However, it is only accurate if the lungs are not obstructed. Helium dilution is used to measure total lung capacity. The height is recorded with a pen to reflect the volume inspired or expired over time. A weight above the chamber changes height with each ventilation according to the circuit volume. The chamber is filled with oxygen and as they breathe, gas increased and reduces the volumes within the circuit. The subject breathes from a closed circuit over water. Of these, height has the greatest influence upon capacities. Measured values are standardised for height, age and sex. However, it cannot measure residual volume. Simple spirometry can measure tidal volume, inspiratory reserve volume and expiratory reserve volume.

Measuring Volumes and Capacities Simple Spirometry This can reflect alveoli that are ventilated but not perfused, for example secondary to a pulmonary embolus. This includes volume in upper and lower respiratory tract up to and including the terminal bronchioles.Īlveolar (distributive) dead space is the volume of air that reaches alveoli but never participates in respiration. Volume of air in lungs after maximum inspirationĪnatomical (serial) dead space is the volume of air that never reaches alveoli and therefore never participates in respiration. Tidal volume + inspiratory reserve volumeĮxpiratory reserve volume + residual volumeĪffected by height, gender, posture, changes in lung compliance. Volume breathed in from quiet expiration to maximum inspiration

Requires adequate compliance, muscle strength and low airway resistance Inspiratory reserve volume + tidal volume + expiratory reserve volume maximum inspiration to maximum expiration) Volume that can be exhaled after maximum inspiration (ie. These are fixed as they do not change with the pattern of breathing. These are composed of 2 or more lung volumes. Volume remaining after maximum expiration Reduced in pregnancy, obesity, severe obstruction or proximal (of trachea/bronchi obstruction) Relies on muscle strength and low airway resistance Relies on muscle strength, lung compliance (elastic recoil) and a normal starting point (end of tidal volume)Įxtra volume that can be expired below tidal volume, from normal quiet expiration to maximum expiration shallow breaths vs deep breathsĮxtra volume that can be inspired above tidal volume, from normal quiet inspiration to maximum inspiration Volume that enters and leaves with each breath, from normal quiet inspiration to normal quiet expirationĬhanges with pattern of breathing e.g.

Lung dead space