Transcutaneous Pco2 to Monitor Noninvasive Mechanical Ventilation in Adults: Materials and Methods

In: Mechanical Ventilation

13 Apr 2014

The study protocol was approved by the Ethics Committee of the University Hospital of Geneva. Patients who had been admitted in the ICU and equipped with an indwelling arterial catheter were considered for the present study. To be included, they had to be in hemodynamically stable condition without vasopressor amine treatment other than low-dose dopamine (£0.2 mg/min) to avoid cutaneous hypoperfusion or vasoconstriction. Twenty-six patients were included (18 men, 8 women, aged 68±10 years). Diagnoses included respiratory failure in eight, monitoring after heart failure or myocardial infarction in eight, neurologic disorders in six, severe systemic hypertension in two, pulmonary hypertension in one, and sepsis in one. At the time of the protocol, systolic BP was 128±24 mm Hg, diastolic pressure was 61 ±13 mm Hg, and mean BP was 83 ±15 mm Hg. Mean body weight was 74± 11 kg. Eight patients were under perfusion of low doses of dopamine.
Transcutaneous measurements of carbon dioxide (TcPco2) were performed with a capnograph (Fastrac; Sensormedics Corp; Yorba Linda, Calif). The sensor uses a Stow-Severinghaus electrode. Briefly, it consists of a pH-sensitive glass electrode, a silver/silver chloride reference electrode, and a heater; the electrodes are bathed in an electrolyte solution, that is covered with a gas-permeable Teflon membrane. The collar of the sensor heats the skin to promote local vasodilatation. Warming the skin also softens the keratin layer, thereby making the physical barrier to diffusion more permeable. C02 diffuses from the skin through the membrane; it reacts with water to form H2C03, which in turn dissociates into H+ and HC03“; the H+ production modifies the pH in the electrolyte solution. Changes in pH are related to TcPco2 according to the Henderson-Hasselbach equation. The signal produced is the voltage difference between the pH electrode and the silver/silver chloride reference electrode. Values of TcPco2 reported by the sensor reflect correction factors used by the system software to compensate for both patient temperature and sensor temperature. canadian neighbor pharmacy

After introducing barometric pressure, the capnograph was calibrated before each measurement, using a two-point dry-gas calibration with 5% and 10% C02. Calibration takes approximately 10 min and is recommended each time the capnograph has been turned off or after a 4-h period of use. The skin was cleansed with alcohol and dried. The sensor was then placed on the skin of the anterior abdomen using a double-sided adhesive ring and a contact gel: the membrane surface temperature was set at 43.5°C (Fig 1). TcPco2 was continuously recorded on a paper chart. Arterial blood gases were determined with a gas analyzer (ABL 520; Radiometer; Copenhagen, Denmark). The study protocol consisted of three parts.

Figure 1. The capnograph (Fastrac) with the sensor placed on the skin of the anterior abdomen.

Figure 1. The capnograph (Fastrac) with the sensor placed on the skin of the anterior abdomen.

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