cholesteatoma obstructing the aditus despite normal middle ear pressure equalization provided by a grommet in the ear.
Micro-CT scanning of temporal bones has revealed numerous retroauricular microchannels, which connect the outer bone surface directly to the underlying mastoid air cells. Their structure and dimensions have suggested a separate vascular supply to the mastoid mucosa, which may play a role in middle ear (ME) pressure regulation. This role may be accomplished by changes in the mucosa congestion resulting in volumetric changes, which ultimately affect the pressure of the enclosed ME gas pocket (Boyle’s law). Further, such mucosa congestion may be susceptible to α-adrenergic stimulation similar to the mucosa of the nose. The purpose of our study was to investigate these hypotheses by recording the ME pressure in response to adrenergic stimulation administered by retroauricular injections at the surface of the microchannels. In a group of 20 healthy adults we measured the ME pressure by tympanometry initially in the sitting position, and then in the supine position over a 5 min period with 30 s intervals. In each subject, the study included 1) a control reference experiment with no intervention, 2) a control experiment with subcutaneously retroauricular injection of 1 ml isotonic NaCl solution, and 3) a test experiment with subcutaneously retroauricular injection of 1 ml NaCl-adrenaline solution. In both control experiments the ME pressure displayed an immediate increase in response to changing body position; this pressure increase remained stable for the entire period up to five minutes. In the test experiments the ME pressure also showed an initial pressure increase, but it was followed by a distinct significant pressure decrease with a maximum after 90 s. The test group was injected with both a 5 and 10 % adrenaline solution, but the responses appeared similar for the two concentrations. Subcutaneous retroauricular injection of adrenaline caused a significant pressure decrease in ME pressure compared with control ears. This may be explained by the microchannels conveying the adrenaline to the underlying mastoid mucosa, where it may result in a vascular constriction and decongestion, ultimately resulting in a ME pressure decrease. These findings suggest that the microchannels contain vascular connections to the mastoid mucosa, and that the mastoid mucosa is susceptible to vasoactive mediators, which may play a role in ME pressure regulation. Further anatomical and physiological experiments should be carried out to confirm these suggestions including pharmacological interactions with the mastoid mucosa.
High injection pressure is one of the warning signs of intraneural injection, with animal models suggesting pressures higher than 69 or 176 kPa as high risk, and is normally detected subjectively and inaccurately. We describe a system improvised from common clinical components that uses Boyle’s law to objectively measure injection pressure. The objectives of the study were to (1) Validate our improvised pressure gauge (IPG) by comparing the injection pressure as calculated by Boyle’s law against the measured pressure and (2) Use the IPG to measure the range of injection pressures by two groups of anesthetic professionals using the “syringe feel” technique. Our IPG system consists of an extended 1 ml syringe attached to a 3-way stopcock, inserted between the syringe containing the local anesthetic injectate and the needle. The IPG was validated against a pressure calibration reference. 20 anesthesiologists and 20 anesthetic assistants were recruited to apply pressure to the 20 ml syringe in vitro while blinded to the attached IPG. The pressures were measured on three separate occasions for each participant. There was good agreement (<8 percent difference) between the measured and theoretical pressure values. Anesthesiologists exceeded the threshold of 69 kPa in 18 of a total of 60 attempts whereas anesthetic assistants exceeded the threshold in 30 attempts out of 60 attempts. Anesthetic assistants exerted a higher overall pressure of 80 kPa compared to 51 kPa for anesthesiologists-this was statistically significant (p = 0.027). Our improvised system is easily and rapidly assembled from common clinical equipment and shows promise as a monitor for inadvertent intraneural injection.
ImportanceIt is well known that altitude ascent with intravitreal gas can cause expansion of gas and intraocular pressure (IOP) elevation. According to Boyle’s law, the gas bubble will not expand unless a higher altitude than the gas insertion site has been reached. We report four cases in which intravitreal gas was injected at an altitude of 790 m (Jerusalem). All four cases developed high IOP even though they did not reach a higher altitude in their post-operative period.ObservationsA report of four patients following vitrectomy with 12% mixture of perfluoropropane and air are presented. All four patients arrived with ocular pain following the ascent by car of 765-1100 m to Jerusalem where the vitrectomy and gas insertion was conducted. Upon examination, all four patients had high IOP (30-55 mm Hg). IOP was well controlled with IOP-lowering medications. None of the patients suffered from long-term complications.Conclusions and RelevanceCaution should be taken with altitude changes in patients with intravitreal gas even if there was no ascent from the altitude in which the vitrectomy was performed.Eye advance online publication, 2 May 2014; doi:10.1038/eye.2014.83.
In accordance with Boyle’s law (as barometric pressure decreases, gas volume increases), thoracostomy is often recommended for patients with pneumothoraces before helicopter EMS (HEMS) transport. We sought to characterize altitude-related volume changes in a pneumothorax model, aiming to improve clinical decisions for preflight thoracostomy in HEMS patients.