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PubMed1996

The accuracy of pneumotachograph measurements during mechanical ventilation.

We hypothesized that differences between the conditions under which a pneumotachograph (PT) is calibrated and those during data collection might lead to large errors in measured flow and volume during mechanical ventilation. A Fleisch No. 1 and Fleisch No. 2 and a screen PT were connected to "ideal" tubing configurations that optimized flow characteristics, and to ventilator tubing with and without a Y-connector and endotracheal (ET) tube. Each PT was also evaluated after water had accumulated in its resistive element. Air was passed through each PT configuration, using both a continuous and a pulsatile flow pattern, and collected in a water-seal spirometer. "Measured" and "true" flow and volume were determined from the PT and the spirometer, respectively. Measured flow and volume were falsely low when the PT was adapted to ventilator tubing. Addition of a Y-connector and ET tube caused measured flow and volume to increase, and, in some cases the relationship between measured and true flow became nonlinear. Water accumulation in the PT did not lead to measurement errors. We conclude that when a PT is used during mechanical ventilation, tubing geometry must be identical during calibration and data collection, and that calibration should be performed over the entire range of relevant flows.

Calibration↗
PubMed1994

Precision and accuracy of Morgan ventilometers at continuous and sinusoidal flows.

The P.K. Morgan Mark II Ventilometer comprises a turbine transducer and a control/readout unit, which contains either a VENTX5A or VENTX6A programme. This equipment is used to measure minute ventilation during oxygen consumption tests. The precision and accuracy of six of these devices (a VENTX5A with two turbines, and two VENTX6As each with two different turbines) were examined during continuous and pulsatile flows; accuracy was affected by the syringe rate during the calibration procedure. The reference standard was a 350 l Collins chain-compensated gasometer. At continuous flows, the two VENTX5A devices showed accuracies of 99.5-100.5% over the range 60-160 l.min-1 whereas those for the four VENTX6As were 99.3-102.1% over the range 60-200 l.min-1. The mean accuracies of sinusoidal flows for the VENTX5A ranged between 96.1-101.3% for minute volumes spanning 15-200 l.min-1; comparable data for the VENTX6As were 95.9-98.9%. These mean accuracies improved to 98.1-101.3% for the VENTX5A at measured minute volumes > 80 l.min-1. Omission of the two lowest tidal volumes (1 and 1.5 l) for the VENTX6As narrowed the range of accuracy to 96.9-98.5%. The coefficients of variation for five trials at each minute ventilation were 0-0.2%. We conclude that, while the P.K. Morgan Mark II Ventilometer has excellent precision, its accuracy is dependent on the calibration syringe rate, minute ventilation and program.

Airway Resistance↗
PubMedDate not supplied

Left ventricular and myocardial perfusion responses to volume unloading and afterload reduction in a computer simulation.

Ventricular assist devices (VADs) have been used successfully as a bridge to transplant in heart failure patients by unloading ventricular volume and restoring the circulation. In a few cases, patients have been successfully weaned from these devices after myocardial recovery. To promote myocardial recovery and alleviate the demand for donor organs, we are developing an artificial vasculature device (AVD) that is designed to allow the heart to fill to its normal volume but eject against a lower afterload. Using this approach, the heart ejects its stroke volume (SV) into an AVD anastomosed to the aortic arch, which has been programmed to produce any desired afterload condition defined by an input impedance profile. During diastole, the AVD returns this SV to the aorta, providing counterpulsation. Dynamic computer models of each of the assist devices (AVD, continuous, and pulsatile flow pumps) were developed and coupled to a model of the cardiovascular system. Computer simulations of these assist techniques were conducted to predict physiologic responses. Hemodynamic parameters, ventricular pressure-volume loops, and vascular impedance characteristics were calculated with AVD, continuous VAD, and asynchronous pulsatile VAD support for a range of clinical cardiac conditions (normal, failing, and recovering left ventricle). These simulation results indicate that the AVD may provide better coronary perfusion, as well as lower vascular resistance and elastance seen by the native heart during ejection compared with continuous and pulsatile VAD. Our working hypothesis is that by controlling afterload using the AVD approach, ventricular cannulation can be eliminated, myocardial perfusion improved, myocardial compliance and resistance restored, and effective weaning protocols developed that promote myocardial recovery.

