CAPNOGRAPHY• Measurment of CO2 in respiratory gases.• Integral part of anesthesia monitoring.• Luft 1943 developed it from knowledge that co2 is absorbs infrared radiation of particular wavelength.• Collier – value of rapid IR CO2 analysis• Value of end tidal sample – Ramwell• 1978 Holland – first country to adopt capnography as standard monitoring in anesthesia
GUIDELINES• ASA included capnography in its standrads for basic monitoring.• ISA designated it as ‘desirable’ monitor in anesthesia.• ISA – “Anesthesia monitoring standards recommended in India” for basic monitering from 1999.
Terminology• Capnography: display of instanteneous CO2 conc. Vs time (time capnogran) or expired volume (volume capnogram)• Capnograph: Machine that generates waveform.• Capnogram: Actual waveform
Terminology• Capnometry: Measurment & numerical display• Capnometer: Device that performs the measurement & display readings• Breath to breath waveform needs to be displayed for continuous monitoring
Methods used for measurement• Raman spectometry – scatering• Gas sample is illuminated by high intensity monochromatic Argon laser beam.• Light is absorbed by molecules which is then excited to unstable vibrational & rotational energy states, called as ‘Raman scattering’• Used to identify molecules in gas phase including CO2 & inhalational agents
Methods used for measurement• Mass spectrometry• Photoacoustic gas measurement• Colorimetric method
TYPES OF CAPNOGRAPH SIDE STREAM MAIN STREAM• CO2 sensor located in main • Cuvette containing CO2 sensor monitor inserted between breathing• Tiny pump aspirates gas from circuit & ETT. patients airway • IR rays detector• Transferred by 6 ft long • No need of sampling & capillary tube in to main unit scavenging.• Rate 50 – 200 ml/min • To prevent condensation of• Contamination of FGF water vapour, heated to 40°C• Water trap • Skin burns• Gas should be retrieved & • Kinking of tube reinjected • False reading if not clean• Spontaneous respiration • Better in children
CALIBRATION• Capnographs must be calibrated periodically• At least daily acc to manufacturers for main stream• Automatic Zeroing – side stream moniter• Main stream calibration sample cell sealed with mixtures of CO2 & N2• Range is up to 100mmHg which is useful in rare cases like malignant hyperthermia, hypoventilation.
PHYSIOLOGY• EXPIRATION• PHASE I – Dead space gas exaled, no CO2• PHASE II – Mixing of alveolar gas with dead space• PHASE III - Alveolar Plateau, CO2 reach• INSPIRATION• PHASE IV – Inspiration starts CO2 becomes zero• α angle – PHASE II & PHASE III 1OO° V/Q• β angle – PHASE III & PHASE IV 90° REBREATHING
TYPE OF CAPNOGRAMSTIME CAPNOGRAM VOLUME CAPNOGRAM
CLINICAL USES• NORMAL VALUE: 35 – 45 mmHg• Metabolism• Increased with increase in metabolism• Increased temp, shivering, convulsions,excess catecholamine, blood & bicabonate administration, release of torniquet with reperfusion, glucose containing iv fluids• Laproscopy, thoracoscopy.
How to interpret capnogram?• Respiratory Rate (speed of paper to be known)• Height of capnogram ( value of max exp & inspiratory conc of CO2)• Shape ( altered in abnormal states)• Baseline of capnogram (rebreathing)
CIRCULATION• Reduced with reduction in cardiac output if ventilation is normal.• Reduced blood flow to lungs reduces as in thoracic surgery, manipulation to heart.• Rapid reduction in ETCO2 in absence of changes in BP, HR, CVP indicates pulmonary embolism.• Large embolism - reduce CO further reduction in CO2
TAKE HOME MESSAGES• Capnography included as standard of basic monitoring.• Side stream & main stream advantages & disadvantages.• Helpful in monitoring of respiratory rate, ETCO2, rebreathing, prediction of PaCO2• Shapes of capnogram help in diagnosis of abnormal conditions.• Must be calibrated periodically.• Must be cleaned or disposed to prevent cross infection.