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A new methodology for intra-breath control of mechanical ventilation
Abstract
Recent improvements in digital control strategies are driving the implementation of innovative pulmonary ventilation modalities able to support the physician in the intensive care through a better management of the patient's respiratory pathologies. New control strategies, such as intra-breath ones, which guarantee successful patient-machine synchronization and dynamic adjustment to the patient, still require improvements in the control of the breathing process. Such improvements can be achieved by decreasing the response time of the actuation system driving the mechanical ventilators. This paper presents a new methodology of intra-breath control to generate traditional and high frequency ventilation techniques. The methodology is implemented through a custom-developed electro-mechanical system, which consists of on/off solenoid valves driven by a fast switching driver circuit. In vitro experimental trials show the system's ability to generate almost continuous flow-rate patterns in different shapes, a time resolution down to 20 ms, flow-rate resolution of 1L/min, and repeatability of 0.5L/min. Experimental results show that the proposed electro-mechanical system can be used as a stand-alone hardware solution for both intra-breath control techniques and high frequency ventilation.
Background and objective:
Fraction of Inspired Oxygen is one of the arbitrary set ventilator parameters which has critical influence on the concentration of blood oxygen. Normally mechanical ventilators providing respiratory assistance are tuned manually to supply required inspired oxygen to keep the oxygen saturation at the desired level. Maintaining oxygen saturation in the desired limit is so vital since excess supply of inspired oxygen leads to hypercapnia and respiratory acidosis which lead to increased risk in cell damage and death. On the other side a sudden drop in oxygen saturation will lead to severe cardiac arrest and seizure. Hence intelligent real time control of blood oxygen level saturation is highly significant for patients in intensive care units.
Methods:
This paper gives statistical pair wise analysis for finding out deeply correlated physiological parameters from clinical data for fixing fuzzy variables. An advisory fuzzy controller using Mamdani model is developed with R programming to predict FiO2 which is to be delivered from the ventilator to maintain SaO2 with in required levels.
Results:
Fuzzy variables for the fuzzy model is fixed using 75% of the clinical data collected. Remaining 25% of the data is used for checking the system. Compared the predictive output of the system with physicians' decisions and found to be accurate with less than five percentage error.
Conclusions:
Based on the comparison the system is proved to be effective and can be used as assist mode for physicians for effective decision making.
New features of mechanical ventilators are frequently introduced, including new modes, monitoring techniques, and triggering techniques. But new rarely translates into any measureable improvement in outcome. We describe 4 new techniques and attempt to define what is a new invention versus what is innovative-a technique that significantly improves a measurable variable. We describe and review the literature on automated weaning, automated measurement of functional residual capacity, neural triggering, and novel displays of respiratory mechanics.
Despite a shift to noninvasive respiratory support, mechanical ventilation remains an essential tool in the care of critically ill neonates. The availability of a variety of technologically advanced devices with a host of available modes and confusing terminology presents a daunting challenge to the practicing neonatologist. Many of the available modes have not been adequately evaluated in newborn infants and there is paucity of information on the relative merits of those modes that have been studied. This review examines the special challenges of ventilating the extremely low birth weight infants that now constitute an increasing proportion of ventilated infants, attempts to provide a simple functional classification of ventilator modes and addresses the key aspects of synchronized ventilation modes. The rationale for volume-targeted ventilation is presented, the available modes are described and the importance of the open-lung strategy is emphasized. The available literature on volume-targeted modalities is reviewed in detail and general recommendations for their clinical application are provided. Volume guarantee has been studied most extensively and shown to reduce excessively large tidal volumes, decrease incidence of inadvertent hyperventilation, reduce duration of mechanical ventilation and reduce pro-inflammatory cytokines. It remains to be seen whether the demonstrated short-term benefits translate into significant reduction in chronic lung disease. Avoidance of mechanical ventilation by means of early continuous positive airway pressure with or without surfactant administration may still be the most effective way to reduce the risk of lung injury. For babies who do require mechanical ventilation, the combination of volume-targeted ventilation, combined with the open-lung strategy appears to offer the best chance of reducing the risk of bronchopulmonary dysplasia.
