CO 2 plays an important role in acid—base buffering. The lungs serve as a pump to promote the activity of ventilation. Gases diffuse from areas of higher concentrations to areas of lower concentration. As a result, oxygen from the alveolus where Po 2 is about mm Hg diffuses into the blood, and CO 2 diffuses from the blood into the alveolus where Pco 2 measures about 40 mm Hg. Because inspired air has negligible CO 2 amounts less than 0. ETco 2 reflects metabolism, circulation, and ventilation.
In patients with normal pulmonary function, CO 2 normally 35 to 45 mm Hg and ETco 2 should correlate closely, with a deviation End Title about 2 to 5 mm Hg. Circulating blood CO 2 is slightly greater than exhaled CO 2 due to a ventilation-perfusion.
Not all alveoli participate in gas exchange at a given time, resulting in some shunting. Dead-space ventilation results in ventilated alveoli with insufficient perfusion, which leads to low ETco 2.
This may result from such ventilatory problems as high mean airway pressure or inadequate exhalation time resulting in overdistentionor from such circulatory problems as shock, massive fluid loss, or pump failure resulting in hypotension. To visualize ventilation and perfusion, see Lung zones. Measured CO 2 also is influenced by CO 2 production, End Title. In critically ill patients, CO 2 production may increase from such conditions as sepsis, fever, seizures, agitation, and carbohydrate overloading.
Because variances may occur over hours, ETco 2 values must be trended for clinical application. All have evolved to be lighter, more accurate, and easier to calibrate.
Each type has certain advantages and limitations. Gas samples are aspirated from exhaled gas flow via the ventilator circuit through a T-adapter and are read at the monitor. A slight delay in End Title retrieval may occur due to the lag time between airway and monitor. Sidestream monitors can be used with noninvasive ventilation and are relatively inexpensive when part of a monitoring package. With these monitors, a sampling window is inserted directly in-line with the ventilator circuit for CO 2 measurement.
This allows a more rapid response time and requires a smaller amount of sample gas than sidestream monitoring. But mainstream monitors increase mechanical dead space, depending on size of the chamber used to collect a gas sample, while adding weight on the airway.
While sidestream and mainstream monitors rely on infrared absorption, the newest type of ETco 2 monitor uses molecular correlation spectrography for greater precision. The Microstream monitor has a rapid response time and may be used with both invasive and noninvasive ventilation. Its main limitations are cost and the need for a monitor separate from the bedside monitor or ventilator.
This section discusses ETco 2 waveforms and the assessment capabilities of ETco 2 monitoring. ETco 2 values may be displayed solely as a numeric value capnometry or with a waveform capnography. Waveforms may be time-based CO 2 over time or volume-based CO 2 plotted over exhaled tidal volume.
See Basic time-based capnogram by clicking the PDF icon above. It is usually presented as a graph of expiratory CO 2 measured in millimeters of mercury, "mmHg" plotted against time, or, less commonly, but more usefully, expired volume. The plot may also show the inspired CO 2which is of interest when rebreathing systems are being used. The capnogram is a direct monitor of the inhaled and exhaled concentration or partial pressure of CO 2and an indirect monitor of the CO 2 partial pressure in the arterial blood.
In healthy individuals, the difference between arterial blood and expired gas CO 2 partial pressures is very small. In the presence of most forms of lung disease, and some forms of congenital heart disease the cyanotic lesions the difference between arterial blood and expired gas increases and can exceed 1 kPa.
During anesthesia, there is interplay between two components: the patient and the anesthesia administration device which is usually a breathing circuit and a ventilator. The critical connection between the two components is either an endotracheal tube or a mask, and CO 2 is typically monitored at this junction.
Capnography directly reflects the elimination of CO 2 by the lungs to the anesthesia device. Indirectly, it reflects the production of CO 2 by tissues and the circulatory transport of CO 2 to the lungs.
When expired CO 2 is related to expired volume rather than time, the area beneath End Title curve represents the volume of CO 2 in the breath, and thus over the course of a minute, this method can yield the CO 2 per minute elimination, an important measure of metabolism. Sudden changes in CO 2 elimination during lung or heart surgery usually imply important changes in cardiorespiratory function. Capnography has been shown to be more effective than clinical judgement alone in the early detection of adverse respiratory events such as hypoventilationoesophageal intubation and circuit disconnection; thus allowing patient injury End Title be prevented.
During procedures done under sedation, capnography provides more useful information, e. Capnography provides a rapid and reliable method to detect life-threatening conditions malposition of tracheal tubesunsuspected ventilatory failure, circulatory failure and defective breathing circuits and to circumvent potentially irreversible patient injury.
Capnography is increasingly being used by EMS personnel to aid in their assessment and treatment of patients in the prehospital environment. These uses include verifying and monitoring the position of an endotracheal tube or a blind insertion airway device. A properly positioned tube in the trachea guards the patient's airway and enables the paramedic to breathe for the patient. A misplaced tube in the esophagus will lead to the patient's death if it goes undetected.
The AHA also notes in their new guidelines that capnography, which indirectly measures cardiac output, can also be used to monitor the effectiveness of CPR and as an early indication of return of spontaneous circulation ROSC. Studies have shown that when a person doing CPR tires, the patient's end-tidal CO 2 ETCO2the level of carbon dioxide released at the end of expiration falls, and then rises when a fresh rescuer takes over.
Other studies have shown when a patient experiences return of spontaneous circulation, the first indication is often a sudden rise in the ETCO2 as the rush of circulation washes untransported CO 2 from the tissues.
Paramedics are also now beginning to monitor the ETCO2 status of nonintubated patients by using a special nasal cannula that collects the carbon dioxide. A high ETCO2 reading in a patient with altered mental status or severe difficulty breathing may indicate hypoventilation and a possible need for the patient to be intubated.
Low ETCO2 readings on patients may indicate hyperventilation. Capnography, because it provides a breath by breath measurement of a patient's ventilation, can quickly reveal a worsening trend in a patient's condition by providing paramedics with an early warning system into a patient's respiratory status. Stories for Film is packaged in a four panel wrap around digipak with an embossed gloss finish and includes a page booklet.
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