Velocimetry

The first quantitative measurements of blood pressure were performed in animals by Hales in 1733 [24 , 25 (link)]. Early reports of intra-arterial pressure measurement in the human are from 1912, when Bleichröder [26 ] cannulated his own radial artery. It is unlikely that he recorded his BP although it would have been possible at that time: Frank developed accurate and fast manometers that could measure pulsatile pressure in 1903 [27 ]. Invasive measurement of BP was confined to the physiology labs for quite some time [28 (link), 29 (link)]. However in the 1950s and 1960s, with the development of refined insertion techniques [30 (link)] and Teflon catheters it became standard clinical practice. High fidelity catheter-tip manometers, such as used to measure pressure gradients across a coronary stenosis, were introduced by Murgo and Millar in 1972 [31 ]. Table 1 gives an overview of BP methods.Table 1

Methods for measurement of blood pressure and cardiac output

System	Method	Company	CO		BP
Nexfin	Finger cuff technology/pulse contour analysis	BMEYE	+	___	+	___
Finometer	Finger cuff technology/pulse contour analysis	FMS	+	___	+	___
LIFEGARD® ICG	Thoracic electrical bioimpedance	CAS Medical Systems, Inc.	+	___	+	…
BioZ Monitor	Impedance cardiography	CardioDynamics International Corporation	+	___	+	…
Cheetah reliant	“Bioreactance”	Cheetah Medical	+	___	+	…
Cardioscreen/Niccomo	Impedance cardiography and impedance plethysmography	Medis Medizinische Messtechnik GmbH	+	___	+	…
AESCULON	Electrical “velocimetry”	Osypka Medical GmbH	+	___	+	…
HIC-4000	Impedance cardiography	Microtronics Corp Bio Imp Tech, Inc.	+	___
NICaS	Regional impedance	NImedical	+	___
IQ2	3-dimensional impedance	Noninvasive Medical Technologies	+	___
ICON	Electrical “velocimetry”	Osypka Medical GmbH	+	___
PHYSIO FLOW	Thoracic electrical bioimpedance	Manatec biomedical	+	___
AcQtrac	Thoracic impedance	Väsamed	+	___
esCCO	Pulse wave transit time	Nihon Kohden	+	___
TEBCO	Thoracic electrical bioimpedance	HEMO SAPIENS INC.	+	___
NCCOM 3	Impedance cardiography	Bomed Medical Manufacturing Ltd	+	___
RheoCardioMonitor	Impedance cardiography	Rheo-Graphic PTE	+	___
HemoSonic™ 100	transesophageal Doppler	Arrow Critical Care Products	+	___
ECOM	Endotracheal bioimpedance	ConMed Corporation	+	___
CardioQ-ODM™	Oesophageal Doppler	Deltex	+	___
TECO	Transesophageal Doppler	Medicina	+	___
ODM II	Transesophageal Doppler	Abbott	+	___
HDI/PulseWave™ CR-2000	Pressure waveform analysis	Hypertension Diagnostics, Inc	+	_ _	+	_ _
USCOM 1A	Transthoracic Doppler	Uscom	+	_ _
NICO	Rebreathing Fick	Philips Respironics	+	…
Innocor	Rebreathing Fick	Innovision A/S	+	…
Vigileo/FloTrac	Pulse contour analysis	Edwards Lifesciences	–	___	–	___
LiDCOplus PulseCO	Transpulmonary lithium dilution/pulse contour analysis	LiDCO Ltd	–	___	–	___
PiCCO2	Transpulmonary thermodilution/pulse contour analysis	PULSION Medical Systems AG	–	___	–	___
MOSTCARE PRAM	Pulse contour analysis	Vytech	–	___	–	___
Vigilance	Pulmonary artery catheter thermodilution	Edwards Lifesciences	–	…	–	___
DDG	Dye-densitogram analyzer	Nihon Kohden	–	…
Truccom	Pulmonary artery catheter thermodilution	Omega Critical Care	–	…
COstatus	Ultrasound dilution	Transonic Systems Inc.	–		+	…
CNAP Monitor 500	Finger cuff technology	CNSystems Medizintechnik AG			+	___
SphygmoCor® CPV System	Applanation tonometry	AtCor Medical			+	_ _
TL-200 T-LINE	Applanation tonometry	Tensys Medical, Inc.			+	_ _