Studies have found that increasing the respiratory frequency during mechanical ventilation does not always improve alveolar minute ventilation and may cause air-trapping.
To investigate the theoretical and practical basis of higher-than-normal ventilation frequencies.
We used an interactive mathematical model of ventilator output during pressure-control ventilation to predict the frequency at which alveolar ventilation is maximized with the lowest tidal volume (V(T)) for a given pressure. We then tested our predicted optimum frequencies and V(T) values with various lung compliances and higher-than-normal frequencies, with a lung simulator and 5 mechanical ventilators (Dräger Evita XL, Hamilton Galileo, Puritan Bennett 840, Siemens Servo 300 and Servo-i).
Compliances between 10 mL/cm H(2)O and 42 mL/cm H(2)O yielded V(T) between 4.1 mL/kg (optimum frequency 75 cycles/min) and 6.0 mL/kg (optimum frequency 27 cycles/min). The intrinsic positive end-expiratory pressure at the optimum frequency was always less than 2 cm H(2)O. All the ventilators except the Hamilton Galileo had an optimum frequency near 50 cycles/min, whereas the predicted optimum frequency was 60 cycles/min.
With these ventilators and pressure-control ventilation, alveolar minute ventilation can be optimized with higher-than-normal frequency and lower V(T) than is commonly used in patients with acute respiratory distress syndrome. We call this strategy mid-frequency ventilation.
Objective:
To theoretically assess the viability of an automatic procedure to support the anesthesiologist in properly setting mechanical ventilators when the operating conditions are switched from volume controlled to pressure controlled ventilation whilst maintaining the preset tidal volume. The procedure is based on a simple linear model of the ventilator breathing system with constant parameters and utilizes the signals gathered by the ventilator without the need to add further equipment. After a short period of stable volume controlled ventilation with the desired tidal volume, the herewith described algorithm allows the calculation of the value of pressure limit to set in pressure controlled mode which assures the previously settled tidal volume with the same breathing frequency and inspiratory-expiratory time ratio.
Methods:
The algorithm allows the online identification of the four parameters necessary for the mathematical model that are obtained by means of a direct comparison between the pressure, flow and volume waveforms generated by the model and the analog signals provided by the ventilator. The theoretical approach was validated by two different ventilators, various settings, two breathing circuits, endotracheal tubes of various sizes and two mechanical simulators of the respiratory system operating in various conditions.
Results:
Errors usually less than 5% (p < 0.05) on the target tidal volume were obtained for various settings typically used for adult ventilation in less than 10 s. The theoretical approach shows its limitations (errors of 10+/- 5%, p < 0.05) at high breathing frequencies (30-40 bpm) and low tidal volumes (200-300 ml).
Conclusions:
The proposed theoretical approach shows the viability, for adult settings, of one of the simplest mathematical model for mechanical ventilation in order to quickly and safely switch from volume controlled to pressure controlled ventilation. The algorithm could easily be in perspective implemented in the software of the ventilator providing the anesthesiologist with an indication on the value of pressure limit to set in order to safely switch ventilation mode.
Pneumatic driving systems are mainly used in industrial applications where the moving parts are usually fixed by the mechanical stops. For flexible and precise positioning tasks of pneumatic drives relatively expensive proportional valves have been implemented. In order to develop cheaper pneumatic servo systems, the employment of low-cost on/off solenoid valves have received considerable attention. But, the traditional technologies used in on/off solenoid valves manifest different problems caused by the electric and thermal effects, inertial forces of their mechanical parts and friction phenomenon. However, it seems that innovative technology used in fast switching solenoid valves gives better possibilities in control of pneumatic systems due to their negligible internal friction and modular architecture. In this paper a control method based on pulse-width-modulation algorithm for position control of a pneumatic actuator with high-speed solenoid valve is considered and experimentally verified.
