Clinical Meetings at RH Year 2004

2004 - A Special Application of Body Plethysmograph

Drs KC Wong, Grace Law, YC Chan, TB & Chest Unit, TWGHs Wong Tai Sin Hospital

Case history
A 45-year-old lady with severe kyphoscoliosis secondary to childhood poliomyelitis presented to our unit in 2001 for progressive exertional dyspnoea, disturbed sleep and ankle oedema. Results of initial investigation were as follows: I) Restrictive ventilatory defect: FEV1/FYC =0.49 LlO.54 L (90.1 %), FEV,- 25% predicted; 2) Type 2 respiratory failure: arterial blood gases (room air) showing pH 7.35, pC02 8.1 kPa, p024.9 kPa, and HC03 34 mM.

Failing LTOT, patient was subsequently started on nocturnal nasal BiPAP: IPAP 12 cmH20, EPAP 4 cmH20, RR 14/min,andTi 1sec.PatienttoleratedBiPAPwell with improvement in her clinical condition and daytime arterial blood gases: (room air) pH 7.37, pC02 7.1 kPa, p02 13.6 kPa, and HC03 30.9 mM.

Around end of 2002, patient was planning for an overseas trip to Japan and she consulted us regarding her fitness for air travel. Results of preliminary evaluation were as follows:

B.W. 33.5kg. Height: 1.34m. SpO2: 95% (room air)

ABGs (room air)


Vitalograph
FEV1: 0.57 L
FVC: 0.70 L
FEV1/FVC: 81.4%

In accordance with the BTS Recommendations on assessment 1,2a hypoxic challenge test was subsequently performed because this patient had, besides a screening Sp02 value of between 93-95%, the following additional risk factors: (i) FEVl <50% predicted, (ii) restrictive lung disease due to severe kyphoscoliosis and old poliomyelitis, and (iii) being on ventilatory support.

A hypoxic challenge test by means of modified body plethysmograph 3 was peiformed:

A communication port was created by dislodging the pneumotachograph from the body plethysmograph (Figures 1 & 2). Tubings for nitrogen infusion, sensor of the oxygen analyzer and oxygen nasal prongs & tubing were placed inside the body plethysmograph through this communication port (Figure 2). A size G cylinder containing 1,250 litres of 99.995% nitrogen at 135 Bar was used to deliver nitrogen into the body plethysmograph through two flow-meters, each at the initial rate of 15 litres per minute. The oxygen analyzer used in this test was a fuel cell electrochemical analyzer. Figure 3 showed the complete set-up for this hypoxic challenge test.




The patient sat quietly inside the plethysmograph with the door closed. The Fi02 inside the body plethysmograph dropped from 21% to 15% after approximately 10 minutes of nitrogen infusion. Her Sp02 was monitored by a wrist-type pulse oximeter, which dropped to a nadir of 84%; but it was quickly restored to above 95% with administration of O2 via nasal cannula at the rate of 1-2 L/minute. The patient reported no increase in dyspnoea or any other discomfort throughout the test. The test was terminated after the target Fi02 of 15% was achieved & maintained for about 10 minutes.

Based on the results of this hypoxic challenge test, the followings were recommended to our patient for air travel:
1. Use of in-flight cannula oxygen ILlmin at rest
2. Use of in-flight cannula oxygen 2L1minduring meal or exertion
3. Self monitoring of in-flight Sp02 by pulse oximeter
4. To arrange portable battery for her BIPAP (for standby)

Discussion
Although the proportion of atmospheric oxygen remains at 21% of the total barometric pressure (PB) at rising altitude, there is considerable fall in logarithmic manner in the partial pressure of atmospheric oxygen due to a corresponding fall of the total barometric pressure at altitude 4 as shown in figure 4.




For example, atmospheric P02 at sea level is 159 mmHg (0.21 PB at sea level) and atmospheric P02 at 8,000 feet is 118 mmHg (0.15 PB at sea level).

The corresponding PI02 fully saturated with water vapour at 37°C can be derived fromtheequation:PI02=H02 (PB - 47 mmHg). 4

Commercial aircrafts normally cruise between 22,000 and 44,000 feet above sea level. The hypobaric effects of such high altitudes are lethal. International Aviation Regulation requires that aircrafts maintain 8,000 feet cabin altitude at the highest operating altitude.5 This equates a pressure of inspired oxygen (PI02) that can be reproduced by an inspired oxygen fraction (FI02) of about 15% at sea level.

