4. Resultados y discusión
4.1 Efectos de la melatonina sobre sobre la funcionalidad de espermatozoides incubados
4.1.2. Efectos sobre la motilidad espermática:
Mechanical ventilation is instituted mainly to improve gas exchange and to decrease respiratory muscle workload. The clinical response to this lifesaving treatment in terms of gas exchange is usually evaluated by means of inter- mittent arterial blood-gas measurements, continuous pulse oximetry monitoring, and, less often, monitoring end-tidal CO 2 . These measurements provide an objective way to titrate therapy. Although gas exchange is the main function of the lungs, the respiratory system also has a muscular pump that is central to its main purposes. The way we evaluate the function of the respiratory muscles clinically during the course of ACV and patient–ventilator interactions is rudimentary. Knowing how much effort a particular patient is making and how much unloading is to be provided is very difficult to ascertain on clinical grounds. Too much or too low respiratory muscle effort may induce muscle dysfunction, and this eventually could delay ventilator withdrawal.
When ACV is first initiated, the ventilator usually over- comes the total breathing workload. How long the period of respiratory muscle inactivity is to be maintained is unknown. When ACV is triggered by the patient, multiple factors interplay between the patient and the ventilator. Although high levels of assistance decrease the sensation of dyspnea, they also increase the likelihood of wasted inspira- tory efforts (see Fig. 6-7 ). How ACV is adjusted, in particular concerning inspiratory flow rate and tidal volume settings, is a major determinant of its physiologic effects. If the settings are selected inappropriately, these may lead the physician to erroneously interpret that the problem lies with the patient and perhaps administer a sedative agent when, in reality, the patient is simply reacting against improper adjustment of the machine.
When patients are receiving ACV, they are at risk of under- going periods of underassistance alternating with periods of overassistance. This is so because of the varying ventilatory demands (see Fig. 6-5 ) and because the mechanical char- acteristics of the respiratory system also change over time. The frequency of such phenomena and their clinical conse- quences are unknown. The effects of permanent monoto- nous tidal volume delivery, as well as whether or not sighs are to be used in this setting, also remain to be elucidated.
The only way to interpret clinically whether the patient is doing well or not during ACV is to evaluate respiratory rate and the airflow and airway pressure trajectories over time. During patient-triggered ACV, muscle effort can be estimated by superimposing the current and the passive airway pressure trajectories. Airway occlusion pressure is an important component of the airway pressure trajectory during patient-triggered breaths. This variable is a good
estimate of the central respiratory drive and is highly cor- related with the inspiratory muscle effort. Such measure- ments would allow clinicians to analyze trends and estimate patient–ventilator interactions objectively. It is surprising that such sound noninvasive monitoring possibilities have yet to be widely implemented, and it is ironic to realize how many new ventilator modes are introduced without having passed rigorous physiologic and clinical evaluations.
SUMMARY AND CONCLUSION
The most widely used ventilator mode in mechanically ventilated patients continues to be ACV. Many of its physi- ologic effects are well characterized, and it is conceivable that, in the main, its purposes are met. ACV is also very versatile because it offers ventilator support throughout the entire period of mechanical ventilation. As with any other mode, the effects depend on the way ACV is imple- mented. The necessity to impose a number of fixed settings, in essence, tidal volume and inspiratory flow rate, implies that the respiratory pump may be unloaded suboptimally and contraction of the respiratory muscles may asynchro- nous with the ventilator. The clinical consequences of these phenomena are not negligible.
Since its introduction, ACV implementation has under- gone considerable changes, and it is presently applied less aggressively than in the past. Thanks to an enormous amount of physiologic and clinically oriented research, we have learned that ACV can be harmful to patients, injuring both the lungs and the respiratory muscles. Future research should help us to deliver ACV in such a manner that a patient’s clinical needs are served more optimally.
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