Crystalloid preload versus rapid crystalloid administration after induction of spinal anaesthesia (coload) for elective caesarean section.

Current methods of crystalloid preload administration prior to spinal anaesthesia for elective caesarean section are relatively ineffective in preventing hypotension. This study examined the relevance of the timing of the fluid administered. Fifty women were randomly allocated to receive either 20 ml x kg(-1) of crystalloid solution during 20 minutes prior to induction of spinal anaesthesia (preload), or an equivalent volume by rapid infusion immediately after induction (coload). Significantly more patients in the coload group did not require vasopressor therapy pre-delivery (P=0.047). The coload group required a lower median dose (P=0.03) and a lower median number (P=0.04) of ephedrine doses for the treatment of maternal hypotension pre-delivery. There was no between-group difference in either the total cumulative dose, or in the total number of doses of ephedrine. Neonatal outcomes among the two groups were similar. Rapid crystalloid administration after, rather than over 20 minutes before the induction of spinal anaesthesia for elective caesarean section, may be advantageous in terms of managing maternal blood pressure prior to delivery.

The performance and calibration of TDH39 earphones fitted with Model 51 and MX41/AR cushions.

Coupler measurements were made to compare the performances of new-style Model 51 cushions and old-style MX41/AR cushions fitted to TDH39 earphones. In addition, the calibration, using an artificial ear, of TDH39 earphones fitted with Model 51 cushions was compared with the calibration, using a reference coupler, of the same earphones fitted with MX41/AR cushions. It is concluded that the two cushion types can be taken as identical when fitted to TDH39 earphones, except at 6 kHz where differences in earphone sensitivity of up to 2.8 dB were observed. Likewise, output hearing levels of TDH39 earphones measured using a reference coupler and an artificial ear were found to be similar, except at 6 kHz where differences of up to 6.4 dB were observed. The data support the decision by ISO to declare the Model 51 equivalent to the MX41/AR cushion, thereby requiring TDH39 earphones to be calibrated using a reference coupler whichever cushion is fitted. However, they also highlight once again the ill-defined performance of TDH39 earphones at 6 kHz.

A constant-flow calorimeter for the measurement of acoustic power at megahertz frequencies.

The calorimeter absorbs, in a flow of castor oil, a fraction of the acoustic power output of an ultrasound transducer. It has been used to measure powere of between 1 mW and 10 W at frequencies from 1-5 to 3 MHz. Examples are given of results obtained for an ultrasound therapy unit and an ultrasonic pulse--echo scanner. Sources of inaccuracy are defined. It appears that constant-flow calorimetry offers a potentially very sensitive method for the absolute measurement of acoustic power.