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Two steps forward in bedside monitoring of lung mechanics: transpulmonary pressure and lung volume.

Cortes GA, Marini JJ - Crit Care (2013)

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For many decades, pressure-based respiratory mechanics have served to aid the judgment of clinicians when monitoring mechanical ventilation and making important decisions in respiratory care... However, measurements based on airway pressure (PAW) alone have limited ability to generate individualized insights for a diverse patient population with varied pathologic conditions... While the passive lungs are the primary target of attention, PAW-based interpretations may be influenced by differences in breathing pattern, alterations in chest wall activity (including diaphragmatic function), changes in lung volume, asymmetry of lung disease, abdominal distension, etc... Absolute values of Pes are not only influenced by the 'mediastinal artifact' as a result of re-positioning from sitting to supine, but also by elevation of intra-abdominal pressure (IAP) and position-related lung volume changes... Recently, Owens et al. concluded that Pes measurement artifacts imposed by mediastinal weight and postural effects are within a clinically acceptable range... Similarly, improvement in respiratory-system compliance was also observed in the "esophageal pressure-guided group"... However, despite trends toward improved survival, this study does not provide uncontestable data supporting reduction in mortality associated with this mechanical ventilation strategy guided by PTP estimations in patients with ALI/ARDS... Nonetheless, PTP monitoring deserves credit for shifting the attention of clinicians to a more individually-tailored physiologic understanding of the respiratory function changes that occur during ALI and, although not perfect, estimations of PTP are of more help in elucidating the interactions between patient characteristics, disease conditions and ventilator settings than are pulmonary mechanics based on airway pressure alone (Table 1)... One example (Engstrom Care-stationtechnology, GE Healthcare Madison, WI) of this approach directly measures the end-expiratory lung volume by slightly altering the delivered FiO2 level (step changes of only 0.1) for short periods of time using its volumetric O2 and CO2 measurement capability... In a previous study we compared this method with ('gold standard') quantitative computer tomographic (CT) imaging and found that this automated method correlated well (across a wide range of end-expiratory lung volumes)... Studies conducted in lung injury models have investigated the relationship between FRC and tidal compliance... In a porcine oleic-acid-injury study, Rylander et al. found that FRC was a more sensitive indicator of PEEP-induced aeration than was compliance... Clinical experience shows that oxygenation is markedly affected by postural changes in certain patients... These hypoxemic episodes may be the result of position-related ventilation/perfusion changes associated with abrupt reductions in FRC or to regional perfusion changes... Moreover, recognizable patterns and trends in correlated indexes of FRC and PTP, in addition to traditional monitoring tools, could help diagnose and/or provide an early warning to the clinician of impending danger in the settings of chest wall abnormalities (e.g., elevated IAP) and the asymmetrically distributed lung diseases often encountered in critical care.

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Potential roles of transpulmonary pressure (PTP) and functional residual capacity (FRC) in bedside monitoring of risk of ventilator-induced lung injury (VILI). Negative values of end-expiratory PTP (total positive end-expiratory pressure [PEEPTOT] - end-expiratory esophageal pressure [PesEXP]) suggest collapse in the region of the balloon. Tidal reversal to positive end-inspiratory PTP values (plateau airway pressure [PPLAT] - end-inspiratory esophageal pressure [PesINSP]) suggests potentially damaging tidal opening/collapse cycles. Resting lung volume (FRC) may be used in a 'strain' equation (see text and Table 1) in which end-inspiratory and end-expiratory absolute lung volumes are required, in addition to PEEPTOT and PesEXP. VT: tidal volume, CL: lung compliance.
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Figure 1: Potential roles of transpulmonary pressure (PTP) and functional residual capacity (FRC) in bedside monitoring of risk of ventilator-induced lung injury (VILI). Negative values of end-expiratory PTP (total positive end-expiratory pressure [PEEPTOT] - end-expiratory esophageal pressure [PesEXP]) suggest collapse in the region of the balloon. Tidal reversal to positive end-inspiratory PTP values (plateau airway pressure [PPLAT] - end-inspiratory esophageal pressure [PesINSP]) suggests potentially damaging tidal opening/collapse cycles. Resting lung volume (FRC) may be used in a 'strain' equation (see text and Table 1) in which end-inspiratory and end-expiratory absolute lung volumes are required, in addition to PEEPTOT and PesEXP. VT: tidal volume, CL: lung compliance.

Mentions: Regarding the role of FRC in interpretation of lung mechanics, changes in lung volume could help characterize the nature and severity of lung disease. Resistance and compliance are expressed in absolute terms (cm H2O/l/s and ml/cm H2O, respectively) that vary with aerated lung dimensions. Knowing the FRC value facilitates the separation of restrictive from obstructive disease and allows better interpretation of parenchymal gas exchanging efficiency [42]. Additionally, since 'specific compliance' and 'specific elastance' account for the resting size (volume) of the aerated lung, the response of the respiratory system to an imposed stress may be best evaluated when FRC is known [43]. By determining the size of the 'baby lung', FRC has the potential to elucidate the mechanical stress incurred during tidal breathing and the risk for VILI in the setting of ALI/ARDS [43,44]. We must recognize that FRC values could lead to subject-specific interpretations of lung stress (PTP), and may be integral for assessing lung strain (tidal volume/FRC) - commonly equated with tissue 'stretch' [44]. With current techniques, valid FRC measurements can be obtained to calculate a strain ratio. The latter references the end-tidal volume to its resting level, with strain ratios exceeding 1.5-2.0 signaling concern for lung overstretch [44] (Figure 1).


Two steps forward in bedside monitoring of lung mechanics: transpulmonary pressure and lung volume.

