Phase III (alveolar plateau) represents alveolar gases. Phase II is the mixture of dead space gases and alveolar gases. Phase I represents dead space gases containing no CO 2. The expiratory segment is further divided into three phases. These units attach directly to the oxygen facemask or nasal cannula, allowing for CO 2 monitoring in a spontaneously breathing patient receiving sedation.Ī typical time capnogram is divided into an inspiratory and expiratory segment. Further advances in mainstream capnography have produced smaller, light-weighted adapters for use in nonintubated patients. The temperature of the sensors is also decreased with better shielding to minimize facial burns. Newer versions, however, are more lightweight and have disposable adaptors. The earlier generations of mainstream capnometers had significant disadvantages, including bulky sensors requiring sterilization after each use and a risk of facial burns from the heated sensor. Condensation of moisture can interfere with the functioning of the unit. The mainstream sensor is heated above body temperature, which prevents condensation and allows the sensor to function in high moisture environments. There is no extra tubing, and there is no delay in the display of the waveform. During the respiratory cycle, respiratory gases flow through the adapter and the amount of CO 2 in the sample is measured immediately. The sensor emits IR light to a photodetector on the other side of the adapter. In this method, a lightweight IR sensor is attached directly to the adapter. Mainstream capnography involves the use of an adapter between the breathing circuit and endotracheal tube. A sidestream capnograph with a display of waveforms and end-tidal CO 2 (ETCO 2) values is shown in Figure 2. Keeping the sample tubing antigravity can also minimize contamination from water vapor or secretions. The use of a filter between the tubing and the unit containing the CO 2 sensor minimizes this problem. A drawback of sidestream capnography is that the tubing may become blocked from water vapor or secretions. For example, there are modified nasal cannulas which allow the sampling of expired gases even while administering supplemental oxygen. One of the main advantages of sidestream capnography is that it can be used in nonintubated patients. Because this gas sample must travel through the tubing to the CO 2 sensor before it is processed, there is a slight delay in the display of the CO 2 waveform. The tubing is connected from the side port on the adapter to a separate unit which contains the CO 2 sensor, and a sample of gas is aspirated through this disposable tubing during the respiratory cycle into the capnograph for measurement.

It involves the use of disposable tubing (usually several feet long) and a T-piece adapter, which is inserted between the breathing circuit and endotracheal tube or other airway device. Sidestream capnography is the most widely used method for continuous CO 2 monitoring.

In the future, given increasing use of capnography during CPR large databases can be analyzed to predict outcomes. Future directions include determining the outcomes based on capnography waveforms PETCO 2 values and determining a reasonable duration of CPR. There is also increasing recognition of the value of capnography in intensive care settings in intubated patients.

There is emerging evidence that PETCO 2 values can guide the initiation of extracorporeal life support (ECLS) in refractory cardiac arrest (RCA). Additional evidence favoring the use of capnography during CPR includes definitive proof of correct placement of the endotracheal tube and possible prediction of patient survival following cardiac arrest, although the latter will require further investigations. Available evidence suggests that there is significant correlation between partial pressure of end-tidal CO 2 (PETCO 2) and cardiac output that can indicate the return of spontaneous circulation (ROSC). Based on an extensive review of available published literature, we selected all available peer-reviewed research investigations and case reports. Most recent Advanced Cardiac Life Support (ACLS) guidelines now recommend using capnography to ascertain the effectiveness of chest compressions and duration of cardiopulmonary resuscitation (CPR). Capnography continues to be an important tool in measuring expired carbon dioxide (CO 2).

Co2 capnography