| Description |
Functional residual capacity (FRC) is the gas volume remaining in the lung following a normal expiration. The size of the FRC may be compromised as result of many pathophysiologic factors, including anesthesia, obesity, acute lung injury, and acute respiratory distress syndrome. Without sufficient FRC volume, both blood oxygenation and carbon dioxide excretion are limited, leading to hypoxemia, carbon dioxide retention, and possible morbidity and mortality. Clinicians have long recognized the potential for improved care from FRC measurement availability, and researchers have been looking for an effective means of bedside FRC assessment during mechanical ventilation for decades. FRC measurement is useful, for example, for guiding ventilation management to improve gas exchange for patients with reduced FRC. Traditional methods of FRC measurement have been valuable for researching disease progression and monitoring ambulatory patients, but are impractical at the bedside. Recent research has proposed better bedside utility through volume-based methods such as nitrogen or oxygen wash-in/ washout to help address the need for FRC measurement. However, the proposed volume-based methods give lower measurement precision during ventilation with spontaneous effort or high airway pressure. Furthermore, these volume-based systems cannot be used with circle breathing systems which are commonly found in the operating room. Thus, the need remains for automated, accurate bedside FRC measurement systems that can be used in the intensive care unit and the operating room during many modes of ventilation, including controlled, assisted, spontaneous and mixed. This dissertation describes the development, clinical feasibility testing and clinical accuracy assessment of two novel bedside models for FRC measurement that use tracer gas washin/washout. The first model, called the modified multiple breath nitrogen washout model, makes use of end-tidal gas measurements to measure FRC. Using endtidal measurements instead of volume reduces errors from signal synchronization. The second model, which is called the partial rebreathing carbon dioxide model, allows FRC measurement during fixed inspired oxygen concentration, making FRC measurement possible in the operating room, where circle breathing systems are common. Both FRC measurement methods demonstrate good accuracy, are compatible with any ventilator brand and can easily be moved from patient to patient for bedside measurement. |