+ noninvasive, – invasive, ___ continuous, _ _ semi-continuous, … intermittent

Practical noninvasive (intermittent) BP measurement became possible when Riva-Rocci presented his air-inflatable arm cuff connected to a manometer in 1896 [32 , 33 (link)]. By deflating the cuff and feeling for the pulse, systolic BP could be determined. In 1905 Korotkoff [34 , 35 (link)] advanced the technique further with the auscultatory method making it possible to determine diastolic pressure as well. In 1903 Cushing recommended BP monitoring using the Riva-Rocci sphygmomanometer for patients under general anesthesia [36 (link)]. Nowadays, automated assessment of BP with oscillometric devices is commonly used. These devices determine BP by analyzing the oscillations measured in the cuff-pressure. The pressure in the cuff is first brought above systolic pressure and then deflated to below diastolic pressure. Oscillations are largest when cuff pressure equals mean arterial pressure. Proprietary algorithms determine systolic and diastolic values from the oscillations. Oscillometers may be inaccurate [37 ], and provided values that are frequently lower than direct BP measurements in critically ill patients, [38 (link), 39 (link)] whereas detection of large BP changes is unreliable [40 (link)]. Due to its intermittent nature hyper- and hypotensive periods may be missed [2 (link)].
“Semi-continuous noninvasive methods” based on radial arterial tonometry require an additional arm cuff to calibrate arterial pressure [41 (link)–43 (link)]. The use of these devices may become problematic under conditions with significant patient motion or surgical manipulation of the limbs [43 (link), 44 (link)]. However, tonometry devices have contributed greatly to the knowledge of the relation between the pressure wave shape and cardiovascular function [45 (link), 46 (link)].

Women who agreed to participate were given follow-up appointments at about 20, 28, and 36 weeks' gestation in the National Institute for Health Research Cambridge Clinical Research Facility (Cambridge, UK). All research scans after the dating scan were done with a Voluson i system (GE Healthcare, Fairfield CT, USA) by one of a team of six sonographers, all of whom received standard training. All ultrasound examinations followed the same protocols as those used in the clinical service.13 , 14 (link) At the 20 week research appointment, participants were given a novel questionnaire we created to obtain details about their medical history and demographic characteristics.¹¹ (link) The 20 week scan had both routine (review of fetal anatomy and biometric measurements) and research (uterine and umbilical artery Doppler flow velocimetry) elements. Women were informed about routine elements (any concerns about the fetal anatomy and of the fetal measurements at the 20 week scan), but women and clinicians were masked to the research elements (results of the uterine and umbilical Dopplers). At the 28 and 36 week research appointments, umbilical and uterine artery Doppler flow velocimetry were repeated, and ultrasonographic measurement of fetal biparietal diameter, head circumference, abdominal circumference, and femur length were also done using standard techniques. An estimated fetal weight (EFW) percentile was calculated by use of the Hadlock equations and reference standard.15 (link), 16 (link) Uteroplacental Dopplers, biometry, and EFW results from the research ultrasound scans at 28 and 36 weeks were not reported to the participant or the clinician. However, both were informed about incidental findings, specifically previously undiagnosed placenta praevia, severe oligohydramnios (amniotic fluid index <5), a previously undiagnosed fetal abnormality, or non-cephalic presentation at the time of the 36 week scan.
Gestational age was defined on the basis of ultrasonographic estimation at the time of the first scan, as recommended.³ Distributions of all measurements in the research scans were similar to previously reported reference cohorts (appendix). Summary statistics for reproducibility and reliability of research scans (assessed by two sonographers, scanning the same woman twice at the same appointment, each masked to the results of the other's scan) are tabulated for 45 women at 20 weeks' gestation and 44 women at 36 weeks' gestation (appendix). Coefficients of variation were less than 5% for fetal biometry and EFW, and between 5% and 10% for uteroplacental Doppler at both timepoints.
Women were selected for additional, clinically indicated scans in the third trimester of pregnancy as per routine clinical care, using local and national guidelines (eg, the NICE Guidelines on low risk women,³ women with diabetes,¹⁷ and women with hypertensive disorders¹⁸ ). Women were also screened with serial measurement of the symphyseal-fundal height. All women carried their maternity notes, which included a chart of the normal range of measurements for fetuses in relation to gestational age. Referral for an ultrasound scan was made by the midwife or doctor providing clinical care. Results of all clinically indicated scans were reported and paper copies were filed in both the participant's hand-held notes and hospital case records.
Screening status in relation to EFW was classified on the basis of the last scan before birth (which could be the 28 week scan or the 36 week scan for universal ultrasonography, depending on the gestational age at delivery). Screen positive was defined as an EFW less than the 10th percentile, using an externally derived reference range15 (link), 16 (link) (for both selective and universal ultrasonography). Screen negative was defined as an EFW of the 10th percentile or more (both selective and universal ultrasonography), or if no clinically indicated scan had been done at gestational age of 26 weeks or later (only selective ultrasonography).

