Simultaneous monitoring of intratidal compliance and resistance in mechanically ventilated piglets: A feasibility study in two different study groups

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Highlights

  • Intratidal resistance–volume profiles change in a characteristic way depending on PEEP, tidal volume and the underlying mechanical properties of the lung.

  • In healthy lungs, intratidal resistance–volume profiles decline linearly.

  • A curved intratidal resistance decline might indicate pathological changes in the underlying mechanical lung properties.

  • Intratidal resistance–volume profiles might help guiding a lung protective ventilation setting.

Abstract

Compliance measures the force counteracting parenchymal lung distension. In mechanical ventilation, intratidal compliance–volume (C(V))-profiles therefore change depending on PEEP, tidal volume (VT), and underlying mechanical lung properties. Resistance counteracts gas flow through the airways. Due to anatomical linking between parenchyma and airways, intratidal resistance–volume (R(V))-profiles are hypothesised to change in a non-linear way as well. We analysed respiratory system mechanics in fifteen piglets with lavage-induced lung injury and nine healthy piglets ventilated at different PEEP/VT-settings. In healthy lungs, R(V)-profiles remained mostly constant and linear at all PEEP-settings whereas the shape of the C(V)-profiles showed an increase toward a maximum followed by a decrease (small PEEP) or volume-dependent decrease (large PEEP). In the lavage group, a large drop in resistance at small volumes and slow decrease toward larger volumes was found for small PEEP/VT-settings where C(V)-profiles revealed a volume-dependent increase (small PEEP) or a decrease (large PEEP and large VT). R(V)-profiles depend characteristically on PEEP, VT, and possibly whether lungs are healthy or not. Curved R(V)-profiles might indicate pathological changes in the underlying mechanical lung properties and/or might be a sign of derecruitment.

Introduction

The continuous volume change during tidal breathing induces a continuous impedance change with its two main components airflow resistance and volume distensibility i.e. compliance. The key component of airways’ mechanical impedance is resistance (R) counteracting gas flow, the key component of the parenchyma, i.e. alveoli and terminal airways, is compliance (C) counteracting distension. It has been shown that the shape of the intratidal compliance–volume (C(V))-profile indicates indirectly if the lung is ventilated at high or low intrapulmonary volume (Mols et al., 1999, Mols et al., 2006) and could therefore be used to adjust PEEP and tidal volume (VT) (Schumann et al., 2009). There seems to be a complex relationship between airway calibre, respiratory parenchyma mechanics, and PEEP-setting (Babik et al., 2012) and anatomical linking of parenchyma and airways leads to a mutual influence of their biomechanical behaviour (Pare and Mitzner, 2012). Constriction of airways stiffens the parenchyma and could therefore reduce compliance (Mitzner et al., 1992). Any increase in lung volume, by contrast, tethers open adjacent airways, increases their volume until the basal airway membrane limits further volume increment and, hence, decreases flow-related airway resistance. The intratidal configuration and volume of airways and parenchyma change rhythmically. It can therefore be assumed that the intratidal resistance of the respiratory system also changes depending on ventilation setting (Mols et al., 2001) and underlying mechanical lung properties. Analyses of single points within the breathing cycle (inspiratory peak, inspiratory pause or end-expiratory pressure point) provide a representation of the lung under minimal or no-flow conditions (Hickling, 1998, Jonson et al., 1999). By its very nature, describing the continuously changing conditions of the lung during ongoing mechanical ventilation can only be achieved by mapping the intratidal, non-linear course of R and C (Lichtwarck-Aschoff et al., 2000, Stahl et al., 2006). We use our gliding-SLICE-method (Schumann et al., 2009) for a combined investigation of intratidal compliance and resistance.

We hypothesised that both intratidal R(V)- and C(V)-profiles can be simultaneously analysed and that intratidal R(V)-profiles change depending on PEEP, VT and possibly the underlying mechanical lung properties. Simultaneous monitoring of intratidal C(V)- and R(V)-profiles could help to assess lung status at bedside.

Section snippets

Animal data

Two different study groups were investigated under a variety of PEEP/VT-settings. Nine lung-healthy piglets (23–29 kg) were premedicated with tiletamine 2.2 mg kg−1 + zolazepam 6 mg kg−1 (Zoletil, Virbac, Carros, France). After IV induction with ketamine 8 mg kg−1 and morphine 1 mg kg−1, an aesthesia was maintained with IV infusion of ketamine 20 mg kg−1 h−1 and morphine 0.5 mg kg−1 h−1. Animals were paralysed with Pancuronium. A 10 mL kg−1 IV bolus of dextran 60 was given to ensure relative normovolemia at all

Results

In the lung healthy group two animals showed clear signs of intrinsic PEEP. With an initial resistance of almost 70 cmH2O s/L, the data of two animals of the lavage group (one only at PEEP = 0 cmH2O/VT = 16 mL/kg BW) showed clear signs of airway narrowing to almost full blocking, probably due to lavage fluid accidently remaining in the airways. As the validity of these data had to be doubted they were discarded from further analysis.

At similar ventilation settings, the average Horovitz-Index (PaO2/FiO2

Discussion

In this study we investigated two different groups of piglets under a variety of PEEP/VT-settings and were able to simultaneously analyse the intratidal course of volume-dependent compliance and resistance on a breath-by-breath basis. As the main results we found the intratidal resistance–volume profile to depend in a characteristic way on PEEP, VT, and possibly on whether the lungs are healthy or not. Intratidal resistance and compliance profiles might be interdependent especially in injured

Conclusions

Curved intratidal resistance might indicate pathological changes in the underlying mechanical lung properties and/or might be a sign of derecruitment at the set PEEP/VT. Monitoring intratidal resistance in addition to intratidal compliance might help finding a lung-protective ventilation setting.

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgements

This study was supported by a grant of the Bundesministerium für Bildung und Forschung, BMBF (Germany) with the project title WiM-Vent (knowledge and model based mechanical ventilation, grant number 01|B10002C).

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