Clinical Importance of Slow Vital Capacity – What the Respiratory Therapist Needs to Know

By Shruti Singh, Sara Z. Khan, Bhakti Patel, Dhawani Pandya, Arunabh Talwar, M.D, FCCP.^1A

Introduction

Spirometry is a pulmonary function test that allows screening, diagnosis, and monitoring of respiratory diseases [Fig1]. This is a simple, non-invasive physiological test that is easy to perform. By quantifying the respiratory volumes and flows, such as forced vital capacity (FVC), forced expiratory volume in the first second (FEV1), and the relationship between these parameters (FEV1/FVC), airway obstruction can be detected with high sensitivity and specificity. In addition, this test allows a clinician to classify the severity and response to the bronchodilator. Clinicians use serial spirometry measurements to monitor treatment response as well as the progression of a disease¹. Reliable interpretation of pulmonary function results relies on the availability of appropriate reference values (obtained from healthy subjects with the same anthropometric variables like age, height, gender, and ethnicity) to help distinguish between health and disease and to assess the severity and nature of any functional impairment².

 

Spirometry essentially allows the clinician to measure the maximal volume of air that an individual can inspire and expire with maximal effort. The primary signal measured in spirometry is either volume or flow as a function of time. From a clinical standpoint, the most relevant measurement is the vital capacity (VC). Vital capacity is defined as the expiratory volume delivered during an expiration cycle made as forcefully and as completely as possible starting from full inspiration. It is the total of tidal volume, inspiratory reserve volume, and expiratory reserve volume (VC = V + IRV + ERV). Vital capacity may be measured as inspiratory vital capacity (IVC), slow vital capacity (SVC), or forced vital capacity (FVC). The FVC is similar to VC, but it is measured as the patient exhales with maximum speed and effort. A low vital capacity is defined as a value below the 5th percentile in adults or less than 80% of predicted capacity in children and adolescents five to 18 years of age.

The SVC can either be measured during a slow, gentle, maximal expiration after a maximal inspiration or, alternatively, during a maximal inspiration following a slow, gentle, maximal expiration^3,4. While performing spirometry, both the FVC and SVC maneuvers are repeated a minimum of three times―and no more than eight times are usually required⁴. In healthy individuals, if performed correctly, the SVC should typically be equal to FVC due to the lack of dynamic compression on the airways. However, it is well established that in obstructive airways disease there may be a difference between FVC and SVC in the presence of increased collapsibility/compressibility of the airways^5,6,7. It is suggested that patients with obstructive disease more frequently have a ∆SVC−FVC > 0.20 L when compared with normal individuals¹⁴.

In patients with obstructive lung disease, SVC is greater than FVC because airway closure occurs earlier at a higher volume than the true residual volume (RV) due to the inflammatory narrowing of the peripheral airways. Also, during the forced maneuver of FVC, the dynamic compression of the airways further worsens the airway closure, resulting in air trapping, with a higher RV and hence lower FVC³. On the other hand, during the relaxed maneuvers of SVC, the airway closure is delayed due to little dynamic compression of the airways and low intrathoracic pressure, resulting in a lower RV and higher SVC as more air gets mobilized. The difference between SVC and FVC serves as an indicator of air trapping and increases further when more airway obstruction is present and the disease is more severe. [Fig 1]

Clinically, obstructive airways disease is diagnosed when an obstructive ventilatory defect is present in the pulmonary function test. Obstruction is defined as the ratio of FEV1 to vital capacity below the lower limit of normal. A potential issue complicating this diagnostic approach is that, as mentioned above, vital capacity can be measured in various ways (i.e., FVC or SVC). In the event of a significant difference between SVC and FVC as seen in obstructed patients, one can potentially encounter a clinical scenario of a “pseudo-normal” FEV1/FVC ratio while the FEV1/SVC ratio may be below the lower limit of normal. For these reasons, The American Thoracic Society (ATS) and The European Respiratory Society (ERS) recommend the use of the largest VC to calculate the FEV1/VC ratio [Tiffineau index] to define the obstructive airways disease¹.

The difference between FVC and SVC is also affected by variables like body mass index (BMI) and age^6,8,9. Obesity can decrease a patient’s vital capacity, as reflected in both FVC and SVC measurements. In individuals with a BMI 25 kg/m2 who do have obstruction (defined as FEV1/FVC ratio lower than LLN), FVC is larger than SVC. In contrast, in overweight and obese individuals, SVC is larger than FVC, and the SVC-FVC difference is positively correlated with BMI. Thus it appears that in overweight and obese individuals, the use of FEV1/FVC alone may lead to underdiagnosis of obstructive airways disease, and in that scenario, one should also evaluate the FEV1/SVC while interpreting these results⁷.

It is well established that the residual volume and the functional residual capacity increase with age, resulting in a decrease in vital capacity. However, FVC decreases with age at a faster rate than SVC. This is due to the reduced chest wall compliance and expiratory muscle strength and a greater tendency of smaller airways to close during the forced maneuver. During the SVC maneuver, the smaller airways remain patent longer during the expiratory maneuver, and hence the noticed difference between SVC and FVC increases with aging. This implies that a low FEV1/SVC but preserved FEV1/FVC might be seen in healthy elderly subjects. Thus an indiscriminate reliance on FEV1/SVC in elderly populations may carry a theoretical risk of overdiagnosis of obstructive airways disorder in that age group.

Respiratory muscle weakness is a major cause of morbidity and mortality in patients with neuromuscular diseases (NMDs) Close monitoring of respiratory function is currently the standard of care. It is well established that serial vital capacity measurements are important to evaluate the progression of neuromuscular disorders like amyotrophic lateral sclerosis (ALS). In NMDs, impairment of respiratory muscle contractile function leads to respiratory pump failure, characterized by decreases in vital capacity (VC), total lung capacity (TLC), and functional residual capacity (FRC). This reduction in VC is further augmented in the supine position, and a drop of greater than 25% is considered 90% sensitive and 79% specific for the diagnosis of diaphragmatic weakness. Paying attention to SVC in patients with neuromuscular disorders is thus also important¹⁰. In this population, slow vital capacity is a more comfortable and convenient test to perform in clinical practice, particularly in patients with upper motor neuron disease with respiratory insufficiency¹¹. On the other hand, FVC is challenging to perform in ALS patients as it requires greater effort and coordination leading to air volume leakage between the mouthpiece and the weak lips.

Conclusion

Though the classical teaching is to pay attention to only FVC while interpreting spirometry results, there are clinical scenarios as highlighted above wherein paying attention to SVC is important. In general, using SVC instead of FVC in the FEV1/VC ratio enhances the yield of spirometry in detecting mild airflow obstruction in younger and obese subjects^12,13. On the other hand, FEV1/SVC ratio should be used with caution in elderly subjects with preserved FEV1/FVC because a low value may represent a false-positive finding for airflow limitation.

In addition, patients with progressive neuromuscular disorders (like ALS), particularly with bulbar involvement, may not be able to perform a proper FVC maneuver, and in that setting, SVC may serve as an alternative measure of vital capacity.

References

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1A. Corresponding Author/Reprint Requests:
      Arunabh Talwar, M.D., F.C.C.P
Northwell Health
Pulmonary, Critical Care and Sleep Medicine
410 Lakeville Rd. New Hyde Park, NY 11040
Phone: (516) 465-5400
Fax: (516) 465-5454
E-mail: arunabhtalwar1@gmail.com