Influence of Ventilator Performance on Assisted Modes of Ventilation
More recent generations of ventilators that are based on state-of-the-art technology tend to be better adapted to the wide range of ventilatory situations usually observed in critically ill patients. They differ from previous generations in many technical
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4.1 Introduction More recent generations of ventilators that are based on state-of-the-art technology tend to be better adapted to the wide range of ventilatory situations usually observed in critically ill patients. They differ from previous generations in many technical aspects, particularly pressurization working principles: e.g., pressurized gas source, piston, turbine, compressor bellows, Venturi system, flow-regulation systems based on microprocessors. These features associated with new software now permit manufacturers to provide attractive modes of ventilation. The rationale claimed for the development of these sophisticated ventilators is to allow a better adaptation of machines to patients' needs. Nevertheless, studies aiming at comparing currently available ventilator performance in vitro as well as in vivo are relatively scarce. In routine practice, respiratory failure under mechanical ventilation is more often attributed to a patient's worsening or to inadequate ventilatory settings, whereas poor technical performance related to the ventilator itself is rarely cited. Recently, several studies have stressed the clinical impact of technical differences regarding either the trigger or the delivered flow. In this chapter we emphasize the importance of ventilator performance in dinical situations where assisted modes are used, such as in the weaning process. We will also review the ways in which ventilators can be compared.
4.2 Physiological Aspects of Patient-Ventilator Interactions Respiratory volume displacement needed for gas exchange, and related to either negative pressure (P mus) generated by the patient during spontaneous breathing or positive pressure (P appl) provided by the ventilator during MV, can be described at each instant by the equation of motion: Pmus + Pappl = (Flow x Rtot)+(Volume x Est) + PEEPi where Rtot represents the resistance and Est the elastance of the respiratory system. In other words, the pressure needed to generate a given flow, and then a volume, has to overcome resistive pressure (Flow x Rtot ) and elastic recoil pressure (Volume x Est), added to the inspiratory threshold load related to intrinsic PEEP (PEEPi). J. Mancebo et al. (eds.), Mechanical Ventilation and Weaning © Springer-Verlag Berlin Heidelberg 2003
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From a clinical point of view, the situation in which the breathing pattern is completely controlled by the ventilator because of sedation and muscle paralysis (i.e., Pmus is abolished) is opposite to the situation where the patient is breathing spontaneously without any ventilatory support (Pappi = 0). Most of the time, the patient is not able to sustain the breathing effort alone, and the total pressure applied to the respiratory system (P tot ) depends on both the negative pressure generated by the patient and the positive pressure provided by the ventilator. Both systems, the ventilator and the patient, should be synchronized as closely as possible in order to improve ventilatory assi
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