square wave fall in pressure (the 'pop' test - Butler and Oldershaw 1986). After calibration of the system, the end of the needle was inserted into a bung on the end of an adapted syringe which contained 2 ml of saline. The plunger was set at a fifteenth of the remaining volume of the syringe. With the system set ready to record pressure at 100 mm Hg range, the syringe plunger was smoothly and rapidly withdrawn. This applied a negative pressure of approximately 1/15 atmosphere (50 mm Hg). The waveform obtained is shown in Figure 2.8. There is almost no overshoot, being less than 3% of the pressure applied.
Calculation of frequency response was performed as below:
fo = 1 where fo = frequency response 27tS S = time constant
The time constant 8 is the time taken to decay to 0.3678 of the step function, where 0.3678 = 1/e.
For the trace in Figure 2.8:
fo = _______ 1 = 7.94 Hz 2 X 3.142 X 0.02
The 'pop test' was repeated 23 times over four occasions. There was no discernible difference between waveforms
I S«CO«N<i F i g u r e 2.8: W a v e f o r m o b t a i n e d in " p o p " t e st , fo r c a l c u l a t i o n of f r e q u e n c y r e s p o n s e . 2.6 S E T T I N G U P A N D C A L I B R A T I O N : T h e M i n g o g r a f w a s s w i t c h e d o n at l e a s t 30 m i n u t e s p r i o r to r e c o r d i n g to a l l o w t i m e for c a l i b r a t i o n a n d s t a b i l i s i n g of t h e r m a l drift . 500 u n i t s of h e p a r i n w e r e a d d e d to 500 ml of s t e r i l e n o r m a l s a li ne . 20 ml of h e p a r i n i s e d s a l i n e w a s t h e n a s p i r a t e d i n t o a 20 ml s y r i n g e . T h i s w a s c o n n e c t e d t o a t h r e e w a y tap, w h i c h w a s t h e n a t t a c h e d to t h e s i d e a p e r t u r e of t h e G B O O l p r e s s u r e dome. A s e c o n d t h r e e w a y t a p wa s a t t a c h e d to t h e u p p e r a p e r t u r e of t h e dome. W i t h t h e t h r e e w a y t a p s t u r n e d so t h a t t h e s y r i n g e c o m m u n i c a t e d t o a i r a c r o s s t h e d o m e a n d s e c o n d t h r e e w a y tap, h e p a r i n i s e d s a l i n e w a s f l u s h e d t h r o u g h t h e dom e , e n s u r i n g t h a t n o a i r b u b b l e s w e r e p r e s e n t ( F i g u r e 2.9). T h e d o m e w a s t h e n l e f t
open to air only, at the sidearm of the upper three way tap (Figure 2.10).
A small volume of water was placed on the pressure transducer, and the dome screwed firmly and securely onto the transducer which was placed in a specially designed holder. The transducer was then connected to one of the pressure amplifiers of the Mingograf. The system was balanced, using the button which balances the Wheatstone bridge within the pressure transducer. The paper transport was turned on and the baseline adjusted, using the shift control, to a suitable line on the paper. The system was balanced every few minutes until the baseline was stable. With the amplifier range set to 100 mm Hg, the test button was depressed. The deviation of the inkjet was checked, and if it did not correspond to 50 mm, the inkjet was adjusted while the test button was depressed using the gain control. The transducer was balanced again, and turned off to air. The side arm of the lateral three way tap was then attached to a mercury sphygmomanometer dedicated to the purpose. The cuff was inflated to a pressure of 100 mm Hg, and connected to the dome (Figure 2.11). The deflection on the paper record should be 50 mm. If this was not so, fine adjustments were made using a screw above the test button on the pressure amplifier. The sphygmomanometer was disconnected, and the system opened to air again and balanced. If there was marked electrical drift, the system was recalibrated.
Figure 2.9: Arrangement of system for initial flushing
Once the system was calibrated, the manometer tube was attached to the forward aperture of the dome via the three way tap, and the system flushed with heparinised normal saline, again ensuring that no air bubbles were present. The system was left open to air only at the forward three way tap until ready for use. The manometer tube was kept
sterile.