The Equivent and lungfunction testing
On this page you will find information about inta-thoracic pressure measuring in horses and pony’s. What are the indications for measuring, how do we perform an intra-thoracic measurement and what are the properties of the different versions of the Equivent.
Intra-thoracic pressure measuring was done in humans in the late 19th century. In horses it was developed in the seventies of the 20th century. Purpose is to establish an abnormal airway resistance. Most patients were chronic coughing horses and the diagnosis was COPD or emphysema. When the normal elasticity of the lung decreases the respiratory muscles must work harder to ventilate the lungs. Factors influencing the resistance of the more cranial airways will also change the normal intra-thoracic pressure difference, the ΔPth. This difference will increase during workout.
In the early days of measuring in horses the diagnosis emphysema or COPD was readily made. Many horses with good treatable asthma were disposed of. Asthma like we know it in humans was not recognized in the horse. In the 80’s the role of Aspergilles and Micropolyspora in low quality hay became evident.
Intra-thoracic pressure was measured inter-pleurally. A large needle was inserted and placed between the pleura and connected to a pressure transducer. As long as the ( unsedated ) horse stood still al went well but the slightest movement corrupted the result. The low grade pneumothrax after the procedure was rarely of any clinical significance. In the early 80’s intra-esofageal measuring proved to be as reliable as the inter-pleural procedure. A 6mm diameter polyethylene tube connected to a little balloon ( finger of a latex or pvc glove ) is placed in the esofagus. The location of the balloon is halfway between heart and cardia. Externally the location is found at the 10th intercostal space ( see instruction video ) and the tube is marked at this point with some pvc tape.
It is important that the measuring system is it self of no influence to the pressure difference. After sucking it slightly vacuum 2-4 ml of air is inflated in the system with help of a 3 way stopcock. To much air will lift the zero pressure and to little may lower the pressure. These parameters were established in a synchronous measuring of inter-pleural and intra-esofageal pressure in 10 horses in the University of Utrecht in the early 80’s.
Intra-thoracic measuring in the non working horse was and is the golden standard. Measuring the working horse has major advantages. A horse with mild bronchitis that is breathing very superficially at rest will have little increase in intra-thoracic pressure The Equivent 200 en 300 measure intra-esofagal pressure during workout. The horse can not compensate his increased resistance anymore. Increased resistance of the upper airways will also be more readily detectable.