Measurements of functional residual capacity during intensive care treatment: the technical aspects and its possible clinical applications

Abstract

Direct measurement of lung volume, i.e. functional residual capacity (FRC) has been recommended for monitoring during mechanical ventilation. Mostly due to technical reasons, FRC measurements have not become a routine monitoring tool, but promising techniques have been presented. We performed a literature search of studies with the key words 'functional residual capacity' or 'end expiratory lung volume' and summarize the physiology and patho-physiology of FRC measurements in ventilated patients, describe the existing techniques for bedside measurement, and provide an overview of the clinical questions that can be addressed using an FRC assessment. The wash-in or wash-out of a tracer gas in a multiple breath maneuver seems to be best applicable at bedside, and promising techniques for nitrogen or oxygen wash-in/wash-out with reasonable accuracy and repeatability have been presented. Studies in ventilated patients demonstrate that FRC can easily be measured at bedside during various clinical settings, including positive end-expiratory pressure optimization, endotracheal suctioning, prone position, and the weaning from mechanical ventilation. Alveolar derecruitment can easily be monitored and improvements of FRC without changes of the ventilatory setting could indicate alveolar recruitment. FRC seems to be insensitive to over-inflation of already inflated alveoli. Growing evidence suggests that FRC measurements, in combination with other parameters such as arterial oxygenation and respiratory compliance, could provide important information on the pulmonary situation in critically ill patients. Further studies are needed to define the exact role of FRC in monitoring and perhaps guiding mechanical ventilation.

Accuracy and precision of end-expiratory lung-volume measurements by automated nitrogen washout/washin technique in patients with acute respiratory distress syndrome

“…This ventilator provides bedside EELV measurements by using the multibreath nitrogen-washout technique (MBNW) [8,15-18]. The oxygenation goal was achieved by adjusting FiO 2 , which was maintained constant during the study.…”

Electrical impedance tomography in the clinical assessment of lung volumes following recruitment manoeuvres

“…[31][32][33] EIT is valid and reliable As mentioned above, the gold standard for monitoring changes in recruitable lung tissue is CT scanning despite the limitations to this modality. 15,16 A highly significant correlation has been established between regional ventilation as determined by EIT when compared to electron beam CT (r 2 values ranging between 0.66 and 0.86). 34 Further, regional ventilation calculated using EIT was compared against CT in the ALI population during a slow inflation manoeuvre, demonstrating that the technique is comparable to the CT in an intensive care population.…”

“…9,11 The current gold standard for monitoring changes in recruitable lung tissue is computerized tomography (CT) scanning. 15,16 Although this technology is available and accurate, it is not readily used in monitoring patients in intensive care due to exposure of patients to substantial doses of ionizing radiation, the need to transport patients out of the intensive care unit, and the high cost of CT scanning. 15 Similarly, it is impractical to repeat CT scans every time a ventilatory manoeuvre is attempted.…”

Two Steps Forward in Bedside Monitoring of Lung Mechanics: Transpulmonary Pressure and Lung Volume

“…For example, rapid and/or irregular respiratory rates with large variations in tidal volume may alter FRC values and/or prevent gas-automated methods from performing the measurement [27]. Abnormal metabolic states because of high fever and/ or agitation, as well as neurological conditions that alter respiration may also infl uence FRC measurements by varying CO 2 production and breathing patterns [27,28].…”

“…Important information can be extracted from the FRC value, because this measurement correlates with 'functional' (aerated and communicating) lung size [28]. Resting aerated lung volume is tightly correlated with oxygenation [36], estimated risk for VILI [15] , work of breathing [38] and gas trapping [39].…”