Division 1

Assessment of the Degree of an Employee's Permanent Impairment Resulting from an Injury

Chapter 2 - The Respiratory System

 

2.0 Introduction

In conducting an assessment, the assessor must have regard to the Principles of Assessment and the definitions contained in the Glossary.

The measure of impairment is the reduction in physiological function below that found in health.

Respiratory impairment is quantified by the degree to which measurements of respiratory function are changed by the compensable injury or injuries, relative to values obtained in a healthy reference population.

Conditions such as chronic obstructive airways disease and chronic bronchitis are to be assessed according to the methods used to measure loss of respiratory function.

Employees who have permanent respiratory limitation secondary to massive pulmonary embolism should be assessed under Chapter 2. Any WPI rating awarded in these circumstances must not be combined with a WPI rating from Table 13.4: Thrombotic Disorders (Chapter 13 - The Haematopoietic System).

2.1 Assessing Impairment to Respiratory Function

2.1.1 Measurements

The most commonly recommended measurements for determining respiratory impairment are:

• spirometry with measurement of the forced expiratory volume at 1 second (FEV₁) and forced vital capacity (FVC); and
• the transfer factor, or diffusing capacity of the lung, for carbon monoxide (TlCO), measured by the single breath method.

However, the measurements used must be derived from either:

• the tests prescribed below where relevant (for example, in assessing asthma); or
• where a test is not prescribed, from tests appropriate to assessing the impairments caused by the particular compensable condition or conditions.

Other measurements commonly used to assess impairment include:

• the lung volumes;
• total lung capacity (TLC) and residual volume (RV); and
• the response to a maximum exercise test including measurement of the oxygen consumption at the maximum workload able to be achieved (vO₂max), and the degree of arterial oxygen desaturation during exercise.

On occasion, other measurements may be needed to define impairment accurately. For example:

• the elastic and flow resistive properties of the lungs;
• respiratory muscle strength;
• arterial blood gases;
• polysomnography (sleep studies);
• echocardiography with estimation of pulmonary artery pressure; and
• quantitative ventilation-perfusion scans of the lung.

Measurement of the partial pressures of oxygen and carbon dioxide in arterial blood (PaO₂and PaCO₂respectively) are not usually required to assign impairment accurately. However, individual variation may result in severe impairment in gas exchange when other measures of function indicate moderate impairment only. Arterial PaO₂of <55 mm Hg and/or PaCO₂>50 mm Hg, despite optimal treatment, is evidence of severe impairment and attracts a WPI rating of 70%.

Measurements of arterial blood gases should be performed on two occasions with the employee seated.

2.1.2 Methods of Measurement

Measurements must be performed in a manner consistent with the methods used by a respiratory function laboratory accredited by one or more of the following bodies:

• the Thoracic Society of Australia and New Zealand;
• the Australian Sleep Society; or
• the Australian Council on Health Care Standards.

Methods of measurement should conform to internationally recognised standards in relation to the equipment used, the procedure, and analysis of the data. Reference values (?predicted? normal values) should be representative of the healthy population and be appropriate for ethnicity where possible. Laboratories providing measurements used to assess impairment should state the source of each method of measurement, and the source of the reference values used.

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2.1.3 Impairment Rating

Several professional groups have published criteria for rating the severity of impairment based on spirometry, gas transfer and vO₂max. These professional groups include the Thoracic Society of Australia and New Zealand (Abramson, 1996), the American Thoracic Society (American Thoracic Society Ad Hoc Committee on Impairment/Disability Criteria, 1986), and the American Medical Association (2001). In general, measurements are expressed as a percentage of the predicted value (%P) and, where several measurements are performed, the most abnormal result is used to classify the degree of impairment.

Severity of impairment is rated as shown in Table 2.1. This generic table can be used to assign WPI ratings using any valid measurement for which there is predicted normal data.

Table 2.1: Conversion of Respiratory Function Values to Impairment

% WPI	Respiratory Function %P
0	>85
10	85 to 76
20	75 to 66
30	65 to 56
40	55 to 51
50	50 to 44
60	45 to 41
70	40 to 36
80	≤35

2.2 Asthma and other Hyper-reactive Airways Diseases

Impairment due to asthma can be confounded by the natural history of occupational asthma, by variably severe airflow obstruction, and therefore variable FEV₁, and by response to treatment.

For hyper-reactivity of airways due to occupational exposures, assessment of impairment is made after:

• the diagnosis and cause are established;
• exposure to the initiating factors eliminated; and
• appropriate treatment of asthma implemented.

