The Reid Index in healthy cats

Asthma is a prevalent respiratory disease in humans and a similar condition has been reported in cats. It is difficult to diagnose feline asthma because the signs are often mistaken for other pulmonary or cardiac diseases, and the gross and microscopic lesions are subtle. The bronchial gland to bronchial wall ratio, also known as the Reid Index (RI), is commonly used to confirm asthma and chronic bronchitis in humans but this method has never been validated in cats. The objectives of this investigation are to establish the normal RI in healthy cats, and investigate the effect of bronchial sectioning and lung lobe on RI. The lungs of forty one clinically healthy cats obtained from P.E.I. Humane Society were examined at postmortem and fixed in formalin. Preselected sections of the right apical, right intermediate, right accessory, right diaphragmatic, left apical, and left diaphragmatic lobes were examined microscopically and the lungs were further classified as having no microscopic lesions or having minor lesions. Bronchi were cut either longitudinally or transversely and the RI was determined by linear measurement using J-image software (IJ) or by two-dimensional measurement using point count stereology (PCS).

Results showed that the RI in healthy cats is 61% ± 11.8 (using the mean of individual measurements in both groups of lungs with and without minor lesions, measured by linear measurement, and cut either longitudinally or transversely on bronchial view that have cartilage and cartilage was excluded) which is notably higher than the 40% reported for healthy human lung. There were minor but significant differences among lung lobes within the group of lungs with no microscopic lesions (p = 0.034) and group of lungs with minor lesions (p < 0.001). There were no significant differences between the lungs with no microscopic lesions and lungs with minor lesions (p < 0.946). There was a significant difference between the right intermediate lobe in the group of lungs with no microscopic lesions and the right intermediate lobe in lungs with minor lesions ( p = 0.029) with a higher RI in the group with minor lesions. The average, of gland to wall ratio in all lobes except RIN lobe in the group of lungs with no microscopic lesions, cut longitudinally and measured by linear measurement was 50.44 ± 0.359.

The correlation between linear measurement and two-dimensional measurement was poor when cartilage was included in the measurements. Linear measurement and bronchial cross sections had more variation than two-dimensional measurement and longitudinal sections. Minimum of 12 measurements per cat (taken from different lobes) and a minimum of 15 measurements per lobe are needed to obtain a meaningful RI.

It was concluded from this investigation that the feline bronchial glands have a notably higher RI than human. Using the right intermediate lung lobe for RI calculation in cats should be avoided. Two-dimensional measurements should be preferred over linear measurements and longitudinal sections of the bronchial tree should be chosen over cross sections of bronchi. The least amount of variation in the RI is found when two-dimensional measurements are made in longitudinal sections. For a practical RI method, it is recommended to use the longitudinal sections measured by linear measurements in bronchial areas without cartilage. The measurements should be made in bronchial areas that do not contain cartilage or if present, cartilage should be excluded from the calculations.