The total PneuView system is comprised of precisely engineered mechanical test lung (or lungs), an electronic sensor and signal conditioning package and a host computer. The test lung is designed to mimic compliance and resistive loads representative of the range human pulmonary physiology, including normal and extreme pathologic conditions. The sensor and signal conditioning board includes a set of pressure transducers and a precision time base. Lung and airway pressure are measured continuously and digitized to 12-bit precision at a sampling frequency of 100 Hz. The board includes a serial interface circuit that creates the data stream transmitted using an RS-232 protocol to the host computer (through an available serial, or “COM” port). A block diagram representation of the Sensor/Interface board is shown in Figure 1. The PneuView software package running on the host computer manages and records the data stream, calculates and analyzes a wide variety of breath parameters and presents the results in various ways to the user or saves reports for later retrieval.
PneuView System Sensor/Interface Board
The software package includes approximately 100 modules containing the algorithms that acquire, condition and store the data stream, parse the breath pattern, calculate defined breath parameters, perform analyses of trends, and finally present the myriad results to the user, numerically and/or graphically, either on the screen or in printed form. Two key elements of the system are the calculation module, which are responsible for converting the raw data into real-time data streams representing airway and lung pressures, lung volume and flow rate, and the breath parsing module, which converts these streams into the logical phases of a breath pattern. Once the breath phases are identified, useful parameters such as Breath Rate, Tidal Volume, and Peak Airway Pressure are calculated relative to the appropriate phase of each breath. Both the real time and breath related parameters are made available for presentation or documentation, either as numeric values or in the form of graphs. A separate module is used to analyze the measured parameters over time, allowing the system to quantify and present trends over comparatively long periods of time.
Like many mechanical and gas systems, the physical characteristics of the test lung may be conveniently modeled as a set of mathematical differential equations. The mechanical and gas dynamics of the device are well understood, quantifiable, and very repeatable, thanks to the built in calibration adjustments that are part of its design. A system of second order differential equations, then, is defined for use as a mathematical representation of the physical test lung within the software. The second order model allows the system to account for inertial, damping and elastic properties of the physical system. Inertial effects include the mass of the lung plates and other moving parts; damping effects include shaft friction and hysteresis in the bellows; elastic effects include the compliance and counterbalance springs. The state variables for the equations are lung and airway pressure, and the first and second derivatives with respect too time of each. The coefficients for each equation are calculated from a set of fourth order polynomial correlation functions, one set for each compliance setting. These correlation functions are produced individually for each lung based on data taken at calibration time.
PneuView System Correlation Functions
Figure 3. presents a block diagram of the software modules involved in the calculation of the real time and breath parameters.
PneuView System Breath Parameter Calculation Engine Overview
The raw data comes to the calculation module as measurements of lung pressure, airway pressure and time. From this data stream, algorithms scale and condition the measurements, and then calculate the first and second time derivatives of the pressures. The next step is to calculate physical lung volume by solving the set of differential equations defining the physical test lung. Once physical volume is known, gas law relationships are used to convert volume to the specified reference system (e.g., NTPD). From here, flow rate is calculated and the data set is prepared for breath parsing.