Volume‐controlled Respirators

Abstract

Equipment used for automatic ventilation of the lungs can be classified in different ways. Commonly used methods are mostly based on the process of cycling or triggering or the way the insufflated volume is controlled. In that respect a common division is made of referring to the final, end-inspiratory pressure as measured in the upper airways of the patient-pressure-controlled machines or to the final insufflated volume where the pressures will be a function of the mechanical characteristics of the lung and the airways and the ventilator itself-volume-controlled machines. Such divisions usually explain only some simple factors of the action but provide little information regarding fundamental mechanical properties of the device which are of great importance for the possibility of the equipment to administer an adequate ventilation even under unfavorable mechanical conditions of the lungs and airways.For such information it is necessary to know the qualities of the driving force on the insufflated gas and the flow, pressure and power characteristics of the device.Examples of different types of force generators and their principle pressureflow characteristics have been given by Mushin, Rendell-Baker, and Thompson ( 1959), Mapleson ( 1962), and Holaday and Rattenborg ( 1962). Recently, a slightly different approach to the analysis of function of ventilators has been used by Engstrom and Norlander (1962), Engstrom (1963), and by Norlander (1964). Their approach was based on the power demand on the ventilator during different clinical conditions of intermittent positive pressure ventilation. The power is equal to the product of flow and pressure. The integrated power gives the work, i.e. :This gives a simple method for determination of the future distribution of the respiratory work as it develops in the lungs and the airways during spontaneous breathing or respirator treatment, and the power demand on the force generator of the ventilator.The product of flow and pressure is easily computed with current electronic techniques. We have utilized a multiplier based on the Hall effect which was developed in our department . Three different volume-controlled ventilators are presented here. They have been analyzed with respect to force, power and gas flow in the generator circuit at conditions similar to those occurring clinically.The force generators have been classified according to the force available for action on the respiratory gas during the inspiratory period. (1) Constant force generator with a nonadjustable flow; and (2) increasing force generators with: ( a ) direct action, and (b) indirect action of the patient circuit.The ventilators were connected to a simulated patient circuit, consisting of a 40-liter glass bottle and with an adjustable flow resistance. The ventilators were adjusted to deliver approximately equal tidal volumes at equal respiratory frequencies. The flow resistance in the "patient" circuit was adjusted so that the peak airway pressure amounted to 40 to 50 cm. of water at a maximum load. This p...