Question

why may someone demostrate lower than average forced expiratory volume in one second (FEV1.0) and vital capacity (VC), and yet have a higher tidal volume (TV) than average?

* note : subject had a cold when tested on

Answer 1

Forced expiratory volume (FEV) measures how much air a person can exhale during a forced breath. The amount of air exhaled may be measured during the first (FEV1), second (FEV2), and/or third seconds (FEV3) of the forced breath.

Forced vital capacity (FVC) is the total amount of air exhaled during the FEV test.

Vital capacity (VC) measures the amount of air you can exhale after inhaling as deeply as possible. It is one of many lung function tests.

Forced expiratory volume and forced vital capacity are lung function tests that are measured during spirometry. Forced expiratory volume is the most important measurement of lung function. It is used to:Diagnose obstructive lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). A person who has asthma or COPD has a lower FEV1 result than a healthy person.

Tidal volume has been described as the volume of air that is transported into and out of the lungs with each respiratory cycle. Tidal volume is clinically important not only for ventilator settings in the critically ill patient, but can also be clinically important during spontaneous respiration. Tidal volume in a healthy adult male is approximately 500 mL and in a healthy adult female is approximately 400 mL, but can also be altered to fit physiological need.

Capacities are the sum of two or more respiratory volumes. The normal resting point of the lung is at the end of a normal, quiet expiration. Functional residual capacity is the volume of air remaining in the lungs after this normal, quiet expiration and is equal to (expiratory reserve volume + residual volume). Inspiratory capacity is the volume of air that can be inspired following a normal, quiet expiration and is equal to tidal volume + inspiratory reserve volume. Vital capacity is the volume of air under voluntary control, equal to (inspiratory reserve volume + tidal volume + expiratory reserve volume). Vital capacity measurement requires maximal effort on the part of the patient and is often called forced vital capacity. Total lung capacity is the amount of air contained within a maximally inflated lung (all four volumes combined).

Mathematically, Lung Capacities

•Total lung capacity (TLC) = TV + IRV + ERV(OR FEV) + RV

•Vital capacity (VC) = IRV + ERV + TV

•Inspiratory capacity (IC) = TV + IRV

•Functional residual capacity (FRC) = RV + ERV

TV, Tidal volume; IRV, inspiratory reserve volume; ERV, expiratory reserve volume; RV, residual volume

The forced expiratory volume in 1 second (FEV1) is the volume of air (in liters) exhaled in the first second during forced exhalation after maximal inspiration. Normally, at least 80% of the forced vital capacity (FVC) is exhaled in the first second. The FEV1 is used with the FVC to differentiate between obstructive lung disease (FEV1/FVC < 70%) and restrictive lung disease (reduced FEV1 and FVC but normal FEV1/FVC relationship). An FEV1 of less than 1 L indicates significant lung disease.

Tidal volume is essentially every breath that you take. However, tidal volume plays a crucial role in calculating a patient's minute ventilation. Minute ventilation is the volume of gas exchanged by a patient's lungs per minute and is expressed as: minute ventilation = tidal volume(respiratory rate).

Intrathoracic Pressure Changes During spontaneous breathing, air enters the lungs because intrapulmonary (intraalveolar) pressure, drops from 760 to 758mm Hg .ie pulmonary pressures are negative During mechanical ventilation air is forced into the lung by applying positive pressure which increases intrathoracic pressure. High inflation pressures can result in barotrauma. High tidal volumes can result in volutrauma. Both of these situations can cause acute lung injury, and worsen respiratory failure.