The PB at 8,000 feet is about 565 mmHg and the corresponding PA02 can be
calculated from the Alveolar gas equation, PA02=PI02 - PAC02/R 4.6and Pa02 can be estimated.

The PA02 and Pa02 at 8,000 feet are about 65 mmHg and 55 mmHg respectively. Pa02 of 55 mmHg just sits at the top of sloping part of the oxyhaemoglobin dissociation curve. Further decrease in Pa02 can lead to precipitous fall in % Hb saturation.

Although healthy passengers do not generally experience symptoms at cabin altitude of 5,000 - 8,000 feet, subtle mental deficits such as altered perception, impaired judgement or vision, learning inefficiency and increased fatigability may be initially detected. 4.5

The primary physiological response to altitude hypoxia is stimulation of peripheral chemo-receptors at carotid bodies leading to hyperventilation and maximizing PA02 and Pa02. 5 The minute ventilation increases as a result of increase in tidal volume rather than tachypnoea. A - a P02 gradient narrows with altitude in people at rest and most important physiological change is loss of head of pressure driving O2from alveoli to blood, as ! in PA02 much greater than that in mixed venous P02 4.7 (because of the shape of the Oxyhaemoglobin Dissociation Curve at low P02). As a result, alveolar - venous gradient for 02 is smaller and equilibration slower than at ground level.

Clinically benign and reversible hypoxia - induced vasoconstriction increases pulmonary arterial pressure and vascular resistance. Cardiac output increases initially with hypoxia in a dose - dependent manner, primarily due to tachycardia. Hypoxia overcomes the cerebral vasoconstrictor effect of hyperventilation and oxygen delivery to the brain is maintained through dilation of cerebral vessels.

Pre-flight evaluation for oxygen therapy
About 10% of in-flight medical emergencies is respiratory in nature. 8 Respiratory problems 8 are the 3rd most common cause of in-flight medical emergency and also the 3rd most common cause of medical diversion of flight after cardiac and neurological events. It is hoped that with proper pre-flight evaluation, at least some in-flight respiratory emergencies can be avoided.

Categorically, the 3 methods currently used to evaluate patient fitness for air travel J are:
1. 50 metre walk tests; 2. Prediction of hypoxaemia from equations; 3. Hypoxic challenge tests.
1. 50 metre walk test
This test evaluates the ability to walk 50 metres. It simulates the stress of altitude hypoxaemia patient will experience at rest in-flight. Failure to finish the walk or moderate to severe dyspnoea alert to possible need for in-flight oxygen. It is a crude assessment and there is evidence validating this test.

2. Prediction of hypoxaemia from equations
These equations are derived' almost exclusively from COPD patients who had Pa02 measured in hypobaric chamber or before/during exposure to simulated altitude while breathing Fi02 of 15% from a reservoir bag.

Some examples of equations I are:
(1) This relates Pa02 at altitude (Alt) to Pa02 at sea level
(Ground): Pa02 Alt(mm Hg) =0.410 x Pa02 Ground (mm Hg) + 17.652

(2)ThisrelatesPa02AlttoPa02GroundandincludesFEVI in litres: Pa02 Alt =0.519 x Pa02 Ground (mm Hg) + 11.855 x FEV, (litres) - 1.760

(3) This relates Pa02 Alt to Pa02 Ground and includes FEVI as % predicted: Pa02 Alt =0.453 x Pa02 Ground (mm Hg) + 0.386 x (FEVI % pred) + 2.44

(4) This relates Pa02 Alt to Pa02 Ground and includes flight or destination altitude:
Pa02 Alt =22.8 - (2.74 x altitude in thousands of feet) + 0.68 x Pa02 Ground (mm Hg) Note:
Thousands of feet should be entered as feet divided by 1000; 8000 feet would thus be entered in the equation as 8.0.