Conventional modes of mechanical ventilation often cause patient-ventilatory asynchrony and may fail to assure adequate gas exchange while attempting to adhere to lung protective ventilatory strategies. Airway pressure release ventilation, partial liquid ventilation, and high-frequency ventilation are alternative ventilatory strategies used for acute respiratory distress syndrome that aim to protect the lung while providing adequate gas exchange. The closed-loop ventilatory strategies use computer algorithm and feedback monitoring of respiratory system mechanics to improve patient-ventilator interaction and comfort.
Proportional strategies for artificial ventilation are the most recent form of synchronized partial ventilatory assistance and intra-breath control techniques available in clinical practice. Currently, the majority of commercial ventilators allowing proportional ventilation uses proportional valves to generate the flow rate pattern. This paper proposes on-off solenoid valves for proportional ventilation given their small size, low cost and short switching time, useful for supplying high frequency ventilation. A new system based on a novel fast switching driver circuit combined with on/off solenoid valve is developed. The average short response time typical of onoff solenoid valves was further reduced through the driving circuit for the implementation of PWM control. Experimental trials were conducted for identifying the dynamic response of the PWM driven on/off valve and for verifying its effectiveness in generating variable-shaped ventilatory flow rate patterns. The system was able to smoothly follow the reference flow rate patterns also changing in time intervals as short as 20 ms, achieving a flow rate resolution up to 1 L/min and repeatability in the order of 0.5 L/min. Preliminary results showed the feasibility of developing a stand alone portable device able to generate both proportional and high frequency ventilation by only using on-off solenoid valves.
New modes of mechanical ventilation with advanced closed loops are now available, and in the future these could assume a greater role in supporting critically ill patients in intensive care units (ICUs) for several reasons. Two modes of ventilation--proportional assist ventilation and neurally adjusted ventilatory assist--deliver assisted ventilation proportional to the patient's effort, improving patient-ventilator synchrony. Also, a few systems that automate the medical reasoning with advanced closed-loops, such as SmartCare and adaptive support ventilation, have the potential to improve knowledge transfer by continuously implementing automated protocols. Moreover, they may improve patient-ventilator interactions and outcomes, and provide a partial solution to the forecast clinician shortages by reducing ICU-related costs, time spent on mechanical ventilation, and staff workload. Preliminary studies are promising, and initial systems are currently being refined with increasing clinical experience. A new era of mechanical ventilation should emerge with these systems.
The major automatic techniques that are available in commercial ventilators are described and a discussion of the recently developed systems along with the future trends in the field is provided.
The major available automatic control techniques for mechanical ventilation are analyzed and the future trends are discussed in view of today's ICU requirements and the recently developed technologies.
Several major automatic techniques are available in commercial ventilators at this time. Those techniques have been in use successfully and are accepted by clinicians. At the same time, more advanced techniques have been and continue to be developed by various researchers that are designed for more aggressive use of automation in control of ventilation and oxygenation in different phases of ventilatory treatment.
Automatic control of mechanical ventilation can significantly improve patient care in the ICUs, reduce the mortality and morbidity rates associated with provision of inappropriate ventilatory treatments, and reduce healthcare costs. Development of more effective and robust systems that can have high noise immunity and provide effective treatment to patients automatically in different phases of treatment will likely gain increasing momentum in the years to come.
In this article, automatic control technology as applied to mechanical ventilation is discussed and the techniques that have been reported in the literature are reviewed.
The information in the literature is reviewed and various techniques are compared.
Automatic control has been applied in many ways to mechanical ventilation since several decades ago. More aggressive techniques aimed at automatic and more optimal control of the main outputs of the machine have emerged and continue to be enhanced with time.
Development of more efficient automatic techniques and/or enhancement of the present methods are likely to be pursued to make this technology more compatible with future healthcare requirements.
