Sustained Maximal Inspiration (SMI) is a specific breathing maneuver that encourages a deliberate, slow, and deep breath to fully inflate the lungs. This technique is facilitated by an incentive spirometer, a handheld medical device that provides visual feedback on the depth of the breath taken. The primary goal of SMI is to mimic a natural sigh or yawn, promoting the expansion of the small air sacs in the lungs, known as alveoli. Performing this maneuver frequently, especially after surgery or during periods of reduced mobility, helps patients maintain or restore their lung capacity.
Measuring Lung Volume
The incentive spirometer translates a physical breath into a measurable volume using a simple mechanical principle. Most common devices are volume-oriented, directly quantifying the total air inhaled during the SMI maneuver. As the patient inhales through the mouthpiece, the breath creates negative pressure, causing a piston or float inside a chamber to rise. This chamber is marked with a numbered scale, typically in milliliters (mL) or cubic centimeters (cc), showing the exact volume inspired. Some devices are flow-oriented, measuring the rate of air flow using balls suspended within chambers, but volume-oriented spirometers are generally favored for providing a more precise, quantifiable measure of the actual volume achieved.
Calculating Personalized Goals
A “good SMI” reading is not a fixed, universal number but a personalized volume goal established by a healthcare provider. This target volume is calculated using predictive equations based on patient-specific factors, including age, height, and sex. The resulting number represents the predicted maximum inspiratory volume for that demographic profile. The provider adjusts a movable marker on the spirometer to this calculated target volume, such as setting a goal of 2,300 mL for a specific patient. Following major surgery, the goal may initially be set lower, such as 500 mL, and gradually increased as the patient recovers.
Mastering the Technique
Proper technique ensures the measured volume is accurate and the breath effectively reaches deep into the lungs. The patient should first sit upright for optimal diaphragm movement, then exhale normally to empty the lungs. After sealing the lips around the mouthpiece, the patient must inhale slowly and deeply, pulling the piston upward in a steady, sustained motion. Once maximum volume is reached, the patient must hold their breath for three to five seconds to allow for complete lung expansion before slowly exhaling and resting. Common errors, such as inhaling too quickly or not sealing the lips, reduce the maneuver’s therapeutic benefit.
Clinical Importance of Achieving Target Volume
Achieving the personalized target volume during SMI is a preventative action with direct medical significance for pulmonary health. A primary function of this therapy is the prevention of atelectasis, the partial collapse of a lung segment that commonly occurs after general anesthesia, prolonged bed rest, or chest/abdominal surgery. By encouraging a deep, slow inspiration, SMI increases transpulmonary pressure, physically forcing collapsed air sacs to open and remain open. This action also helps clear secretions and reduces the likelihood of secondary complications like pneumonia. Consistently reaching the goal volume is a measurable indicator of effective lung expansion and supports a faster, complication-free recovery.