Efficacy of Positive vs Negative Pressure Ventilation in Unloading the Respiratory Muscles: Methods (1)Patients
Five normal male volunteers and six ambulatory patients with COPD from the Outpatient Clinic participated in the study. The patients with COPD were medically stable without any exacerbation of their respiratory condition within the previous three months. All subjects gave their informed consent prior to the study. The protocol was approved by the Human Subjects Research Committee of Cedars-Sinai Medical Center, Los Angeles.
Measurements
On the day of the study, patients underwent spirometric measurements using a dry seal spirometer (Cardio Pulmonary Instruments, Houston).
Diaphragmatic electromyogram (EMG) was measured using surface electrodes placed in the right sixth and seventh intercostal spaces. An external pacing electrode pad (Padeco, Lake Oswego, Ore) placed over the left scapular area served as the ground. The EMC signal was amplified using a PN22 AC amplifier (TECA, Pleasantville, NY) and filtered using an active band-pass filter (10 to 1,000 Hz) and third-order Paynter filter (time constant = 10 ms). Buy Asthma Inhalers Online
Transdiaphragmatic pressure (Pdi) was obtained by means of the two-balloon technique with balloons positioned in the esophagus and stomach for measurement of esophageal (Pes) and gastric (Pga) pressures using pressure transducers (Validyne MP45-1-871 ± 50 cm HaO, Northridge, CA). The esophageal and gastric catheters were connected to the negative and positive ports, respectively, of a third differential pressure transducer to obtain Pdi (Pdi = Pga-Pes).
A nasal CPAP mask (Respironics, Inc, Monroeville, PA) was modified to accommodate the balloon catheters. An attachment in the mask allowed continuous sampling of end tidal C02 (etCOJ with a mass spectrometer (Perldns-Elmer 1100 Medical Gas Analyzer, Pomona, CA). Udal volume (Vt), breathing frequency (fb), and minute ventilation (Ve) were measured from a turbine flowmeter (ventilation measurement module, Sensormedics, Anaheim, CA) attached to the mask.