The flow around healthy Cassiopea sp. was imaged using the Particle Image Velocimetry (PIV) methods from^{3 (link)}, such that both the incurrent and excurrent flow of the feeding current could be observed. The imaging setup consisted of a 45 × 45 × 45 cm aquarium, filled with artificial seawater. The water was seeded using 10 μm reflective hollow glass spheres for particle image velocimetry (PIV). An Edgertronic high-speed camera filming at 50 frames per second provided a field of view ca. 30 × 30 cm. Two 2-W continuous wave DPSS lasers (wavelength = 520 nm), spread through cylindrical lenses to produce narrow light sheets, were staggered one above the other to illuminate a single coronal plane across the entire field of view. One jellyfish at a time (n = 9) was placed on the bottom in the center of the aquarium, such that the laser sheet crossed the center of the animal. After allowing it to settle for about 10 min, 30 s of video were recorded at 50 frames per second. Analysis using the LaVision software package produced a PIV time-average over the 30 s. PIV was processed with interrogation windows between 48 and 64 pixels, at 50% overlap.

Raw laser speckle images can be analyzed with temporal autocorrelation to find the decorrelation time and the corresponding velocity. According to established mathematical model [40] , the laser speckle intensity autocorrelation function $g_2\left(\tau \right)$ and the intensity decorrelation time ${\tau }_c$ can be related directly as $g_2\left(\tau \right)=1+{\beta \left[e^{-{\left(\tau /{\tau }_c\right)}^n}\right]}^2,$ where $\beta$ is the correction factor associated with the illumination geometry and $n$ depends on the light scattering regime and motion types of the light scatterers. As the scattered light from the transparent sample was dominated by single scattering, we chose $n=1$ in this study. The movement velocity can be converted from the decorrelation time ${\tau }_c$ using the formula $v=\frac{\lambda }{\pi *\mathit{NA}{*\tau }_c},$ where $\lambda$ is the laser source wavelength, and $\mathit{NA}$ is the numerical aperture of the detection (imaging) objective. Autocorrelation was performed on the image intensity pixel by pixel, with a moving time window [41] that typically covered 20 frames.
Particle Image Velocimetry (PIV) is a velocity measurement technique, which calculates a displacement vector for each interrogation window in the FOV with the aid of autocorrelation or cross-correlation techniques. PIVlab is a user-friendly, graphical user interface (GUI) based digital particle image velocimetry software embedded in Matlab [42] . In this study, we applied PIV analysis on raw speckle images to identify the local moving direction of scatterers. More specifically, we used PIVlab to generate vector velocity maps.