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This Essentials of Mechanical Ventilation, 3rd Edition is edited by Dean R. Hess and Robert M. [PDF] Essentials of Mechanical Ventilation 3E eBook Free. Ed By Dean R Hess Read Online Or Download In Secure Pdf Or Secure Epub Formatessentials Of Mechanical Ventilation Third Edition Dean. ISBN [otw_shortcode_button href=”medical-site.info /Xwy/Essentials-of-Mechanical-Ventilation,medical-site.info”.
In this sense it is important to underscore the concept of controlled hypoventilation as a general ventilatory strategy in application to both diseases — limiting the objectives to maintaining the oxygenation and minute-volume essential for avoiding severe acidosis. To this effect it is necessary to reach deep sedation, and particularly in the case of asthma the administration of short half-life neuromuscular relaxants entails a risk of myopathy in these patients particularly when on corticosteroids ; bolus dose administration with train of four TOF monitoring is therefore preferable.
The indication of bicarbonate to resolve respiratory acidosis is questionable to say the least, and should be accompanied by caution—particularly if the patient suffers hemodynamic instability or intracranial hypertension.
The guiding parameter is pH, not pCO2, avoiding alkalemia, particularly in chronic retainers. The benefits of flow versus pressure trigger are clear in the intermittent mandatory ventilation IMV mode, not so decisive in the pressure support ventilation PSV mode, and without differences in the rest of ventilatory modes.
Dynamic Hyperinsufflation and Auto-PEEP Dynamic hyperinsufflation appears when the pulmonary volume at the end of expiration is greater than the functional residual capacity FRC as a consequence of insufficient emptying of the lung, by starting inspiration before completing the preceding expiration.
The only measure that has been shown to predict the complications of hyperinsufflation is the determination of end-inspiratory volume Vei over functional residual capacity, calculating the total volume of gas exhaled in a patient with muscle paralysis after 60s of apnea.
Assuming that the compliance of the respiratory system remains constant, Vei could be calculated from the following formula, though the latter has not yet been validated: Because of the limitations in calculating Vei in clinical practice, iPEEP and Pplateau are used in its place, since both increase in situations of dynamic hyperinsufflation.
Ppeak also increases, but lacks clinical value, since it increases with the rise in airway resistance and high inspiratory flows—with dissipation upon reaching the alveolus. It is the lowest mean alveolar pressure reached during the respiratory cycle, and is best measured during the sustained expiratory valve occlusion maneuver in the relaxed not necessarily paralyzed patient. The presence of auto-PEEP can also be detected when the inspiratory efforts of the patient for activating the trigger are excessive or even ineffective 26 ; when active expiration continues at the start of the next inspiration; and, using the flow-time plot of the ventilator, when expiratory airflow persists at the end of inspiration Fig.
Occlusion of the expiratory valve.
The calculation of iPEEP in a spontaneously breathing patient is much more complex and difficult to extend to clinical practice, due to the frequent contribution of the respiratory muscles during active expiration.
Through the determination of esophageal pressure Pesoph , an approximation can be obtained via the negative inflexion of Pesoph immediately before activating the trigger in each inspiration.
The existence of hyperinsufflation is suspected, since inspiration begins before the expiratory flow returns to zero. The value calculated by this procedure is usually lower than that confirmed from sustained expiration on detecting the lowest iPEEP value, while the second yields the mean value.
There is general agreement that in order to minimize the risk of volutrauma and barotrauma, Pplateau must be kept below 30mmHg. The attempts to exceed iPEEP constitute one of the main causes of respiratory failure in COPD exacerbation, and moreover represent a major cause of failure in the weaning of patients with airflow obstruction.
However, ePEEP should not be applied to patients with COPD while under controlled ventilatory modes with the purpose of maximizing exhalation and of preventing regional overdistension—reserving it for when the assisted mode is resumed, to thus reduce the patient triggering effort. Taking into account the waterfall principle, equaling ePEEP to iPEEP would imply a maximum decrease in inspiratory effort without the risk of hyperinsufflation.
If the above were not enough, there is still another problem: iPEEP varies in relation to the respiratory parameters respiratory frequency and ventilation , the position of the patient, and the changes in respiratory mechanics related to the course of the disease and treatment response. Therefore, the values of iPEEP measured under controlled ventilation in a relaxed patient may be very different from those calculated under spontaneous ventilation—the situation in which we wish to apply ePEEP.
In clinical practice it is useful to ask the patient about his or her degree of comfort and dyspnea while we very gradually increase ePEEP, so that the patient is personally aware of the relief in effort as a result of counterbalancing of the iPEEP until hyperinflation increases the dyspnea will be proportional to Vei, as a result of which the patient can notice the increase in hyperinsufflation.
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