Automatic Ventilation Control

Introduction
This article describes the design of a portable automatic ventilation control system that also processes arterial oxygen saturation (SpO2) data from a pulse oximeter. The system, built around Freescale’s 8-bit MC9S08QG4 microcontroller (MCU), is designed to be a portable, automatic replacement for manual pulse oxygen and ventilation monitoring and control, requiring less human interaction during surgery. It can be particularly useful during surgery on infants. Overview All tissues are fed by oxygen transported through the blood vessels, which is needed to grow the tissue and repair damage. Normally, the body regulates a constant oxygen concentration in extra-cellular fluids, relying primarily on the chemical characteristics of the hemoglobin in red blood cells. For most surgeries, however, the doctors must introduce a certain amount of anesthesia, which also acts as a muscle inhibitor. This automatically reduces the patient’s breathing, and when combined with the blood loss during surgery, can result in hypoxia, or lack of sufficient oxygen in the patient’s body. That’s why the doctors must use an anesthetic ventilator and a meter to measure oxygen saturation in the blood (pulse oximeter) to regulate the anesthesia-oxygen mixture applied during surgery. With mechanical ventilation, an external device is connected directly to the patient and artificially exchanges respiratory gases. Mechanical ventilation is designed to maintain an adequate exchange of gases, even through diminished breathing rates and reduced myocardial use. However, it could also be used to provide adequate lung expansion with reduced respiratory effort, the correct combination of anesthetic sedation and musclerelaxing related drugs and to stabilize the thoracic wall. Ventilators can be divided into five classifications, depending on how the inhaling process is terminated: • Pressure-cycled ventilator: Inspiration of gas is stopped when a designated pressure is achieved. • Volume-cycled ventilator: Inspiration ends when the desired volume of gas has been introduced.


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