Addition of FEVI as a variable in these equations may improve the accuracy of the predicted values

Limitations of the equations:
i. Validity of applying the equations to patients with respiratory diseases other than COPD is doubted.
ii.90% confidence limits are 1 IkPa (12-4% Sp02)' I But predictions are usually reliable enough to establish upper & lower thresholds for "no in-flight O2 needed" (Sp02 >95%) or "in-flight O2required" (Sp02 <92%). iii. Flight duration & cabin condition are not reproduced.
iv. No relevant information about individual cardiovascular, neuropsycholgic and other symptomatic responses at rest or during mild exertion or the effectiveness of supplemental O2at altitude.

3. Hypoxic challenge tests
Two categories of hypoxic challenge tests are available: (i) the hypobaric chamber test and (ii) the hypoxic gas inhalation test.

The hypobaric chamber test appeared to be the ideal simulation test as it simulates
not only the flight altitude hypoxia but also the hypobaria and air density of altitude.
However, it is not widely available and is generally considered as neither practical nor cost-effective.

In the hypoxic gas-inhalation test, also known as hypoxia-altitude simulation test, patients are asked to breathe hypoxic gas mixture at sea level. It is performed in lung function laboratory to evaluate acute altitude - equivalent responses in gas- exchange, symptoms and cardiovascular function. It assumes that (i) altitude hypoxia is the primary stress to patients with cardiopulmonary disorders and Fi02 at altitude can be replicated according to known pressure-altitude relationships and (ii) breathing hypoxic gas mixtures at sea level, normobaric hypoxia e~uates to the hypobaric hypoxia of altitude. In fact, in a study by Dilland et aI, it has been shown that Pa02 changes in patients with COPD with hypoxic gas-inhalation test of Fi02 15% at sea level were comparable to that with hypobaric chamber exposure equivalent to 8,000 feet of altitude above sea level.

In the hypoxic gas inhalation test, subjects are usually asked to breathe the hypoxic gas mixture for 20 minutes or until equilibration, Sp02 and ECG are monitored and ABGs are measured before and on completion. The test can also involve exercise and supplemental O2.

There are several ways to administer 15% oxygen. Oxygen and nitrogen can be mixed in reservoir bag or cylinder. Gas mixnues are given with non-rebreathing valve with mouthpiece and noseclips or tight fitting face mask.

Filling body plethysmograph with pure nitrogen to reduce Fi02 to 15%, like in our case study is another method. No face mask or mouthpiece is needed. It also allows oxygen requirement to be titrated accurately using nasal prongs to supply O2 within the body plethysmograph.

Alternatively, hypoxic gas mixture can be administered with commercial 40%
Venturi mask if nitrogen is used as the driving gas. Entrained air dilutes the
nitrogen giving a Fi02 of 14-15%. Similarly, Fi02 of 15-16% can be produced if 35% Venturi mask is used.

Limitation of hypoxic gas inhalation tests:
(i) Flight duration and cabin conditions are not reproduced.
(ii) Additional stressors such as dehydration, sleep and exercise are usually excluded, although it is possible to involve exercise by walking on treadmill at the slowest speed.

The level of evidence of the BTS Recommendations is at the best Grade B with the AHCPR Scale. The Recommendations are based on consensus opinions and data derived from a few randomised control trials that involved small number of study subjects.

In 2002, BTS air travel walking party acknowledged 8 the need for further research in the following areas: (i) Predictive value of spirometry, regression equations and hypoxic challenge (ii) Walk tests in different disease groups and (iii) Risk of air travel for patients with diffuse parenchymal lung disease.

In a recent study 10of hypoxic challenge of patients with interstitial lung disease and COPD, it was found that even in the presence of satisfactory ABG (PaOz> 9.3 kPa) at sea level, the oxygen saturation in the majority of patients in both groups fell below recommended level of 85% when cabin altitude was simulated. The desaturation was worsened to below acceptable level in almost all patients with challenge of a 50-metre walk test. The results of this study highlight the need for a prospective evaluation of a large number of respiratory patients planning to fly. It is hoped that the on-going UK Flight Outcomes Study 8which is targeted towards this aim can help answer some of the questions in this area.

Acknowledgement: The authors would like to thank the Hong Kong Oxygen Company for lending the oxygen analyser and the Asia Cardiovascular Products Ltd. for providing technical support in this test.





References

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