Portable ventilators (PVs) are used for patient transport with increasingly frequency. Due to design differences it would not be unexpected to find differences among these ventilators in the imposed work of breathing (WOBI) during spontaneous respiratory efforts. The purpose of this investigation was to compare the WOBI characteristics during spontaneous breathing of seven PVs; Bird Avian, Bio-Med Crossvent 4, Pulmonetics LTV 1000, Hamilton Max, Drägerwerk Oxylog 2000, Impact Uni-Vent 750, and Impact Uni-Vent 754 using a model of spontaneous breathing. Differences between the PVs in regards to the measured parameters increased with increases in simulated breathing demand. WOBI, peak inspiratory pressure, and pressure-time product were consistently less with the LTV 1000 over the range of simulated breathing conditions. During pressure support ventilation these parameters were significantly less with the LTV 1000 compared with the Crossvent 4. Only the WOBI produced by the LTV was consistently lower than the physiologic work of breathing across the simulated spontaneous breathing conditions. Based on these results it is predicted PVs with flow triggering and positive end-expiratory pressure compensation will consistently offer the least WOBI. Clinicians should be aware of these characteristics when using PVs with spontaneous breathing patients.
A new proportional assist ventilation (PAV) method using a proportional solenoid valve (PSV) to control air supply to patients suffering from respiratory disabilities, was studied. The outlet flow and pressure from the proportional solenoid valve at various air supply pressures were tested and proven to be suitable for pressure and flow control in a PAV system. In vitro tests using a breathing simulator, which has been proven to possess the general characteristics of human respiratory system in spontaneous breathing tests, were conducted and the results demonstrated the viability of this PAV system in normalizing the breathing patterns of patients with abnormally high resistances and elastances as well as neuromuscular weaknesses. With a back-up safety mechanism incorporated in the control program, pressure "run-away" can be effectively prevented and safe operation of the system can be guaranteed.
Recognition that volume, not pressure, is the key factor in ventilator-induced lung injury and the association of hypocarbia and brain injury dictate the need to better control delivered tidal volume. Volume-controlled ventilation, though much improved, still suffers from loss of volume due to endotracheal tube leak and gas compression in the circuit. Recent microprocessor-based modifications of pressure-limited, time-cycled ventilators combine advantages of pressure-limited ventilation with the ability to deliver a more consistent tidal volume. Each of the modes has advantages and disadvantages, with limited data available to judge their effectiveness. The Volume Guarantee mode, studied most thoroughly, provides automatic weaning of peak pressure in response to improving lung compliance and respiratory effort. More consistent tidal volume, fewer excessively large breaths, lower peak pressure, less hypocarbia and lower levels of inflammatory cytokines have been documented. It remains to be seen if these short-term benefits translate into shorter duration of ventilation or reduced incidence of chronic lung disease.
Understanding the regulation of breathing in the critical care patient is multifaceted, especially in ventilator-dependent patients who must interact with artificial respiration. Mechanical ventilation originally consisted of simple, manually-driven pump devices, but it has developed into advanced positive pressure ventilators for continuous support of patients in respiratory failure. This evolution has resulted in mechanical ventilators that deliver assist intermittently, attempting to mimic natural breathing. Recently, modes of mechanical ventilation that synchronize not only the timing, but also the level of assist to the patient's own effort, have been introduced. This article describes the concepts related to proportional assist ventilation and neurally adjusted ventilatory assist, and how they relate to conventional modes in terms of patient-ventilator synchrony.
The development of a fast, accurate, and inexpensive
position-controlled pneumatic actuator that may be applied to a variety
of practical positioning applications is described. A novel pulse width
modulation (PWM) valve pulsing algorithm allows on/off solenoid valves
to be used in place of costly servo valves. The open-loop characteristic
is shown both theoretically and experimentally to be near symmetrical. A
comparison of the open- and closed-loop responses of standard PWM
techniques and that of the novel PWM technique shows that there has been
a significant improvement in the control. A linear process model is
obtained from experimental data using system identification. A
proportional integral derivative controller with added friction
compensation and position feedforward has been successfully implemented.
A worst case steady-state accuracy of 0.21 mm was achieved with a rise
time of 180 ms for step inputs from 0.11 to 64 mm. Following errors to
64-mm S-curve profiles were less than 2.0 mm. The controller is robust
to a sixfold increase in the system mass. The actuator's overall
performance is comparable to that achieved by other researchers using
servo valves
Midfrequency ventilation: unconventional use of conventional mechanical ventilation as a lung-protection strategy
- Mireles-Cabodevilla