Compliance↗
PubMed1993

Effects of distension of the urinary bladder on the cardiovascular reflexes from the carotid baroreceptors in the dog.

1. The hindlimb vasoconstrictor effects of distension of the urinary bladder were studied at different levels of input from the carotid sinus baroreceptors in the dog anaesthetized with a mixture of chloralose and urethane. 2. The vascularly isolated hindlimb was perfused at constant blood flow through its femoral artery, so that a change in pressure gradient (mean femoral arterial perfusion pressure minus mean inferior vena caval pressure) indicated a similar directional change in vascular resistance. The vascularly isolated carotid sinus regions were perfused with blood at a constant pulsatile flow. 3. Raising the carotid sinus mean perfusion pressure in randomly selected steps of 30 mmHg from 60 to 210 mmHg had little effect on heart rate unless the blood pressure was controlled, when a progressive bradycardia occurred, but caused a progressive reduction in arterial blood pressure and vasodilatation in the perfused hindlimb. Distension of the bladder at each level of carotid sinus pressure resulted in tachycardia, hypertension and hindlimb vasoconstriction. 4. The cardiac responses to bladder distension were the same at all carotid sinus pressures. When the blood pressure was controlled, however, the response was reduced at high and low sinus pressures. 5. The relationship between the carotid sinus perfusion pressure and hindlimb perfusion pressure (i.e. vascular resistance) was affected by distension of the bladder in two ways. In the one, hindlimb perfusion pressure increased by approximately the same amount at all levels of carotid sinus pressure indicating resetting of the carotid sinus baroreceptor reflex control of hindlimb vascular resistance towards vasoconstriction without change in gain of the reflex. In the other, the pressure increases were diminished at the higher levels of carotid sinus pressure indicating both resetting and an increase in gain of the reflex. 6. Both types of response occurred in the spontaneously breathing animal, in animals artificially ventilated, while pacing the heart, with the arterial blood pressure maintained constant at about 100 mmHg, and after division of the cervical vagosympathetic nerves. The frequency of occurrence of each type of response, however, varied under the different conditions. 7. The possible reasons for the two types of vascular response are discussed.

Animals↗
PubMed2003

Arterial wall adaptation under elevated longitudinal stretch in organ culture.

Arteries in vivo are subjected to large longitudinal stretch which may change significantly due to vascular disease and surgery. However, little is known about the effect of longitudinal stretch on vascular function and wall remodeling, although the effects of tensile and shear stress from blood pressure and flow have been well documented. To study the effect of longitudinal stretch on vascular function and wall remodeling, porcine carotid arteries were longitudinally stretched 20% more than in vivo for 5 days while being maintained in an ex vivo organ culture system under conditions of pulsatile flow at physiologic pressure. Vessel viability was demonstrated by strong vasomotor responses to norepinephrine (NE, 10(-6) M), carbachol (10(-6) M), and sodium nitroprusside (10(-5) M), as well as by dense staining for mitochondrial activity and a low occurrence of cell necrosis. Cell proliferation was examined by incorporation of bromodeoxyuridine (BrdU). Results showed that arteries maintain normal structure and viability after 5 days in organ culture. Both the stretched and control arteries demonstrated significant contractile responses. For example, both stretched and control arteries showed approximately 10% diameter contraction in response to NE. Stretched arteries contained 8% BrdU-positive cells compared to 5% in controls (p<0.05). These results indicate that longitudinal stretch promotes cell proliferation in arteries while maintaining arterial function.

Adaptation, Physiological↗
PubMed1993

Increased heart rate can cause underestimation of regurgitant jet size by Doppler color flow mapping.

