Flow-through versus sidestream capnometry for detection of end tidal carbon dioxide in the sedated patient.

BACKGROUND: End tidal carbon dioxide (ETCO(2)) in non-intubated patients can be monitored using either sidestream or flow-through capnometry [Yamamori et al., J Clin Monit Comput 22(3):209-220, 2008]. The hypothesis of this validation study is that, flow-through capnometry will yield a more accurate estimate of ETCO(2) than sidestream capnometry when evaluated in a bench study during low tidal volumes and high oxygen administration via nasal cannula. Secondarily, when ETCO(2) from each is compared to arterial CO(2) (PaCO(2)) during a study in which healthy, non-intubated volunteers are tested under normocapnic, hypocapnic and hypercapnic conditions, the flow-through capnometer will resemble PaCO(2) more closely than the sidestream capnometer. This will be especially true during periods of lower minute ventilation and high oxygen flow rates via mask in non-intubated, remifentanil sedated, healthy volunteers whose physiologic deadspace is small. METHODS: The performance of a flow-through (cap-ONE, Nihon Kohden, Tokyo, Japan) and a sidestream (Microcap Smart CapnoLine Plus, Oridion Inc., Needham, MA) capnometer were compared in a bench study and a volunteer trial. A bench study evaluated ETCO(2) accuracy using waveforms generated via mechanical lungs during low tidal volumes and high oxygen flow rates. A volunteer study compared the ETCO(2) for each capnometer against PaCO(2) during sedation in which 8 l O(2) was delivered via mask rather than the nasal cannula. RESULTS: In the bench study, the flow-through capnometer gave slightly higher values of ETCO(2) during high-flow oxygen and no discernable differences during variable tidal volumes. Bland and Altman plots comparing ETCO(2) to PaCO(2) showed essentially equal performance between the two capnometers in the volunteers. CONCLUSIONS: Within a wide limit of agreement between the volunteer and bench study, flow-through and sidestream capnometry performed equally well during bench testing and in non-intubated, sedated patients.

Rapid recovery from sevoflurane and desflurane with hypercapnia and hyperventilation.

BACKGROUND: Hypercapnia with hyperventilation shortens the time between turning off the vaporizer (1 MAC) and when patients open their eyes after isoflurane anesthesia by 62%. METHODS: In the present study we tested whether a proportional shortening occurs with sevoflurane and desflurane. RESULTS: Consistent with a proportional shortening, we found that hypercapnia with hyperventilation decreased recovery times by 52% for sevoflurane and 64% for desflurane (when compared with normal ventilation with normocapnia). CONCLUSION: Concurrent hyperventilation to rapidly remove the anesthetic from the lungs and rebreathing to induce hypercapnia can significantly shorten recovery times and produce the same proportionate decrease for anesthetics that differ in solubility.

Hypercapnia shortens emergence time from inhaled anesthesia in pigs.

BACKGROUND: Anesthetic clearance from the lungs and the circle rebreathing system can be maximized using hyperventilation and high fresh gas flows. However, the concomitant clearance of CO2 decreases PAco2, thereby decreasing cerebral blood flow and slowing the clearance of anesthetic from the brain. This study shows that in addition to hyperventilation, hypercapnia (CO2 infusion or rebreathing) is a significant factor in decreasing emergence time from inhaled anesthesia. METHODS: We anesthetized seven pigs with 2 MACPIG of isoflurane and four with 2 MACPIG of sevoflurane. After 2 h, anesthesia was discontinued, and the animals were hyperventilated. The time to movement of multiple limbs was measured under hypocapnic (end-tidal CO2 = 22 mm Hg) and hypercapnic (end-tidal CO2 = 55 mm Hg) conditions. RESULTS: The time between turning off the vaporizer and to movement of multiple limbs was faster with hypercapnia during hyperventilation. Emergence time from isoflurane and sevoflurane anesthesia was shortened by an average of 65% with rebreathing or with the use of a CO2 controller (P < 0.05). CONCLUSIONS: Hypercapnia, along with hyperventilation, may be used clinically to decrease emergence time from inhaled anesthesia. These time savings might reduce drug costs. In addition, higher PAco2 during emergence may enhance respiratory drive and airway protection after tracheal extubation.

Hypercapnic hyperventilation shortens emergence time from isoflurane anesthesia.

BACKGROUND: To shorten emergence time after a procedure using volatile anesthesia, 78% of anesthesiologists recently surveyed used hyperventilation to rapidly clear the anesthetic from the lungs. Hyperventilation has not been universally adapted into clinical practice because it also decreases the Paco2, which decreases cerebral bloodflow and depresses respiratory drive. Adding deadspace to the patient's airway may be a simple and safe method of maintaining a normal or slightly increased Paco2 during hyperventilation. METHODS: We evaluated the differences in emergence time in 20 surgical patients undergoing 1 MAC of isoflurane under mild hypocapnia (ETco2 approximately 28 mmHg) and mild hypercapnia (ETco2 approximately 55 mmHg). The minute ventilation in half the patients was doubled during emergence, and hypercapnia was maintained by insertion of additional airway deadspace to keep the ETco2 close to 55 mmHg during hyperventilation. A charcoal canister adsorbed the volatile anesthetic from the deadspace. Fresh gas flows were increased to 10 L/min during emergence in all patients. RESULTS: The time between turning off the vaporizer and the time when the patients opened their eyes and mouths, the time of tracheal extubation, and the time for normalized bispectral index to increase to 0.95 were faster whenever hypercapnic hyperventilation was maintained using rebreathing and anesthetic adsorption (P < 0.001). The time to tracheal extubation was shortened by an average of 59%. CONCLUSIONS: The emergence time after isoflurane anesthesia can be shortened significantly by using hyperventilation to rapidly clear the anesthetic from the lungs and CO2 rebreathing to induce hypercapnia during hyperventilation. The device should be considered when it is important to provide a rapid emergence, especially after surgical procedures where a high concentration of the volatile anesthetic was maintained right up to the end of the procedure, or where surgery ends abruptly and without warning.