A closed-loop system for control of the fraction of inspired oxygen and the positive end-expiratory pressure in mechanical ventilation

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Abstract

A system for automatic control of the fraction of inspired oxygen (FIO2), and positive end-expiratory pressure (PEEP) for patients on mechanical ventilation is presented. In this system, FIO2 is controlled by using two interacting mechanisms; a fine control mechanism and a fast stepwise procedure used when patient's oxygen saturation level (SpO2) falls abruptly. The PEEP level is controlled automatically and in relation to FIO2 to prevent hypoxemia. The system has been tested by using bench studies and computer simulations. The results show the potential of the system as an aide in effective oxygenation of patients on mechanical ventilation.

Introduction

Supplemental oxygen treatment is used in mechanical ventilation to prevent hypoxemia. In this process, in addition to adjustment of the fraction of inspired oxygen (FIO2), the level of the positive end-expiratory pressure (PEEP) is also adjusted to prevent collapse of the lungs and to provide additional lung volume at the end of expiration for better gas exchange and oxygenation. In most ventilator systems, FIO2 and PEEP are manually set and adjusted by the intensivists. However, this manual control can be a challenging task for many medical personnel if the patient's oxygenation requirements change significantly with time. Control of patient's oxygen level needs to be done in an effective manner to prevent low blood oxygen levels and the serious untoward effects of hypoxemia. At the same time, allowing patient's oxygen level to rise too high or subjecting him to prolonged high level oxygenation can cause many undesired consequences including collapse of the lungs, oxygen toxicity, and in the case of infants, can lead to blindness [1]. Also, control of PEEP needs to be done in coordination with changes in FIO2, and by avoiding the untoward effects of high PEEP levels on the cardiac output and barotraumas [2], [3]. Therefore, automatic and effective control of FIO2 and PEEP can be a valuable technique in assisting the intensivists to treat patients on mechanical ventilation.

Many closed-loop systems for control of various ventilatory parameters have been presented to date that use different targeting schemes and methodologies [4]. A number of those systems have been designed for automatic control of patients' oxygenation [5]. Among the oxygenation controllers, many have been designed to control FIO2 e.g., [6], [7], [8], [9], [10], [11], [12], some have focused on automatic control of PEEP e.g., [13], [14], and some are designed to control both FIO2 and PEEP e.g., [15]. However, in most closed-loop systems that have been presented to date, the control of FIO2 is not robust enough to respond to rapid disturbances of oxygen balance of the patient, or control of FIO2 and PEEP are not combined, or look up tables are used to control the two parameters while a change in one is bound to affect the required level of the other based on the patient's individual response to treatment.

The purpose of this paper is to introduce and describe a system for automatic control of FIO2 and PEEP that can be robust and effective in responding to rapid disturbances in oxygen balance of patients. This system is designed to automatically adjust FIO2 and PEEP in relation to each other and by considering the response of the patient to previous changes in these parameters. The system is designed to prevent hypoxemia as well as hyperoxemia by adjusting FIO2 and PEEP in a robust and safe manner.

Section snippets

System description

A block diagram of the control system is shown in Fig. 1. The arterial oxygen saturation of the patient is measured by a pulse oximeter in this system. This data (SpO2) is applied to a microprocessor through an analog to digital converter (A/D). The microprocessor also receives a signal representing the level of PEEP or CPAP from the ventilator. The control algorithm of the microprocessor determines the next levels of FIO2 and PEEP (or CPAP). Based on these determinations two signals are

Results

In the computer simulation tests, the incremental step change in PEEP was at a conservative level of ±1 cm H2O, and the minimum time required for changing the PEEP level was 5 min to allow for the effects of changing PEEP to appear in blood gases. The prescribed allowed range for the PEEP/FIO2 ratio was 12–24 and the cycle time was 0.75 s.

Fig. 6, Fig. 7, Fig. 8 show three different sets of simulation results.

Fig. 6A–C shows a set of simulation results for a 72 Kg patient with an alveolar–arterial

Discussion

In Fig. 6A–C, an initial period of 30 s of no oxygen breathing is imposed along with hypoventilation to induce acute hypoxemia. As shown in Fig. 6A, the PaO2 level falls sharply at the beginning due to lack of oxygen in the inspired gas. This prompts a stepwise response of the FIO2 controller as soon as the initial 30 s of no oxygen breathing is ended and the controller intervenes. With the intervention of the controller, PaO2 rises to about 132 mm Hg in the next 80 s. Then, the PaO2 level falls

Summary

A new system for automatic control of the fraction of inspired oxygen (FIO2), and the level of positive end-expiratory pressure (PEEP) for patients on mechanical ventilation, or alternatively, for control of FIO2 and the level of continuous positive airway pressure (CPAP) for patients receiving non-invasive mechanical ventilation is presented. In this system, FIO2 is controlled by using two interacting mechanisms. There is a fast stepwise procedure that is used when patient's oxygen saturation

Conflict of interest statement

No conflict of interest.

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