Relationship Between Expired Capnogram and Respiratory System Resistance in Critically III Patients During Total Ventilatory Support – Methods

Relationship Between Expired Capnogram and Respiratory System Resistance in Critically III Patients During Total Ventilatory Support - MethodsModern ventilator monitoring includes the measurement of airway pressures and derived intermittent calculations such as respiratory system resistance (Rrs). Patients in the ICU receiving mechanical ventilation often exhibit high Rrs due to bronchospasm, structural airway abnormalities, low lung volume, and retained bronchial secretions both attached to the airways or to the wall of the endotracheal tube. The measurement of Rrs is useful to assess the effects of bronchodilator drugs, to characterize the various diseases, to alert clinicians to the risk of barotrauma, and to improve patient treatment.
Measurements of expired capnogram have been proposed as a continuous noninvasive method to determine end-tidal Pco2 (PetCO£) in order to assess alveolar ventilation in patients without pulmonary dynamic hyperinflation. The factors that influence the waveform of the expired capnogram are the ventilation perfusion mismatch of some lung zones, airway gas mixing, and CO£ sequential expiration. Therefore, the increasing expired C02 slope observed in determinate patients may be a result of the serial mixing of gas flowing from units with different time constants. In fact, during general anesthesia, the expired C02 slope has been associated with increased respiratory resistance in patients with normal lungs. mycanadianpharmacy

The purpose of the present study was to investigate the relationship between Rrs and expired capnogram in a population of critically ill patients receiving mechanical ventilation admitted in a general intensive care service. In addition, we assessed whether the expired C02 slope and arterial minus end-tidal Pco2 gradient (PaC02-PETC02) gradient could be a sensitive parameter to predict Rrs during total ventilatory support.
Subjects
Forty-one consecutive patients aged between 23 and 89 years (mean ± SEM, 66.5 ± 2.34 years) who had been admitted to the general ICU of the Hospital of Sabadell (Spain) were studied. Mechanical ventilatory support had been initiated for a variety of medical and/or surgical processes. The diagnosis was COPD in 21, ischemic heart disease in 10, septic shock in 5, adult respiratory distress syndrome (ARDS) in 2, acute asthma in 2, and coma in 1. Informed consent was obtained from the patients’ relatives according to the requirements of the Clinical Research Committee of the Hospital of Sabadell.
Material
Patients were orally intubated with a cuffed endotracheal tube (Hi-lo Evac Mallinckrodt, Athone, Ireland) with an inner diameter ranging from 8 to 9.5 mm. At the time of the study, the number of days with the same endotracheal tube in place ranged from 1 to 18 (mean ± SEM, 5.6 ± 0.76). Mechanical ventilation in assist/control mode was carried out with volume-cycled ventilators (Servo 900C and Servo 300, Siemens, Solna, Sweden). Inspired gas was humidified with disposable heat and moisture exchangers (Edith Flex, Gambro Engstr;auom, Bromma, Sweden) that were changed daily. Patients had indwelling radial or femoral artery catheters for blood gas collection and hemodynamic monitoring purposes.

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