OBJECTIVES: This study addressed the hypothesis that at a constant peak flow rate, an increasing heart rate could decrease the maximal apparent jet size by Doppler color flow mapping. BACKGROUND: Recent studies have attempted to predict the severity of regurgitation from maximal jet area by Doppler color flow mapping, which correlates with flow rate for free jets at constant driving pressure and steady flow. In patients, however, maximal jet area exists for only a limited time per beat and the likelihood of visualizing it by Doppler color flow mapping depends on its duration relative to the color frame sampling rate. Increased heart rate could potentially diminish apparent jet size, particularly at slow frame rates that may not permit visualization of the maximal jet area in all beats. METHODS: This interaction was examined in pulsatile flow, holding orifice size and peak flow rate constant and varying pump pulse rate (70 to 180 beats/min) and frame rate (three rates) for jets of low and high momentum. Maximal jet area was measured in 10 consecutive beats at each pulse rate and frame rate and averaged. RESULTS: For the low momentum jet, the 10-beat average of peak jet area decreased progressively with increasing pulse rate. As pulse rate increased from 70 to 180 beats/min, maximal jet area decreased 23% at the fastest frame rate and 42% at the slowest frame rate, with prominent beat to beat variability. Jet area decreased 13% to 20% at pulse rates as low as 90 beats/min. In contrast, for the high momentum jet, maximal jet area decreased by < or = 9% from low to high pulse rate at any frame rate. CONCLUSIONS: Increased heart rate can cause underestimation of apparent jet size by Doppler color flow mapping for a given peak flow rate, particularly for jets with low momentum and delayed penetration into the receiving chamber. This observation may be relevant to acute severe regurgitation with increased heart rate in which such underestimation has been reported, as well as to right-sided lesions and children with rapid heart rates. It will also affect new techniques proposed to quantify regurgitation on the basis of velocities derived from Doppler color flow images. In practice, this effect can be reduced by increasing frame rate and selecting maximal apparent jet size at rapid heart rates and should be considered in relating jet size to the severity of regurgitation.

Blood Flow Velocity↗
PubMed2006

Effects of arterial bifurcation geometry on nucleotide concentration at the endothelium.

A mathematical and computational analysis of nucleotide concentration for a two-dimensional artery bifurcation has been developed. This region of the vasculature is known to be exposed to spatially varying wall shear stress (WSS), hence the variation of adenosine nucleotides is of interest. A previously derived similarity solution for mass transport in blood boundary layers for arbitrary wall shear stress function has been used. For the analytical model, the geometric condition has been incorporated into the system using the theory for flow past a wedge. As the bifurcation angle varies different characteristics are exhibited, such as maxima in ADP concentration and the existence of a low wall shear stress region around the stagnation point, for large angles. In the limiting case of two-dimensional stagnation point flow the concentration was constant throughout the domain length. The computational simulations provided a more detailed understanding into how nucleotides vary. Similarities existed between the two solution methods in the vicinity of the stagnation point, but deviated as the fully developed condition prevailed. Additionally, the effect of pulsatile flow has been included, leading to considerable gradients in wall shear stress, both temporal and spatial. However, the resulting nucleotide concentration is determined by the time-averaged wall shear stress. The effects of flow-induced ATP release have also been included, leading to significant changes in ATP concentration. Under rapid release, the concentration at the surface increased relative to the bulk concentration.

Adenine Nucleotides↗
PubMed1979

[Evaluation of liver haemodynamics by spleno-portography in portal hypertension (author's transl)].

In 68 patients arterial and portal blood flow were measured by angiodensitometry in the course of routine, indirect splenoportography. Results were compared with those already published, with investigations of a model and with animal experiments. It was shown that, given certain underlying conditions, reliable results can be obtained even in the presence of a pulsatile flow and incomplete contrast mixing. Compared with other procedures, the method has the advantage of providing a complete picture of liver haemodynamics in the course of routine splenoportography in a simple way and without affecting the patient further. As a result the indications for performing a shunt and the choice of the shunt technique can be more accurately determined.

Blood Flow Velocity↗
PubMed2001

Power Doppler US: evaluation of the morphology of stenoses with a flow phantom.

