# Simulating Lung Mechanics in Lung Simulators

Lung mechanics are typically described as multi-compartment models with mechanical elements such as springs and dash-pots (friction) along with other, electrical elements such as resistors and capacitors. Using these characterizing elements, lung mechanics is usually described in lumped parameter models, and it is never described in its anatomical complexity. The equation of motion describes the interdependence of the variables, which include pressure, flow, and volume, and the effects of the parameters compliance and resistance, as illustrated in Figure 1.

Figure 1. Elements constituting the equation of motion; “const” is an arbitrary value. Image Credit: neosim AG

Compliance is measured in volume per pressure and can be either constant or sigmoid. Lung compliance (CL) and chest wall compliance (CW) are arranged in series. Together, they form the respiratory compliance, written Crs.

1/Crs = 1/ CW + 1/ CL

Resistance (Raw) is the pressure drop per unit of flow. Although it is regularly assumed to be constant, in reality, it is never constant. Pleural pressure refers to pressure in the pleural cavity, and although it is variable along the gravitational axis, only one pressure is typically taken to represent a mean pleural pressure.

Figure 2. Pressure-volume curve for static conditions. For explanations, read on. Image Credit: neosim AG

Pressure-equilibrium can be achieved in the lungs at a range of different volumes depending on several factors, from muscle tension and lung elasticity, to fluid status, and more. Figure 2 demonstrates the pressure-volume curve with the equilibrium position at 2000 ml (zero PEEP i.e. [email protected], open airways, no muscle tension, zero airway pressure). This [email protected], also termed minimal FRC (FRCmin) is a parameter utilized in TestChest®. Users are able to model lung collapse by setting FRCmin lower than FRC predicted (FRCpred).

Figure 2 illustrates the ideal-typical sigmoid pressure-volume curve, beginning with lung volume that is equal to FRC at zero PEEP. Compliance is shown in the slope at each point of the pressure-volume curve, and as follows:

• Crs : compliance of the respiratory system, between lower and upper inflection point
• C1: compliance below the lower inflection point
• C3: compliance above the upper inflection point, also called Cend
• LIP: lower inflection point, first point of maximal curvature on the pressure volume curve
• UIP: upper inflection point, second point of maximal curvature on the pressure volume curve
• [email protected], also termed minimal FRC (FRCmin), is lung volume at atmospheric pressure (or 0 PEEP)
• Pthreshold: recruitment threshold
• Pcollapse: collapse threshold
• RClh: time constant of lung-heart interaction
• RCrecol: time constant of lung recruitment and collapse

Lung collapse: in supine position, a small portion of the lungs will collapse temporarily. As a result of this collapse, the lung volume will be temporarily reduced. Typically, an inherent vascular reflex will close the collapsed areas and re-distribute blood to well perfused areas (known as hypoxic vasoconstriction). But, when this reflex is not triggered, the collapsed areas of the lung will continue to be perfused, but only with blood that has not been oxygenated.

Lung collapse and recruitment: collapsed lungs can sometimes be recruited by increasing the pressure above a certain threshold. In Figure 4, this threshold is called Pthreshold. If the pressure in the TestChest® bellows (alveolar pressure) increases above that threshold, the apparent compliance of the lung increases to Cr (Compliance of recruitment) and the lung volume increases. If alveolar pressure is lower than Pcollapse, then the lung volume is reduced at the rate determined by the RCcollapse.

Effect of recruitment on Crs: Crs is increased autonomously when lungs are recruited and decreased when lungs are collapsing, if the Pthreshold is exceeded, for instance, if the lung volume is being recruited.

The calculation mechanics are described below.

Enter the following parameters to define your patient case:

• Crs, is called Crsentered below
• Predicted FRC: FRCpred
• Lung volume at ambient pressure: FRCmin
• Pthreshold: recruitment threshold
• Pcollapse: collapse threshold
• RClh: time constant of lung-heart interaction
• RCrecol: time constant of lung recruitment
• RCcol: time constant of collapse

The actually effective compliance Crsactual is calculated as follows:

Crsactual = Crsexpected/ FRCpred*( FRCmin+Vrecruited)

with Crsexpected = Crsentered/ FRCmin* FRCpred

the equation becomes

Crsactual = Crsentered/ FRCmin* ( FRCmin+Vrecruited)

with Vrecruited being the actually recruited lung volume, gained by increasing the inspiratory pressure over the entered threshold (Pthreshold) for a sufficient amount of time. The  “sufficient amount of time” is defined by the “time constant recruitment”.

The maximal value of Crsactual is Crsexpected.

It is important to note that Crs remains unchanged if the minimal recruitment pressure is set high enough that it is never reached.

For example, when no lung volume is recruited, Crsactual is Crsentered.

If the lungs are recruitable, Crs will rise up to Crsexpected, as demonstrated in the formula above.

For example, if a 50 percent collapse is assumed (2*FRCmin = FRCpred) and Crs is entered as 30 ml/mbar, then Crsactual will vary between 30 ml/mbar and 60 ml/mbar. This will depend on the amount of recruited volume.

With the above parameters in the hands of an experienced trainer, TestChest provides unlimited combinations of respiratory mechanics and makes it a versatile device for respiratory care training.

## Acknowledgment

TestChest is a registered Trade Mark of Organis-Gmbh, Landquart, Switzerland

neosim is a Swiss company founded by experts with strong background in lung physiology and mechanical ventilation of intensive care patients. The mission of neosim is to bring high-fidelity physiology and pathophysiology to the patient simulator community.

For training and education of clinicians, especially respiratory therapists and intensive care professionals, neosim simulators create realistic breathing in health and disease. In contrast to other simulators, neosim’s simulators can be treated with intensive care therapy methods and responds like a real human patient. The result manifests itself clinically and can be measured quantitatively with state-of-the-art monitoring in real-time.

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Last updated: Jul 9, 2020 at 3:45 AM

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