Abstract
Background: The slow vital capacity (SVC), forced vital capacity (FVC) difference, and forced expiratory volume in one second (FEV1)/SVC ratio are crucial for understanding respiratory health, especially in mild airway obstruction. These parameters are more reliable than traditional measures and are influenced by age and body mass index (BMI). Investigating their relationship can improve diagnostic accuracy and health-care interventions.

Aims and Objectives: (i) To record peak expiratory flow rate (PEFR), FVC, FEV1 of expiration, FEV1/FVC ratio, forced expiratory flow rate (FEF25-75%), mean forced expiratory flow rate from 0.2 to 1.2 l of volume expired (FEF.2-1.2), SVC, and find the difference between SVC and FVC, ratio of FEV1/SVC. (ii) To correlate SVC, SVC-FVC, FEV1/SVC, and other pulmonary function tests (PFTs) with age, gender, and BMI emphasizing their importance in detecting airway obstruction. (iii) To assess these known indicators of airflow obstruction in healthy individuals to gain knowledge of changing lung health with respect to advancing age, gender, and BMI.

Materials and Methods: The study included 200 individuals, comprising both men and women aged between 18 and 60 years, who were in good health. The sample was divided into five age groups, and participants were classified into categories of underweight, normal weight, overweight, and obese based on their BMI. Standard procedures were employed to record anthropometric measurements. PEFR was measured using Wright’s peak flow meter, whereas FVC, FEV1, FEV1/FVC ratio, forced expiratory flow between 25 and 75% of FVC (FEF25-75%), forced expiratory flow at 2-1.2 l (FEF.2-1.2), and SVC were assessed using an expirograph (Helios 401, RMS, India). Adequate statistical analysis was conducted on the gathered data.

Results: There was a highly significant difference in the means of SVC, SVC-FVC, FEV1/SVC, and other PFTs (P < 0.000) over the five age groups by ANOVA and a significant difference between younger and older age groups analyzed by post hoc test. It also showed a highly significant difference in the mean values of SVC, FEV1/SVC, and other PFTs over the categories of BMI and a significant difference between normal and obese groups by multiple comparisons. A small increase in mean values of SVC-FVC was seen in overweight, obese, and underweight compared to the normal BMI group. Females showed reduced mean values of SVC, FEV1/SVC, and other PFTs compared to males. (P < 0.000). The mean value of SVC-FVC in females is greater than in males (P < 0.01), which suggest that the tendency for airway obstruction may be greater in women compared to men due to smaller airway diameter and diffusion surface. PEFR, FVC, FEV1, FEV1/FVC, FEF25-75%, FEF.2-1.2, SVC, and FEV1/SVC were negatively correlated with age. SVC-FVC was positively correlated with age (r = 0.338, P < 0.000). There was a negative correlation between PEFR, FVC, FEV1, FEF25-75%, FEF.2-1.2, SVC, and BMI.

Conclusions: PFT serves as a crucial diagnostic tool in assessing respiratory health, with parameters such as FVC and FEV1 commonly utilized. However, SVC is gaining recognition for its nuanced insights into lung function. This article delves into the significance of SVC, highlights the differences between SVC and FVC, and explores the relationship between FEV1/SVC ratio, age, and BMI in healthy individuals. The current research was undertaken due to a scarcity of existing studies and demonstrated that SVC, the ratio of FEV1/SVC, and other PFTs diminish with age progression and increasing BMI. In addition, females exhibited decreased lung function, with a greater disparity between SVC and FVC than males.

Key words: Airway Obstruction; Slow Vital Capacity; Difference between Slow and Forced Vital Capacity; Ratio of FEV1/FVC