PURPOSE: To determine the importance of technical settings at power Doppler ultrasonography (US) for the evaluation of stenoses. MATERIALS AND METHODS: A computer-controlled phantom was used to induce a reproducible flow across a calibrated 71% stenosis in an 8.4-mm-diameter tube. Two transducers, 2-4 and 5-10 MHz, working at depths of 3.0 and 11.5 cm, respectively, with different beam angles (40 degrees, 60 degrees, and 90 degrees ), were used to simulate evaluation of pulsatile flow across normal and stenotic vessels in various conditions. For each condition, gain, pulse repetition frequency, and wall filter were progressively turned from low to high values. Two observers measured in a blinded fashion the apparent lumen of the stenotic and normal vessels on longitudinal and transverse images with the use of power Doppler US. RESULTS: When the high-frequency transducer was used, gain significantly affected both stenotic and feeding vessel measurement, whereas pulse repetition frequency and filter only affected feeding vessel evaluation. When the low-frequency transducer was used, all factors, including flow velocity and beam angle, played a significant role (P <.001). In most conditions, overestimation of the lumen and underestimation of the lumen of the feeding vessel led to severe underestimation of the degree of stenosis. CONCLUSION: Power Doppler US cannot be used to measure stenoses accurately. Underestimation of the degree of the stenosis was significantly higher with the low-frequency probe than with the high-frequency probe.

Blood Flow Velocity↗
PubMed1996

Flow-induced detachment of adherent platelets from fibrinogen-coated surface.

A study of the shear forces under which adherent platelets of different morphologies can be detached from surfaces was carried out using a newly designed tapered flow chamber, which covered the entire shear range of physiological circulation. Platelets that naturally settled on a fibrinogen-coated surface were exposed to shear flow and were subsequently processed for scanning electron microscopic observation. We found that 1) the density of platelets remaining after flow exposure decreased with local shear stress, 2) adherent platelets of different morphologies withstood different levels of shear stress: most round cells and 40% of the cells that had a few short pseudopods were detached at < 10 dyn/cm2, whereas most spread cells could withstand 50 dyn/cm2, 3) pulsatile flow was more effective in removing adherent platelets than equivalent steady flow, 4) cytochalasin D and colchicine retarded platelet shape change and made them more easily detached by shear forces, and 5) metabolic energy-depleted platelets spread readily and formed shear-resistant clumps. Our observations indicated that adherent platelets of different morphologies on a fibrinogen-coated surface could withstand different levels of flow shear stress.

Blood Circulation↗
PubMed1995

The effect of topical beta-adrenoceptor blocking agents on pulsatile ocular blood flow.

Thirty-three ocular hypertensive patients (21 with primary open angle glaucoma and 12 glaucoma suspects) were randomly assigned to receive either timolol, levobunolol or betaxolol in one eye. Pulsatile ocular blood flow (POBF) was measured before treatment (baseline) and 2 hours after drop administration. After 1 week of regular twice-daily dosage, POBF was measured again both immediately before and 2 hours after drop instillation. All measurements were made by an investigator masked to treatment. POBF increased by 11% (p = 0.09) at week 0 after levobunolol administration, and by 22% (p = 0.20) at week 1 before drop administration compared with baseline. It dropped by 23% and 25% (p = 0.04 and 0.06, respectively) before and after betaxolol administration at week 1. Although POBF was reduced in the timolol group, this change was not significant. These results can not be explained uniformly by changes in intraocular pressure or blood pressure. The relevance of these measurements to visual function in glaucoma is not known.

Adrenergic beta-Antagonists↗
PubMed1979

Boundary layer separation in models of side-to-end arterial anastomoses.

We performed an investigation of boundary layer separation in models of side-to-end anastomoses, used clinically in axillofemoral and femorofemoral grafting. Boundary layer separation occurs when momentum causes a fluid to flow against a local pressure gradient. Models of side-to-end anastomoses were constructed from Dacron grafting material and from clear plastic blocks and tubing. Fluid energy loss across the anastomosis was small and physiologically insignificant. Local adverse pressure gradients were demonstrated near the anastomosis. Flow visualization studies demonstrated characteristic areas of boundary layer separation in the region of the adverse pressure gradients. The separation region involved both the main limb and the side arm. The separation forms a shell or ring of slow-moving fluid around the mainstream. Preliminary studies with pulsatile flow and with blood demonstrate that boundary layer separation occurs under clinical flow conditions. Boundary layer separation may play a role in the development of anastomotic hyperplasia and atherosclerotic deposits in the vicinity of surgical anastomoses.

