A closed loop cardiopulmonary model composed of a four-chamber varying-elastance heart, a pericardium, a systemic circulation, a pulmonary circulation, airways mechanics, baroreceptors, gas exchange, blood gas handling, coronary circulation, peripheral chemoreceptors and selectable interventions.

Description

 The model simulates physiological features such as the arterial blood pressure waveform, 
 end-diastolic left ventricular volume, coronary capillary flow, pleural pressure, and 
 arterial oxygen concentration. 

 The JSim project file also includes a customized interface that allows the user to 
 simulate interventions (injuries, maneuvers, Valsalva maneuver, pericardial removal, 
 penetrating wounds to heart, septal defect, etc.). Interventions not listed in the 
 customized interface can also be simulated by changing certain parameters in the model. 
 For example, the parameter for pulmonary valve resistance (Rpuv) can be increased to 
 simulate pulmonary valve constriction. 

Equations

The equations for this model may be viewed by running the JSim model applet and clicking on the Source tab at the bottom left of JSim's Run Time graphical user interface. The equations are written in JSim's Mathematical Modeling Language (MML). See the Introduction to MML and the MML Reference Manual. Additional documentation for MML can be found by using the search option at the Physiome home page.

Validation

Presently, this model has only been partially validated.

• Unstressed steady-state physiology: The cardiovascular system has been tuned to produce blood pressures, blood volumes, blood flows and heart rate within the limits of normal human physiological variation. Running the model after loading the 'unstressed' parameter set will produce these 'textbook' results. Reference values used to tune the model were compiled from various literature sources.
• Valsalva maneuver: The arterial pressure response to a simulated Valsalva maneuver produces the 4 distinct phases associated with a sudden, sustained increase in pleural pressure (sustained, in this case, for 15 seconds) followed by its quick decrease to 'normal' values. Heart rate shifts in accord with a typical response; however, its maximum level during phase 3 is slightly lower than what has been previously used to validate other models (Lu et al. 2001, Bannister 1980).

• Forced vital capacity maneuver: To simulate a forced vital capacity (FVC) maneuver, experimental pleural pressure data (Liu et al. 1998) is used to drive the airway mechanics system. The parameters of the airway mechanics system were hand tuned so that the decrease in O₂ and increase in CO₂ at the mouth during the long expiratory phase of the maneuver matched experimental data (Liu et al. 1998). Although the general trends for O₂ and CO₂ in this simulation are similar to data, there are differences in the shape of the curves. Further tuning the resistance-volume and pressure-volume relationships in the collapsible and alveolar segments of the airways mechanics model will likely be needed to better match O₂ and CO₂ trends.

• Removal of pericardium: Removal of the pericardium produces qualitatively valid physiological responses in the model: venous pressure falls, arterial pressure rises, and cardiac output increases (Kuno 1915). These responses appear to be slightly lower in magnitude than what has been found by experiments in dogs (Kuno 1915). To increase these responses, the model must be parameterized so that the pericardium generates more pressure for a given enclosed volume during the normal state (the pressure-volume curve of the pericardial compartment must be shifted to the left).

Notes

• Because the system is highly integrated, numerical solutions for variables in this model converge at very low time steps. The highest time step recommended for a precise solution is 0.001 seconds. A higher time step can be used, and the model will run faster and will use less memory; however, there will be noticeable differences between these results and those produced at lower time steps.
• A reduction in contractile strength is represented by a parameter change. One way to do this here is to use the structural injury choices, for example the decrease in cardiac elastance due to the severance of coronary perfusion, e.g. percent lost set to 95% , starting at 4 seconds and running the total time at 30 seconds.
  
  Observe the changes in Elv, Vlvr or EmaxlvINJ and the overall circulatory responses on plotpage for heart chamber pressures.
• Assumptions and limitations.
• Differences betweeen this model and its literature sources.
• This model is informally refered to as VS001 since it was initially created as part of The Virtual Soldier Project.
• Components of this system are based on models already published in the literature. Various people coded these components into MML so they could be integrated together:

  CODE COMPONENT              CODED BY              BASED ON 
   ----------------------------------------------------------------
   systemic circulation        Dan Beard             Lu et al. (2001)
   pulmonary circulation       Dan Beard             Lu et al. (2001)
   baroreceptor                Dan Beard             Lu et al. (2001)
   heart                       Jim Bassingthwaighte  Rideout (1991)
   cardiac activation function Andy Matuszkiewicz    Heldt et al. (2002)
   pericardium                 Maxwell Neal          Sun et al. (1997)
   airways mechanics           Melissa Krueger       Athanasiades et al. (2000)
   gas exchange                Kalyan Vinnakota      Lu et al. (2001)
   physiological changes       Maxwell Neal          Poiseiulle, Reynold's laws
   blood gas handling          Ranjan K. Dash        Woodbury (1974)
   peripheral chemoreceptors   Maxwell Neal          Lu et al. (2002)
                                                    Duffin et al. (2000)
   coronary circulation        Maxwell Neal          Zinemanas et al. (1994)
  

• This model was published in 
  Neal ML and Bassingthwaighte JB; Subject-specific model estimation of cardiac output and blood volume during hemorrhage. Cardiov457.54 KB