New insights from pressure-volume loops
Lung function measurements with the flexiVent frequently include Pressure-Volume (PV) loops for insight on the intrinsic elastic properties of the respiratory system. The typical maneuver slowly inflates the lungs in steps, from Positive-End Expiratory Pressure (PEEP; 3 cmH2O) to 30 cmH2O, and then deflate them in a similar way to generate a partial PV loop (see Figure 1). The deflation arm is used to derive outcomes, such as static compliance (Cst), and the shape of the curves is known to change in characteristic ways with different disease states. For instance, the curve becomes right-shifted with fibrosis relative to a control condition, and left-shifted in presence of an emphysematous phenotype (see Figure 1). Partial PV loops from the flexiVent are frequently featured in scientific publications. Recent examples include the characterization of novel genetic models1, the effect of exercise during aging2 or the impact on the function of damages occurring during lung development3.
PV loops are rich in information and the depth of it greatly expands when the range over which the PV relationship is assessed spans across the entire volume of the lungs. In this maneuver, known as a full-range PV curve (see Figure 2), the inflation of the lungs starts from complete collapse and thus allows the determination of the subject’s total lung capacity (TLC). Upon deflation, information on the compliance of the respiratory system (C) will be obtained, in addition to the Residual Volume (RV)4, 5. The full-range PV curve has now integrated the flexiVent platform6 and its automation is not only facilitating the measurement, but it is also contributing to deepening the insight available to researchers with additional outcomes like the Functional Residual Capacity (FRC) or Vital Capacity (VC), among others. The automated version of the technique, referred to as Lung Volumes within the flexiWare software, is proving to be helpful as it has already found a valued place in disease model characterization7 and exposure studies8.
In their most recent work, Robichaud et al. (2020)9 demonstrate the utility of analyzing not only the deflation part of partial or full-range PV loops but also that of the inflation arm. In a full-range PV curve, this latter portion could allow the determination of airway compliance (Caw) as well as the pressure required to re-open the lungs following complete collapse (Pop) (see Figure 2). Similarly, the analysis of the inflation limb of a partial PV loop could provide the work of breathing through the calculation of the area under the inflation curve.
1Jansing et al. 2020. miR-21-KO Alleviates Alveolar Structural Remodeling and Inflammatory Signaling in Acute Lung Injury. International Journal of Molecular Sciences; 21:822. DOI: 10.3390/ijms21030822
2Veldhuizen et al. 2019. The effects of aging and exercise on lung mechanics, surfactant and alveolar macrophages. Experimental Lung Research; 45:(5-6): 113-122. DOI: 10.1080/01902148.2019.1605633
3Gremlich et al. 2020. Tenascin-C inactivation impacts lung structure and function beyond lung development. Scientific reports; 10(1): 1-3. DOI: 10.1038/s41598-020-61919-x
4Soutiere et al. 2004. Differences in alveolar size in inbred mouse strains. Respiratory Physiology & Neurobiology;140(3): 283-291. DOI: 10.1016/j.resp.2004.02.003
5Limjunyawong et al. 2015. Measurement of the pressure-volume curve in mouse lungs. Journal of Visualized Experiments; (95): e52376. DOI: 10.3791/52376
6Robichaud et al. 2017. Automated full-range pressure-volume curves in mice and rats. Journal of Appliedl Physiology; 123(4): 746-756. DOI: 10.1152/japplphysiol.00856.2016
7Dimori et al. 2020. Respiratory defects in the CrtapKO mouse model of Osteogenesis Imperfecta. Lung Cellular and Molecular Physiology; 318(4): L592-L605. DOI: 10.1152/ajplung.00313.2019
8Caballero et al. 2019. Immune modulation by chronic exposure to waterpipe smoke and immediate-early
Gene regulation in murine lungs. Tobacco Control; 29(2): s80-s89. DOI: 10.1136/tobaccocontrol-2019-054965
9Robichaud et al. 2020. Insights from the inflation limb of pressure-volume curves. Am J Respir Crit Care Med 201: A3003.