Capnography is useful for:
Ans. C. Determining the appropriate placement of endotracheal tube. (Ref. Clinical Anesthesia by Paul 5th/pg. 670-671)Clinical Anesthesia by Paul 5th Edition/pg. 671.........."Capnography is an essential element in determining the appropriate placement of endotracheal tubes. The presence of a stable ETCO2 for three successive breaths indicates that the tube is not in the esophagus. A continuous, stable CO2 waveform ensures the presence of alveolar ventilation but does not necessarily indicate that the endotracheal tube is properly positioned in the trachea. For example, the tip of the tube could be located in a main-stem bronchus. Capnography is also a monitor of potential changes in perfusion or dead space, is a very sensitive indicator of anesthetic circuit disconnects and gas circuit leaks, and is a method to detect the quality of CO2 absorption".Endotracheal intubation or laryngeal mask airway insertion requires qualitative identification of carbon dioxide in the expired gas. During general anesthesia, capnography and end-tidal carbon dioxide analysis are encouraged. Capnometry is the measurement and numeric representation of the C02 concentration during inspiration and expiration. A capnogram is a continuous concentration-time display of the C02 concentration sampled at a patient's airway during ventilation.Capnography# is the continuous monitoring of a patient's capnogram.# The capnogram is divided into four distinct phases (Fi. The first phase (A-B) represents the initial stage of expiration. Gas sampled during this phase occupies the anatomic dead space and is normally devoid of CO2. At point B, CO2- containing gas presents itself at the sampling site, and a sharp upstroke (B-C) is seen in the capnogram. The slope of this upstroke is determined by the evenness of ventilation and alveolar emptying. Phase C-D represents the alveolar or expiratory plateau. At this phase of the capnogram, alveolar gas is being sampled. Normally, this part of the waveform is almost horizontal. Point D is the highest CO2 value and is called the end-tidal CO2 (ETCO2). ETCO2 is the best reflection of the alveolar CO2 (PACO2). As the patient begins to inspire, fresh gas is entrained and there is a steep downstroke (D-E) back to baseline. Unless rebreathing of CO2 occurs, the baseline approaches zero.# The utility of capnography depends on an understanding of the relationship between arterial CO2 (PaCO2), alveolar CO2 (PACO2), and ETCO2. If the PaCO2-PACO2 gradient is constant and small, capnography provides a noninvasive, continuous, real-time reflection of ventilation. During general anesthesia, the ETC02-PaC02 gradient typically is 5 to 10 mm Hg.# Maldistribution is a common cause of an increased PaCO2-PACO2 gradient. Other patient factors that may influence the accuracy of ETCO2 monitoring by widening the PaCO2-ETCO2 gradient include shallow tidal breaths, prolongation of the expiratory phase of ventilation, or uneven alveolar emptying.# Increases in ETCO2 can be expected when CO2 production exceeds ventilation, such as in hyperthermia or when an exogenous source of CO2 is present# A sudden drop in ETCO2 to near zero followed by the absence of a CO2 waveform is a potentially life-threatening problem that could indicate malposition of an endotracheal tube into the pharynx or esophagus, sudden severe hypotension, pulmonary embolism, a cardiac arrest, or an artifact resulting from disruption of sampling lines. During life-saving cardiopulmonary resuscitation, the generation of adequate perfusion can be assessed by the restoration of the CO2 waveform.# Whereas abrupt decreases in the ETCO2 are often associated with an altered cardiopulmonary status (e.g., embolism or hypoperfusion), gradual reductions in ETCO2 more often reflect decreases in PaCO2 that occur after increases in minute ventilation where ventilation overmatches CO2 production.# Several methods for the quantification of CO2 have been applied to patient monitoring systems. One of the most commonly used methods is based on infrared absorption spectrophotometry (IRAS).Additional Educational points:# ETCO2 is the best reflection of the alveolar CO2 tension.# Increase in ETCO2 is the most sensitive early sign of malignant hyperthermia.6Most Sensitsive|||||Transesophageal echocardiographyPrecordial DopplerPulmonary artery catheter (| PAP)Capnography (|ETCO2)Mass spectrometry (| ETN2 most specific and quantitative)Least SensitivePAP= pulmonary artery pressure; ETCO2= end-tidal carbon dioxide; ETN2=end-tidal nitrogen.
determining Vaporizer malfunction or contamination
determining circuit hypoxia
determining the appropriate placement of endotracheal tube.
detetcing concentration of oxygen in the anesthetic circuit
Anaesthetic Equipments