APPROACH TO MECHANICAL VENTILATION. Chapter 4. Monitoring of ventilatory assistance (and II)
Abstract
The monitoring of ventilatory assistance should analyze four fundamental pathophysiological areas: gas exchange, ventilatory mechanics, respiratory neurophysiological activity, and hemodynamic state of the patient. This issue reviews the essential aspects of ventilatory mechanics. This refers to the set of properties of the respiratory system that determine the ventilatory capacity of the patient, but also the application of mechanical ventilation when this is necessary.
It is based on two basic principles: that all displacement of gas is a consequence of a pressure gradient, and that any pressure gradient is the result of a difference of forces. These forces are of two types: those that tend to collapse the respiratory system, derived from the characteristics of the pulmonary parenchyma; and those that tend to the expansion of the respiratory system, derived from the characteristics of the thoracic wall. This set characterizes a series of resistive and elastic properties of the respiratory system that will condition these pressure gradients.
The difference between the pressure in the proximal airway and the alveolar pressure will be responsible for the generation of the airflow necessary to mobilize the necessary inspiratory volume. We see here three of the four variables that govern the ventilatory mechanics: pressure, ventilatory flow and ventilatory volume. The fourth variable, or more exactly constant, is time. The management of inspiratory and expiratory times, that is, the inspiratory-expiratory relationship (I: E) is fundamental when it comes to achieving the goal of a ventilation adapted to the needs of the patient, but above all safe for this. It is necessary to control volumes and pressures, but also ventilatory flows, in order to avoid the iatrogenia derived from the application of mechanical ventilation. This is; barotruma, volutrauma, and alveolar overdistension.
Modern respirators offer numerous ventilatory modes, very sophisticated at times, with modifiable and measurable parameters of all kinds. Variables, curves, multivariate diagrams, etc. All this, with the purpose of making available to the patient who cares for the best information, so that the decision making is appropriate. However, technology should not make us lose sight of the fact that everything is limited to knowing how to control volume, pressure, flow and time.
