Invasive blood pressure measurements
The invasive techniques, being more accurate, were not readily available for human patients until about 1900 [Frank 1905, Hansen 1949]. In the 18th century, however, Hales described the first measurement of blood pressure with a glass manometer connected to an artery of a living horse [Hales 1733] аnd in 1876 Mayer already used invasive measurements in rabbits for determination of low-frequency blood pressure fluctuations [Traube 1865, Mayer 1876].
Invasive blood pressure measurements are always performed by indwelling of a measurement device in the artery or vein in which the pressure has to be known. The measurement device may either consist of a pressure transducer on the tip of a catheter or of a cannula through which the pressure waves are conducted to a pressure transducer outside the patient. Measurement by a tip-catheter is preferable when high frequency components are investigated, because of its excellent frequency response. However, for use in neonates the catheter has to be very thin [less than 4 French]. Although such catheters exist they still are very expensive. When the device is used for continuous measurements, problems occur due to blood clotting around the tip [Webster 1988]. Clinically, often the possibility of blood sampling is more important than the measurement of accurate blood pressure values. A catheter manometer system [CMS], applied primarily for blood sampling, contains a cannula that is coupled to a pressure transducer. It can be used for at least a week, preventing the baby from being
cannulated again аnd again, аnd allows a continuous monitoring of the arterial blood pressure. A CMS only is applied at clinical indication, taking into account the possible risks: thrombus forming, infection аnd transient ischaemia. The risks of umbilical artery cannulation are higher than the risks of peripheral artery cannulation [Schober 1990, Aldridge аnd Gupta 1992]. Accurate measurements of blood pressure require a transmission of pressure wave frequencies to at least the 10th harmonic of the base frequency [Weindling 1989, Hack et al., 1990a]. In premature neonates this corresponds to a bandwidth of approximately 30 Hz. The transmission of high frequency components is often corrupted by air bubbles or blood clots in the system.
A catheter manometer system
If a clinical indication for an indwelling catheter exists, routinely blood pressure is measured with a fluid-filled catheter manometer system. The system consists of a cannula, an extension tube, two stopcocks, аnd a pressure transducer chamber, that is connected to an infusion pump for flushing [see figure 4-1]. The cannula is brought into the radial artery of the neonate. In the period following delivery normally a
longer cannula is used, which is brought into the aorta via the umbilical artery. The total length of the system is about 50 cm аnd it is flushed with heparinised 0.65% saline solution at approximately 1 ml/h. This continuous flushing prevents the blood from clotting around the catheter tip. The risks for complications, i.e., partial obstruction of the artery, are acceptable [Hack et al., 1990b]. In the pressure
transducer chamber a strain gauge is used to convert the pressure variations to electric signals.
