Heat generation as an index of exhaustion of soda lime

The wall temperatures of the absorptive chambers of a divided soda lime canister were measured in 70 patients to determine the relationship between the difference in temperature of the two chambers and CO2 passage through the first chamber. CO2 passage through the first chamber was detected when the temperature of the second chamber became equal to that of the first. A significant correlation (R = 0.94; P less than 0.001) was found between the magnitude of CO2 passage through the first chamber and the difference in temperature between the chambers. When the maximal absorptive capacity of soda lime was reached, the pH of the surface of soda lime granules was still too high to change the indicator color. Exhaustion of soda lime is more reliably recognized by measuring wall temperatures of the chambers than by observing color change of the soda lime granules.     

Evaluation of a small soda lime canister in a to-and-fro system

The main disadvantages of the to-and-fro system (the bulky canister and the progressive increase in apparatus deadspace) may be overcome by the use of a smaller canister. In this laboratory study, we have evaluated a 160 g canister in a low-flow to-and-fro system (fresh gas flow 1 litre/minute). Two carbon dioxide productions of 150 and 200 ml/minute were simulated. The mean times to exhaustion, defined here as a 0.5 kPa rise in end-tidal PCO2, were 112 and 79 minutes in the 150 and 200 ml/minute carbon dioxide groups respectively. Ventilation to normacapnia or hypocapnia did not affect the times to exhaustion. The soda lime absorbed 16 litres of carbon dioxide before exhaustion, and this was not affected by minute volume or carbon dioxide production. A small soda lime canister is suitable for carbon dioxide absorption in a low-flow to-and-fro system for ventilated adults.     

Performance characteristics of a 'to and fro' disposable soda lime canister

The performance of the Intersurgical disposable soda lime canister was compared to British Pharmacopoeia standards for carbon dioxide absorption and to other carbon dioxide absorber systems. This canister system more than adequately fulfilled the equivalent of the British Pharmacopoeia standard for CO₂ absorption. It performed efficiently for over 3 h of continuous use, absorbing 200 mlmin⁻¹ at varying combinations of tidal volume and ventilation rate. Efficiency was not dependent on close matching of tidal volume with canister volume and there was no channelling of gases. Heat was generated by the reaction between soda lime and CO₂ and the maximum temperature recorded in the system was 42.1 °C. Under clinical conditions this should pose no threat of thermal injury to the patient.     

Clinical use of a small soda lime canister in a low-flow to-and-fro system

It may be possible to overcome the classical disadvantages of a to-and-fro system by the use of a small soda lime canister. We report our experience of such a system in a series of 13 adult patients requiring mechanical ventilation of the lungs during surgery. The system was found to be convenient to use and the charge of soda lime had sufficient absorptive capacity to last throughout the great majority of surgical cases. Using a fresh gas flow of 1 l.min-1, the ratio of inspired isoflurane concentration to isoflurane vaporizer setting was found to be approximately one half after 10 min of anaesthesia. This ratio was maintained throughout the procedure, except for a few minutes following each change in vaporizer setting. A to-and-fro system using a small soda lime canister is a convenient alternative to the circle system for low-flow anaesthesia.     

Liquid crystal temperature monitoring for microsurgery

Postoperative monitoring of free tissue transfers remains a problem for the microsurgeon. Liquid crystal temperature probes (LCT) are used by anesthesiologists to monitor patient core temperature and to indicate changes in temperature trends as an indicator of pending malignant hyperthermia. By placing an LCT monitor on the flap and adjacent tissue at the completion of surgery, temperature differentials can be reliably monitored. If the temperature differential exceeds 2 degrees C, the flap is re-explored. The LCT readout resembles a standard thermometer and can easily be recorded by even inexperienced personnel. LCTs are a convenient, inexpensive, and easy method to monitor both free muscle and free fasciocutaneous flaps.     

Water vapour in a closed anaesthesia circuit reduces degradation/adsorption of halothane by dried soda lime

Dry lime causes a loss of volatile anaesthetics by degradingand adsorbing them. Degradation produces toxic substances andheat. Rehydration of lime stops degradation. If humidified breathinggases rehydrate lime, closed anaesthesia-circuits may reducethe loss of anaesthetics. To test this hypothesis we ventilateda reservoir bag with PhysioFlex^®-devices using fresh (F)and dried (D) soda lime both in the presence (+H) and absence(–H) of halothane. We measured halothane delivery, humidity,temperature, and lime weight. Halothane was lost for 13 minin D+H. Humidity increased steeper with fresh lime, whereasabsorbent weight increased more with dried lime; halothane increasedboth variables (F+H: 99%, 8 g; F–H: 93%, 6 g; D+H: 58%,17 g; D–H: 24%, 15 g). Surprisingly, temperature remainedconstant, probably because of the high gas flow (70 litres min^–1)generated inside the Physioflex^®. These findings indicaterehydration of dried lime by humid gases and a rapid cessationof the loss of halothane in the PhysioFlex^®. Br J Anaesth 2000; 85: 308–10     

The CO2 absorption capacity and color indication of a newly developed soda lime (Wakolime)

The CO2 absorption capacity and the color change of ethyl violet (color indicator) of Wakolime A (newer type). Wakolime (older type), and Sorb 800 were studied under the clinical anesthetic condition. A two chamber canister was filled with soda lime and anesthetic gas flowed upward through the canister at the rate of 4 ml.min-1. The wall temperature of each chamber was measured with surface temperature sensing device. After 30 hours of use, the colored soda lime in the first chamber was examined with a color analyzer. The change of wall temperature indicated that the CO2 absorption capacity of Sorb 800 was the largest and that of Wakolime was the smallest among the three. The indicator color of both Wakolime A and Sorb 800 was clearer and of more vivid violet than that of Wakolime, which was dull and grayish. These results indicated that Wakolime A is superior in the CO2 absorption capacity and color indication to Wakolime.     

Ultrasound is a reliable method for determining jugular bulb dominance.

Despite widespread application of jugular oximetry devices, the optimal side to cannulate for monitoring cerebral oxygenation is controversial. For most monitoring strategies, the dominant or larger internal jugular vein gives the most representative values for venous oxygen saturation. However, there is little information on how to best determine the dominant side. The purpose of this study was to compare the results of an ultrasound examination to two other standard methods for determining the dominant internal jugular vein, the jugular vein compression test and the computed tomographic (CT) approach. Seventeen patients with severe head injury (GCS <8) were studied. The ultrasound examination showed the mean internal diameter of the right and the left internal jugular veins to be 1.27 cm (standard deviation [SD] 0.16 cm) and 1.21 cm (SD 0.36 cm), respectively. The right internal jugular vein was larger than the left in 11 (65%) of the patients. The diameter of the dominant or larger internal jugular veins were 1.44 cm (SD = 0.22), compared with 1.04 cm (SD = 0.18) on the opposite side (P < .05). The results of the ultrasound method were in agreement with the CT scan method in 94% of the comparisons and with the jugular vein compression test in 82% of comparisons. These studies demonstrate that the ultrasound method provides useful information about the side of the dominant cerebral venous drainage, comparable to other standard methods, without the need for a CT scan or manipulation of intracranial pressure.