Appropriate treatment follows the guidelines in the Asthma Management Handbook 2002 (National Asthma Council, 2002, 5th ed, Melbourne: National Asthma Council of Australia), a later edition of those guidelines, or later guidelines widely accepted as representing best practice by the medical profession.

Permanent impairment should not be assessed until 2 years after cessation of exposure as severity may improve during this period.

An impairment rating scale is set out in Figure 2-A and Table 2.2. The scale used in Figure 2-A and Table 2.2 is modified to account for frequency of increased impairment from asthma despite optimal treatment.

A score reflecting impairment from asthma is calculated by:

• adding the points scored for reduction in FEV₁%P;

• and either

• and

The number of days on which any valid measurement of FEV₁or PF is less than 0.85 x the mean of the six highest values of FEV₁or PF during the monitoring period is to be expressed as a percentage of total days in the monitoring period.

The maximum impairment score from Figure 2-A is 11. One additional point is given, yielding a score of 12, if asthma cannot be controlled adequately with maximal treatment. The score from Figure

2-A is converted to a WPI rating using Table 2.2.

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Figure 2-A: Calculating Asthma Impairment Score

Score	FEV1, % P After Bronchodilator	DFEV1, % Change in FEV1with Bronchodilator	PD20 ORmmol	% of Days Lowest FEV1* is ≤ 0.85 Highest FEV1
0	>85	<10	>4.0	<6
1	76 to 85	10 to 19	0.26 to 4.0	6 to 24
2	66 to 75	20 to 29	0.063 to 0.25	25 to 34
3	56 to 65	≥ 30	≤ 0.062	35 to 44
4	≤ 55			≥ 45

Table 2.2: WPI Derived from Asthma Impairment Score

% WPI	Asthma Impairment Score
0	0
10	1
20	2
30	3
40	4
45	5
50	6
55	7
60	8
65	9
70	10
75	11
80	12

 

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2.3 Lung Cancer and Mesothelioma

Employees with lung cancers (other than mesothelioma) are considered severely impaired at the time of diagnosis and are given a WPI rating of 70%.

If there is evidence of tumour, or if tumour recurs one year after diagnosis is established, then the employee remains severely impaired and the WPI rating is increased to 80%.

Employees with mesothelioma are considered severely impaired and a WPI rating of 85 % is awarded upon diagnosis.

2.4 Breathing Disorders Associated with Sleep

Some disorders such as obstructive sleep apnoea, central sleep apnoea, and hypoventilation during sleep, can cause impairment which is not quantifiable by standard measurements of respiratory function such as spirometry, diffusing capacity, or response to exercise.

Obstructive sleep apnoea should be assessed using Table 2.4 below. Central sleep apnoea should be assessed using Table 12.1.3: Sleep and Arousal Disorders (Chapter 12 ? The Neurological System).

An overnight sleep study is used to define the severity of sleep-related disorders of breathing and can be used to define impairment after appropriate treatment has been implemented. During the overnight sleep study there is continuous monitoring of breathing pattern, respiratory effort, arterial oxygen saturation, electrocardiogram, and sleep state. Results of sleep studies cannot readily be expressed in terms of a percentage of predicted values. Consequently, impairment is rated by assigning scores to the degree of abnormality at sleep study (Figure 2-B, and Table 2.4). These ratings are based on frequency of disordered breathing, frequency of sleep disturbance, degree of hypoxaemia and, as appropriate, hypercapnoea during sleep. In addition, degree of daytime sleepiness is assessed using the Epworth sleepiness scale (Johns, 1991).

Where vascular morbidity is present (for example, high blood pressure, myocardial infarction, or stroke) and is attributable to sleep apnoea, impairment should be assessed against the relevant table in Chapter 1 - The Cardiovascular System.

The total score derived from Figure 2-B below is the sum of the scores from each column: the maximum score is 12. This score is converted to a WPI rating using Table 2.4.

Figure 2-B: Calculating Obstructive Sleep Apnoea Score

Score	Epworth Sleepiness Score	Apnoeas + Hypopnoeas/hr of Sleep	Respiratory Arousals*/hr of Sleep	Cumulative Sleep Time, mins, with SaO2<90% #
0	<5	<5	<5	0
1	5 to 10	5 to 15	5 to 15	<15
2	11 to 17	16 to 30	16 to 30	15 to 45
3	>17	>30	>30	>45

Table 2.4: WPI Derived from Obstructive Sleep Apnoea Score

% WPI	Sleep Apnoea Score
0	0
10	1
20	2
30	3
40	4
45	5
50	6
55	7
60	8
65	9
70	10
75	11
80	12