Microstream® Technology Overview
Oridion took a completely new approach to capnography in developing its Microstream® technology and FilterLine® components. In order to understand how these products conquer past problems with capnography, it helps to understand how conventional capnography works.
Capnography is based on the principle that CO2 molecules absorb infrared radiation at specific wavelengths. The capnograph contains special photodetectors tuned to these wavelengths that enable the calculation of CO2 levels in the breath sample.
Conventional capnographs typically use a heated element called a blackbody emitter for the infrared radiation source. Unfortunately, this type of emitter is both imprecise and inefficient because it produces a broad infrared spectrum. As a result, the capnograph requires a large sample cell and high flow rate, which causes occlusion and accuracy problems. Blackbody emitters also generate large amounts of heat, creating hardware challenges that restrict monitor portability and ruggedness.
Microstream® - A Unique CO2 Emission Source
Microstream® employs a unique, laser-based technology called molecular correlation spectroscopy (MCS™) as the infrared emission source. Operating at room temperature, the Microstream® emitter is electronically activated and self-modulating, which eliminates the need for moving parts.
Unlike the broad infrared spectrum produced by a blackbody emitter, MCS™ creates an infrared emission precisely matching the absorption spectrum of CO2. The Microstream® emitter radiates a focused beam of infrared energy characterized by the narrow region (0.15 µm wide) of the spectrum where CO2 molecules absorb infrared radiation. A blackbody emission is typically 135 times broader. Because MCS™ is highly accurate with all gas samples, there is no need to create special algorithms within the monitor to correct for high concentrations of oxygen or anesthetic gases.
Small Sample Cell
The highly efficient and CO2-specific emission source used in Microstream® technology results in an extremely short light path. This sets the stage for a number of technological advantages and clinical benefits. Because of the short light path, the breath sample cell can be greatly reduced in size (down to 15 µl) compared to sample cells used in conventional capnography.
Accuracy in Monitoring Neonates
The advantage of a small sample cell is most apparent with neonatal patients who have high respiratory rates and small tidal volumes. A large sample cell can cause the inspired and expired breath to blend within the cell, resulting in slow response time, falsely low EtCO2 measurements and a distorted waveform shape. With Microstream®, a small sample cell designed for laminar flow, accurate monitoring can be attained with a much lower flow rate.
Minimal Flow Rate
A low flow rate is important because it prevents moisture and humidity from entering the sample line and obstructing the pathway, a problem common in sidestream technology. Microstream® operates at a flow rate of only 50 ml/min. Other capnography systems typically require flow rates two or three times as high. As with the small sample cell, the low flow rate ensures accurate and responsive monitoring for neonates and infants, despite their small tidal volumes.
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Physics of capnography
Bhavani Shankar Kodali MD
Side stream capnometers using low aspiration flow rates (50ml.min-1)
Minimizes dispersion of gases in the sampling tubes
Less likely to aspirate condensed water and secretions thereby minimizing chances of occlusion
Despite low flows, the response time is preserved by using highly CO2-specific infrared source and maintaining laminar gas flow throughout the breathing circuit
PETCO2 values obtained with this monitor accurately predicted PaCO2 values in adults
Studies in newborns also showed that Microstream technology accurately measured alveolar CO2 in newborns without pulmonary disease, as demonstrated by normal PetCO2 -PaCO2 gradients .
Traditional high flow side-stream capnometers (150 ml.min-1) result in erroneous PETCO2 measurements and distorted waveforms in neonates and infants with small tidal volumes and high respiratory rates. In addition, high flow capnometers also likely to aspirate water and secretions into the sampling tubes resulting either erroneous PETCO2 values or in total occlusion of sampling tube. To obviate these disadvantages, recently, Microstream capnometers such as NBP-75®, Nellcor Puritan Bennett, Plesanton, CA,USA), and Oridion Capnostream 20 with an aspiration flow rate of 50 ml.min-1, have been developed. It uses a highly CO2-specific infrared source where the IR emission exactly matches the absorption spectrum of the CO2 molecules. This facilitates the sample cell to utilize a much smaller volume (15 µml) that permits a low flow rate without compromising response rate or accuracy. Low aspiration flow rates also minimize dispersion of gases in the sampling tube and prevent mixing of the small inspiratory and expiratory volumes observed in newborns. Rapid response time is preserved by laminar gas flow throughout the breathing circuit
End-tidal PCO2 values obtained by this monitor provided a sufficiently accurate estimation of PaCO2 in intubated mechanically ventilated as well non-intubated, spontaneously breathing adults.1,2 Studies have also shown that microstream technology is superior in neonates and children.3,4 Hagerty et al4 evaluated the accuracy of a new low-flow sidestream capnography technology and analyzed components of the capnograms in mechanically ventilated newborns with and without pulmonary disease. Twenty patients were prospectively identified. Eligible infants were mechanically ventilated and had an indwelling arterial catheter. Two groups were identified: newborns who received mechanical ventilation for pulmonary diseases, and newborns who received postoperative mechanical ventilation for nonpulmonary conditions. Respiratory gas sampling was performed using low flow (50 mL/min), sidestream, hand held device, Microcap by Oridion Medical Inc, which utilized MicrostreamTM sampling technology. End-tidal CO2 (PetCO2) was measured for 1-minute pre- and post-arterial blood sampling, and PetCO2 and PaCO2 were compared for each patient. Eight quantitative waveform parameters were also measured on all patients. Newborns in the pulmonary group (n=13) (persistent pulmonary hypertension of the newborn/meconium aspiration syndrome, respiratory distress syndrome, pneumonia) and newborns in the control group (n=7) were matched for birth weight, gestational age, and postnatal age. PetCO2-PaCO2 gradient values were higher in the pulmonary group (7.4+/-3.3 mm Hg) than controls (3.4+/-2.4 mm Hg). Four waveform parameters (ascending slope, alveolar angle, alpha angle, descending angle) were identified, which independently differentiated patients with pulmonary disease from controls. Low-flow capnography with Microstream technology accurately measured alveolar CO2 in newborns without pulmonary disease, as demonstrated by normal PetCO2-PaCO2 gradients. The measured PetCO2 -PaCO2 gradient, as expected, was significantly higher in newborns with pulmonary disease. The authors also identified four quantitative waveform parameters that may be useful in differentiating between mechanically ventilated newborn patients with and without lung disease.
1. Casti A, Gallioli G, Scandroglio M, Passaretta R, Borghi B, Torri G. Accuracy of end-tidal carbon dioxide monitoring using the NBP-75 microstream capnometer. A study in intubated ventilated and spontaneously breathing nonintubated patients. European J Anesthesiology 2000;17:622-626.
2. Casti A, Gallioli G, Passaretta R, Scandroglio M, Bignami E, Torri G. End tidal carbon dioxide monitoring in spontaneously breathing nonintubated patients. A clinical comparison between conventional sidestream and microstream capnometers. Minerva Anestesiol 2001;67(4):161-4.
3. Sing S, Venkataraman ST, Saville A, Bhende MS. NPB-75: A portable quantitative microstream capnometer. Am J Emerg Med 2001;19(3):208-10.
4. Hagerty JJ, Kleinman ME, Zurakowski D, Lyons AC, Krauss B. Accuracy of new low-flow sidestream capnography technology in newborns: a pilot study. J Perinatol 2002;22(3):219-25.
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