Arteriosclerosis↗
PubMed2006

Coordination between ventilatory pressure oscillations and venous return in the cephalopod Sepia officinalis under control conditions, spontaneous exercise and recovery.

Venous blood flow was measured for the first time in a cephalopod. Blood velocity was determined in the anterior vena cava (AVC) of cuttlefish S. officinalis with a Doppler, while simultaneously, ventilatory pressure oscillations were recorded in the mantle cavity. In addition, magnetic resonance imaging (MRI) was employed to investigate pulsatile flow in other major vessels. Blood pulses in the AVC are obligatorily coupled to ventilatory pressure pulses, both in frequency and phase. AVC peak blood velocity (v(AVC)) in animals of 232 (+/- 30 SD) g wet mass at 15 degrees C was found to be 14.2 (+/- 7.1) cm s(-1), AVC stroke volume (SV(AVC)) was 0.2 (+/- 0.1) ml stroke(-1), AVC minute volume (MV(AVC)) amounted to 5.5 (+/- 2.8) ml min(-1). Intense exercise bouts of 1-2 min resulted in 2.2-fold increases in MV(AVC), enabled by 1.6-fold increments in both, AVC pulse frequency (f (AVC)) and v(AVC). As increases in blood flow occurred delayed in time by 1.7 min with regard to exercise periods, we concluded that it is not direct mantle cavity pressure conveyance that drives venous return in this cephalopod blood vessel. However, during jetting at high pressure amplitude (> 1 kPa), AVC blood flow and mantle cavity pressure pulse shapes completely overlap, suggesting that under these conditions, blood transport must be driven passively by mantle cavity pressure. MRI measurements at 15 degrees C also revealed that under resting conditions, f (AVC )and ventilation frequency (f (V)) match at 31.6 (+/- 2.1) strokes min(-1). In addition, rates of pulsations in the cephalic artery and in afferent branchial vessels did not significantly differ from f (AVC) and f (V). It is suggested that these adaptations are beneficial for high rates of oxygen extraction observed in S. officinalis and the energy conserving mode of life of the cuttlefish ecotype in general.

Animals↗
PubMed2000

A preliminary study of the application of the transductal velocity ratio for assessing persistent ductus arteriosus.

OBJECTIVE: To compare the transductal velocity ratio (TVR) of the persistent ductus arteriosus (PDA) with other echocardiographic criteria for haemodynamic significance of a PDA. METHODS: This was a prospective study (from January 1997 to August 1998) in the nurseries of the Royal Women's Hospital, Melbourne. Infants with a clinically suspected PDA were eligible and included if the echocardiogram showed a PDA with a structurally normal heart and the TVR had been measured. The PDA was assessed for evidence of left heart dilatation, the presence of reverse or absent diastolic flow in the descending aorta, the pattern of Doppler flow velocity waveform in the ductus arteriosus, and subjective assessment of ductal diameter on the real time image. The peak systolic velocity (PSV) was obtained from the pulmonary and aortic ends of the PDA, and the TVR calculated by dividing the PSV at the pulmonary end by the PSV at the aortic end. RESULTS: Forty two infants had 59 echocardiographs with their TVR calculated. Mean (SD) birth weight was 1008 (362) g. Mean (SD) gestational age at birth was 27.4 (2.2) weeks with a mean (SD) corrected gestational age of 28.7 (2.7) weeks. The mean TVR was decreased in those infants with a high left atrial diameter/aortic diameter (LA/Ao) ratio (1.9 v 2.8, p = 0.0032) or reverse/absent diastolic flow in the descending aorta (2.1 v 3.0, p = 0.02). This difference was greater if those two criteria were combined (1.7 v 3.4, p = 0.0027). The mean TVR was decreased in infants with a wide open duct seen on two dimensional imaging (1.5 v 3.0, p < 0.0001) or pulsatile flow seen on pulsed Doppler in the PDA (1.9 v 3.4, p = 0.0001). The LA/Ao and left ventricle internal diameter/aortic diameter (LVIDd/Ao) ratios were higher in the group with a TVR < 1.8 than in the other two groups; these differences were statistically significant. CONCLUSIONS: The TVR as a measure of the degree of constriction of a PDA is associated with other echocardiographic criteria for a haemodynamically significant PDA. A low TVR (signifying a poorly constricted duct) is associated with echocardiographic features of a significant left to right shunt, and vice versa. Further research is required to determine the usefulness of the TVR in predicting closure or likely continuing patency of a PDA and the need for treatment.

