The infrasound blood-pressure recorder. A clinical evaluation

History of blood pressure measurement in newborns and infants

The development of methods for measuring blood pressure (BP) in newborns and small children has a rich history. Methods for BP measuring in adults had to be adapted to this age group. For measuring BP in direct invasive way, a suitable approach had to be found to access the arterial circulation through the umbilical and later radialis artery. Currently, results obtained from direct invasive BP measurement are considered the "gold standard". The development of non-invasive methods for BP measuring in newborns and children began with the use of von Basch's sphygmomanometer (1880). In 1899, Gustav Gärtner constructed the device, which was the basis for the flush method. After the discovery of the palpation and auscultation methods, these methods were also used for BP measurement in newborns and children, however, the BP values obtained in these ways were typically underestimated using excessively wide cuffs. From the auscultation method, methods utilizing ultrasound and infrasound to detect arterial wall movement and blood flow were later developed. The oscillometric method for BP measurement was introduced by E. J. Marey so early as in 1876. In 1912, P. Balard used the oscillometric technique to measure blood pressure in a large group of newborns. Through different types of oscillometers using various methods for detecting vascular oscillations (such as xylol method, impedance and volume plethysmography, etc.), the development has continued to assessment of vascular oscillations by modern sensor technology and software. For continuous non-invasive blood pressure measurement, the volume-clamp method, first described by Jan Peňáz in 1968, was developed. After modification for use in newborns, application of the cuff to the wrist instead of the finger, it is primarily used in clinical physiological studies to evaluate beat-to-beat BP and heart rate pressure variability, such as in the determination of the baroreflex sensitivity.

Methods for assessing blood pressure values in humans

In clinical medicine and epidemiology, measurements of blood pressure largely rely upon the use of cuff inflation and Korotkoff sounds. Although still the most practical, this method has been recognized to have important limitations. This paper focuses on two limitations of the cuff method that have been found while recording 24-hour intraarterial blood pressure in free-living normotensive and hypertensive patients. First, the 24-hour blood pressure is characterized by large long- and short-term variabilities whose magnitudes vary according to the patient's basal blood pressure and age. This is likely to reduce the possibility that a few isolated cuff measurements are accurate and representative of the patient's average blood pressure. Second, during cuff blood pressure assessment by the doctor (and to a lesser degree by the nurse), the patient's blood pressure normally rises due to an alarm reaction, with a large peak within the first 4 minutes and a subsequent decline. The magnitude of the peak rise, as well as its large and unpredictable difference among subjects may be responsible for seriously and variably overestimating the blood pressure. A 10-minute wait from the beginning of the doctor's visit usually avoids this inconvenience. Finally, the paper briefly considers alternative methods to the cuff method, including invasive intraarterial 24-hour recording in ambulatory subjects, which provides a large amount of information but is impractical, and noninvasive automatic blood pressure devices, which offer a promising practical approach but must wait for technical validation.

Continuous vs intermittent blood pressure measurements in estimating 24-hour average blood pressure

In the past few years noninvasive automatic blood pressure (BP) recorders have been increasingly used to estimate patients' 24-hour BP more accurately than by one or few isolated measurements. However, these recorders only allow BP to be intermittently measured at intervals between 5 to 30 minutes, which means that the number of values collected over 24 hours (10 to 100) remains a tiny fraction of the thousands of values that occur during the same period. To determine whether this represents a limitation to this approach, BP was recorded intraarterially for 24 hours (Oxford method) in 20 ambulant hypertensive patients. A beat-to-beat analysis of the BP recording was provided by a computer, and the average 24-hour systolic, diastolic, and mean BP values were compared with those obtained by analyzing single BP waves of the same recording at intervals of 5, 10, 15, 30, and 60 minutes. In each subject the average 24-hour BP values obtained by the beat-to-beat analysis closely corresponded to those obtained by the analysis performed at 5-, 10-, 15-, or 30-minute intervals. In most subjects, this was the case also when the analysis was performed at 60-minute intervals. These findings demonstrate that intermittency of measurements does not limit the accurate assessment of true average BP. Indeed, accurate assessment can be achieved at intervals as much as 30 or 60 minutes apart.

Clinical evaluation of the Dinamap non-invasive blood pressure monitor in pre-term neonates

The performance of the Dinamap 847 non-invasive blood pressure monitor was evaluated in a group of 10 severely ill pre-term newborn babies. The mean gestational age was 30 weeks and mean birthweight of 1610 gm. The values of systolic, diastolic and mean arterial pressure obtained by the Dinamap were compared with the corresponding values of actual pressure through a right radial artery catheter. All these values showed a high degree of correlation. No complications were noted. It was concluded that the Dinamap 847 should be considered as an alternative for monitoring the arterial blood pressure in pre-term neonates where arterial catheterisation is either not justified or impossible.

The methodology of blood pressure recording

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Comparison of intra-arterial and indirect blood pressures at rest and during isometric exercise in hypertensive patients before and after metoprolol

1. Blood pressure was measured both directly and indirectly in seven hypertensive patients before and after a single oral dose of 100 mg metoprolol, at rest and during sustained handgrip. 2. Intra-arterially measured systolic and diastolic blood pressure increased linearly with time during sustained handgrip at 50% of maximal voluntary contraction. This linearity persisted for 60 s or more in most cases. Heart rate increased linearly for the first 30 s. 3. Indirectly measured blood pressure using an observer bias minimizing Auto-Manometer, under-read systolic and over-read diastolic pressure both at rest and during handgrip. 4. By exact timing of recorded values during handgrip and linear extrapolation (or interpolation) from base-line readings, mean values at 30 and 60 s of handgrip were calculated. The relationship between direct and indirect values remained the same at base-line and 30 s of handgrip. At 60 s of handgrip, this was true only for diastolic pressure. For systolic pressure, indirect and direct values almost coincided. 5. After metoprolol, directly recorded pressure fell slightly (7--12 mm Hg, 0.02 less than P less than 0.10), both at rest and during handgrip, and heart rate fell by 15--18 beats/min (P less than 0.01). The systolic blood pressure and heart rate effect of metoprolol at 1 min handgrip correlated with peak plasma drug levels. Indirectly measured blood pressure did not change significantly. 6. The rate of rise in heart rate and blood pressure from base-line to 60 s handgrip was not significantly influenced by metoprolol.