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The oldest method for determining airway resistance is based on the measurement of ventilation pressure in a branch of the tracheotomy tube (cf. Einthoven, 1892) during artificial ventilation of an anesthetized animal with a Starling pump. The difficulty in this method is the optimal setting of stroke volume on the respiration pump. Setting the stroke volume initially to the upper physiologically acceptable limit may cause rupture in the bronchial tree in case of bronchoconstriction. The Starling pump always forces the set volume into the lung. An overpressure valve or a better respiratory pressure limiter which is set to 50 cm H2O can provide a remedy to release excess pressure delivered to the animal. Alternatively the respiration pump should be set to the lower, physiologically acceptable limit. This value may be rather difficult to find (pCO2 measurement in the expired air is required).
This test is generally performed on more than one animal. The Ventilation Sequencer Module has especially been designed to create a ventilator which can handle up to eight animals. The timing (Rate and Insp. duration) is the same for all the animals, the inspiratory flow can be set individually for each animal. The module can drive up to 8 ventilation heads. If more than one head is used they are connected to the module using a special connecting block. It consists of an electronic and a pneumatic section and requires a compressed air supply. The module VSM with the ventilation head operates on the principle of intermittent constant flow ventilation. An adjustable continuous air flow is passed by a main valve either to the animal (during inspiration) or to an overflow outlet (during expiration). The design of the main valve provides an adjustable, pressure controlled, protection for the animal against excessive pressure rise in the lung. The main valve is controlled electronically according to the selected respiration rate (RATE) and the selected inspiratory cycle (INSP. CYCLE %). The safety pressure as the maximum allowed inspiratory pressure can be adjusted on the VSM module. The timing and the safety pressure is the same for all the ventilation heads, the constant air flow during inspiration can be adjusted individually on each ventilation head. The ventilation head is equipped with the connection for the tracheal pressure transducer.
HUGO SACHS ELEKTRONIK-HARVARD
APPARATUS GmbH
Gruenstrasse 1 · D-79232 March-Hugstetten · GERMANY
Phone: +49 (0) 76 65 - 92 00-0 · Fax: +49 (0) 76 65 - 92
00-90