呼气末二氧化碳监测 讨论

昨天群里一位老师的话题引起了大家的热烈讨论

*采样方式分类

根据仪器的采样方式不同，可分为主流型和旁流型。主流型仪器特点为气流直接经过测量室，检测管路为人工气道的一部分。优点在于检测结果受气道内水汽和分泌物影响较小。缺点在于持续监测仅可用于密闭气道，部分厂家产品明显增加气道管路负重和增加呼吸死腔。旁流型仪器气流被动进入测量室。呼出的气体经由抽气泵抽取部分至测量室进行测量，抽气流速度为20～300ml/min。优点在于可用于非密闭开放气道，采样部位多样。缺点在于采样口易受气道内水汽和分泌物影响，对于低流速通气或小儿，抽吸采样产生的气流丢失可能影响潮气量测定和呼吸机触发。

Carbon dioxide (CO2) monitors measure gas concentration, or partial pressure, using one of two configurations: mainstream or sidestream. Mainstream devices measure respiratory gas (in this case CO2) directly from the airway, with the sensor located on the airway adapter at the hub of the endotracheal tube (ETT). Sidestream devices measure respiratory gas via nasal or nasal-oral cannula by aspirating a small sample from the exhaled breath through the cannula tubing to a sensor located inside the monitor (picture 1).

Mainstream systems are configured for intubated patients. Sidestream systems are configured for both intubated and nonintubated patients.

Sidestream systems are configured to use high flow rates (around 150 cc/min) or low flow rates (around 50 cc/min). Flow rates vary according to the amount of CO2 needed in the breath sample to obtain an accurate reading. Low-flow systems have a lower occlusion rate (from moisture or patient secretions) and are accurate in patients with low tidal volumes (eg, neonates, infants, and adult patients with hypoventilation and low tidal volume breathing). Low-flow systems are also resistant to dilution from supplemental oxygen. High-flow systems sampling at ≥100 cc/min have been shown to be inaccurate in neonates, infants, young children, and in hypoventilating adult patients [1-3].

CO2 monitors are either quantitative or qualitative. Quantitative devices measure the precise end-tidal CO2 (EtCO2) either as a number (capnometry) or a number and a waveform (capnography). Qualitative devices report the range in which the EtCO2 falls (eg, 0 to 10 mmHg or >35 mmHg) as opposed to a precise value (eg, 38 mmHg).

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