Cortes GA, Marini JJ - Crit Care (2013)

Potential roles of transpulmonary pressure (PTP) and functional residual capacity (FRC) in bedside monitoring of risk of ventilator-induced lung injury (VILI). Negative values of end-expiratory PTP (total positive end-expiratory pressure [PEEPTOT] - end-expiratory esophageal pressure [PesEXP]) suggest collapse in the region of the balloon. Tidal reversal to positive end-inspiratory PTP values (plateau airway pressure [PPLAT] - end-inspiratory esophageal pressure [PesINSP]) suggests potentially damaging tidal opening/collapse cycles. Resting lung volume (FRC) may be used in a 'strain' equation (see text and Table 1) in which end-inspiratory and end-expiratory absolute lung volumes are required, in addition to PEEPTOT and PesEXP. VT: tidal volume, CL: lung compliance.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3672500&req=5

Figure 1: Potential roles of transpulmonary pressure (PTP) and functional residual capacity (FRC) in bedside monitoring of risk of ventilator-induced lung injury (VILI). Negative values of end-expiratory PTP (total positive end-expiratory pressure [PEEPTOT] - end-expiratory esophageal pressure [PesEXP]) suggest collapse in the region of the balloon. Tidal reversal to positive end-inspiratory PTP values (plateau airway pressure [PPLAT] - end-inspiratory esophageal pressure [PesINSP]) suggests potentially damaging tidal opening/collapse cycles. Resting lung volume (FRC) may be used in a 'strain' equation (see text and Table 1) in which end-inspiratory and end-expiratory absolute lung volumes are required, in addition to PEEPTOT and PesEXP. VT: tidal volume, CL: lung compliance.
Mentions: Regarding the role of FRC in interpretation of lung mechanics, changes in lung volume could help characterize the nature and severity of lung disease. Resistance and compliance are expressed in absolute terms (cm H2O/l/s and ml/cm H2O, respectively) that vary with aerated lung dimensions. Knowing the FRC value facilitates the separation of restrictive from obstructive disease and allows better interpretation of parenchymal gas exchanging efficiency [42]. Additionally, since 'specific compliance' and 'specific elastance' account for the resting size (volume) of the aerated lung, the response of the respiratory system to an imposed stress may be best evaluated when FRC is known [43]. By determining the size of the 'baby lung', FRC has the potential to elucidate the mechanical stress incurred during tidal breathing and the risk for VILI in the setting of ALI/ARDS [43,44]. We must recognize that FRC values could lead to subject-specific interpretations of lung stress (PTP), and may be integral for assessing lung strain (tidal volume/FRC) - commonly equated with tissue 'stretch' [44]. With current techniques, valid FRC measurements can be obtained to calculate a strain ratio. The latter references the end-tidal volume to its resting level, with strain ratios exceeding 1.5-2.0 signaling concern for lung overstretch [44] (Figure 1).

View Article: PubMed Central - HTML - PubMed

AUTOMATICALLY GENERATED EXCERPT
Please rate it.

For many decades, pressure-based respiratory mechanics have served to aid the judgment of clinicians when monitoring mechanical ventilation and making important decisions in respiratory care... However, measurements based on airway pressure (PAW) alone have limited ability to generate individualized insights for a diverse patient population with varied pathologic conditions... While the passive lungs are the primary target of attention, PAW-based interpretations may be influenced by differences in breathing pattern, alterations in chest wall activity (including diaphragmatic function), changes in lung volume, asymmetry of lung disease, abdominal distension, etc... Absolute values of Pes are not only influenced by the 'mediastinal artifact' as a result of re-positioning from sitting to supine, but also by elevation of intra-abdominal pressure (IAP) and position-related lung volume changes... Recently, Owens et al. concluded that Pes measurement artifacts imposed by mediastinal weight and postural effects are within a clinically acceptable range... Similarly, improvement in respiratory-system compliance was also observed in the "esophageal pressure-guided group"... However, despite trends toward improved survival, this study does not provide uncontestable data supporting reduction in mortality associated with this mechanical ventilation strategy guided by PTP estimations in patients with ALI/ARDS... Nonetheless, PTP monitoring deserves credit for shifting the attention of clinicians to a more individually-tailored physiologic understanding of the respiratory function changes that occur during ALI and, although not perfect, estimations of PTP are of more help in elucidating the interactions between patient characteristics, disease conditions and ventilator settings than are pulmonary mechanics based on airway pressure alone (Table 1)... One example (Engstrom Care-stationtechnology, GE Healthcare Madison, WI) of this approach directly measures the end-expiratory lung volume by slightly altering the delivered FiO2 level (step changes of only 0.1) for short periods of time using its volumetric O2 and CO2 measurement capability... In a previous study we compared this method with ('gold standard') quantitative computer tomographic (CT) imaging and found that this automated method correlated well (across a wide range of end-expiratory lung volumes)... Studies conducted in lung injury models have investigated the relationship between FRC and tidal compliance... In a porcine oleic-acid-injury study, Rylander et al. found that FRC was a more sensitive indicator of PEEP-induced aeration than was compliance... Clinical experience shows that oxygenation is markedly affected by postural changes in certain patients... These hypoxemic episodes may be the result of position-related ventilation/perfusion changes associated with abrupt reductions in FRC or to regional perfusion changes... Moreover, recognizable patterns and trends in correlated indexes of FRC and PTP, in addition to traditional monitoring tools, could help diagnose and/or provide an early warning to the clinician of impending danger in the settings of chest wall abnormalities (e.g., elevated IAP) and the asymmetrically distributed lung diseases often encountered in critical care.

Show MeSH
Related in: MedlinePlus