Anesthesia Toolbox Question:

An 80-year-old woman with no significant past medical history is scheduled to undergo a cystoscopy due to nephrolithiasis. Preoperatively, she is noted to have an SpO2 of 94% on room air. Which of the following is the MOST likely cause of the decrease in baseline SpO2?

A. Decreased functional residual capacity  X

B. Increased closing capacity  ✔

C. Decreased compliance of small airways  X

Read the discussion and key information below.

Discussion:

Closing capacity is the lung volume at which small airway closure begins to occur. It is defined as the residual volume + the closing volume. Increasing the closing capacity increases the risk of atelectasis, air trapping, and hypoxemia. Conditions that cause parenchymal or small airway changes within the lung-like aging, chronic bronchitis, chronic asthma, smoking, and pulmonary edema affect the ability of small airways to stay open and thus increase closing capacity.

Aging results in a series of anatomic and functional changes in the respiratory system. The closing volume, which is the lung volume at which more distal, small airways begin to close during expiration increases with age. These closed airways do not contribute as much to gas exchange and are likely to result in atelectasis. An increase in closing volume is related to a decrease in the supportive tissues surrounding the distal airways.

If the closing capacity equals or exceeds the functional residual capacity (FRC), normal tidal volume respirations will result in large portion of distal dependent airways becoming atelectatic or have such reduced flows they do not contribute to gas exchange. This is a critical factor in declining arterial oxygen (PaO2) with age. Closing volume increases linearly with age (as does residual volume); closing capacity exceeds FRC in the standing patient at about age 65 and exceeds FRC in the supine patient at about 45 years of age.

Key Information:

Closing volume is the lung volume at which more distal, small airways begin to close during expiration. Closing capacity is the residual volume + closing volume. Closing volume increases linearly with age; closing capacity exceeds FRC in the standing patient at about age 65 and exceeds FRC in the supine patient at about 45 years of age; thus, normal tidal volume respirations occur with a large portion of distal dependent airways not contributing to gas exchange.

References: 

Janssens JP, Pache JC, Nicod LP. Physiological changes in respiratory functional associated with aging. Eur Respir J. 1999; 13(1): 197-205.

Sprung J, Gajic O, Warner DO. Review article: age-related alterations in respiratory function - anesthetic considerations. Can J Anaesth. 2006;53(12):1244–57.

Everything you wanted to know about Functional Residual Capacity – CA 1 Podcast 12.3