Birth Weight↗
PubMed2002

Simulation of flow through a Miller cuff bypass graft.

Unnatural temporal and spatial distributions of wall shear stress in the anastomosis of distal bypass grafts have been identified as possible factors in the development of anastomotic intimal hyperplasia in these grafts. Distal bypass graft anastomoses with an autologus vein cuff (a Miller cuff) interposed between the graft and artery have been shown to alleviate the effects of intimal hyperplasia. In this study, pulsatile flow through models of a standard end-to-side anastomosis and a Miller cuff anastomosis are computed and the resulting wall shear stress and pressure distributions analysed. The results are inconclusive, and could be taken to suggest that the unnatural distributions of shear stress that do occur along the anastomosis floor may not be particularly important in the development of intimal hyperplasia. However, it seems more likely that the positive effects of the biological and material properties of the vein cuff, which are not considered in this study, somehow outweigh the negative effects of the shear stress distributions predicted to occur on the floor of the Miller-cuff graft.

Anastomosis, Surgical↗
PubMed1989

Flow/velocity characteristics of arterial bypass stenoses.

In the carotid system with relatively constant blood flow, peak systolic velocity within a stenosis (PSVST) can characterize the degree of hemodynamic stenosis. We have studied flow/velocity characteristics in an in vitro model of stenosis within conduits of varying diameters in an attempt to quantify the degree of stenosis from flow/velocity profiles in peripheral vein bypasses. A Harvard pulsatile flow pump (70 BPM) pumped human blood (HCT, 35-45%) through thin-walled polytetrafluoroethylene (3-6 mm in i.d.) into a variable peripheral resistance maintaining a constant mean blood pressure of 80 mm Hg over a flow range of 0-500 ml/min. A Diasonics DRF400 duplex scanner with a 10-MHz imager and 4.5-MHz Doppler probe was used to image and Doppler the conduits and measure flow through them. Validation of Doppler flow measurements (DF) was performed comparing them with flow measured (MF) by timed collection. PSVST within and pressure drop across a 50% stenosis was measured for each of the conduit's sizes over a range of 0-500 ml/min MF. The results show a good correlation between DF and MF (r = 0.99, P less than 0.001) for the whole range of internal diameters. In each 50% stenosed conduit, PSVST correlated well with MF (r = 0.95, P less than 0.001). Curves were constructed of MF vs PSVST for each 50% stenosed conduit. We conclude that measurement of volumetric flow, conduit diameter, and peak systolic velocity within a vein bypass can objectively predict bypass stenoses of 50% or greater.

Arterial Occlusive Diseases↗
PubMed2006

Computational investigation of subject-specific cerebrospinal fluid flow in the third ventricle and aqueduct of Sylvius.

The cerebrospinal fluid flow in the third ventricle of the brain and the aqueduct of Sylvius was studied using computational fluid dynamics (CFD) based on subject-specific boundary conditions derived from magnetic resonance imaging (MRI) scans. The flow domain geometry was reconstructed from anatomical MRI scans by manual image segmentation. The movement of the domain boundary was derived from MRI brain motion scans. Velocimetric MRI scans were used to reconstruct the velocity field at the inferior end of the aqueduct of Sylvius based on the theory of pulsatile flow in pipes. A constant pressure boundary condition was assigned at the foramina of Monro. Three main flow features were observed: a fluid jet emerging from the aqueduct of Sylvius, a moderately mobile recirculation zone above the jet and a mobile recirculation below the jet. The flow in the entire domain was laminar with a maximum Reynolds number of 340 in the aqueduct. The findings demonstrate that by combining MRI scans and CFD simulations, subject-specific detailed quantitative information of the flow field in the third ventricle and the aqueduct of Sylvius can be obtained.

Adult↗