Monitoring Expired CO2 Kinetics to Individualize Lung-Protective Ventilation in Patients With the Acute Respiratory Distress Syndrome

Abstract

Mechanical ventilation (MV) is a lifesaving supportive intervention in the management of acute respiratory distress syndrome (ARDS), buying time while the primary precipitating cause is being corrected. However, MV can contribute to a worsening of the primary lung injury, known as ventilation-induced lung injury (VILI), which could have an important impact on outcome. The ARDS lung is characterized by diffuse and heterogeneous lung damage and is particularly prone to suffer the consequences of an excessive mechanical stress imposed by higher airway pressures and volumes during MV. Of major concern is cyclic overdistension, affecting those lung segments receiving a proportionally higher tidal volume in an overall reduced lung volume. Theoretically, healthier lung regions are submitted to a larger stress and cyclic deformation and thus at high risk for developing VILI. Clinicians have difficulties in detecting VILI, particularly cyclic overdistension at the bedside, since routine monitoring of gas exchange and lung mechanics are relatively insensitive to this mechanism of VILI. Expired CO2 kinetics integrates relevant pathophysiological information of high interest for monitoring. CO2 is produced by cell metabolism in large daily quantities. After diffusing to tissue capillaries, CO2 is transported first by the venous and then by pulmonary circulation to the lung. Thereafter diffusing from capillaries to lung alveoli, it is finally convectively transported by lung ventilation for its elimination to the atmosphere. Modern readily clinically available sensor technology integrates information related to pulmonary ventilation, perfusion, and gas exchange from the single analysis of expired CO2 kinetics measured at the airway opening. Current volumetric capnography (VCap), the representation of the volume of expired CO2 in one single breath, informs about pulmonary perfusion, end-expiratory lung volume, dead space, and pulmonary ventilation inhomogeneities, all intimately related to cyclic overdistension during MV. Additionally, the recently described capnodynamic method provides the possibility to continuously measure the end-expiratory lung volume and effective pulmonary blood flow. All this information is accessed non-invasively and breath-by-breath helping clinicians to personalize ventilatory settings at the bedside and minimize overdistension and cyclic deformation of lung tissue.

Capnodynamic monitoring of lung volume and blood flow in response to increased positive end-expiratory pressure in moderate to severe COVID-19 pneumonia: an observational study

“…This represents a lack of alveolar recruitment where additional PEEP contributes to increased lung stress without any benefit in gas exchange. The capacity of capnodynamic monitoring at the bedside to facilitate an individualised setting of PEEP warrants further clinical investigation to evaluate if it can contribute to minimising ventilator induced lung injury in C-ARDS and non-COVID ARDS [ 30 ].…”

Intraoperative Ventilator Management of the Critically Ill Patient

“…In addition, VCap provides information about pulmonary perfusion, end-expiratory lung volume, and pulmonary ventilation inhomogeneities, which can be used for optimization of ventilation at the bedside. 58 , 59 , 60 Although further investigation is needed to establish the optimal use of VCap in the intraoperative setting, it is a potentially useful noninvasive tool to measure dead space and optimize intraoperative ventilation.…”

Effects of apparatus dead space on volumetric capnograms in neonates with healthy lungs: a simulation study

“…Third, obtaining capnograms of normal shape by removing any extra V D app provides the chance of calculating other aspects of pulmonary physiology beyond dead space like the breath-by-breath and noninvasive assessment of end-expiratory lung volume and pulmonary perfusion employing capnodynamics. [4][5][6][7] The capnodynamic equation relies on the correct estimation of P a CO 2 , which depends heavily on the size and slope of phase III. These parameters are especially attractive for evaluating ventilated newborns due to the problem of lacking adequate means for cardiorespiratory monitoring.…”

“…[3][4][5][6] The correct calculation of these clinical VCap variables clearly depends on the shape of this waveform. [3][4][5][6][7] However, the widespread occurrence of deformed capnograms has raised doubts about the clinical usefulness of mainstream VCap in preterm and term newborns. 8 Many researchers described distorted volumetric capnograms with long phases I and II and short, or even absent, phase III in preterm neonates with surfactant deficit and bronchopulmonary dysplasia.…”

“…We consider this information clinically relevant because we deal with this kind of fragile patients in the operating theater every day; episodes of severe hypercapnia during anesthesia are seen in such small babies, [21][22][23] and volumetric capnography provides useful noninvasive monitoring only if waveforms maintain a reasonable shape. [3][4][5][6][7] From all causes of deformation of the volumetric capnograms, the one related to V D app seems to be the most relevant problem in healthy babies. 8 We commonly see in our daily practice that volumetric capnograms recovered their normal shape after removing any additional V D app and that the technical capabilities of commercial volumetric capnographs seem to be adequate for monitoring even the smallest anesthetized babies with healthy lungs."…”