Duration Of Reverse Flow Research Articles

Effect of nucleation frequency and compressible buffer volume on the thermofluidic characteristics for flow boiling through a microchannel heat sink

A 1-D transient model of flow boiling in a rectangular microchannel is developed to assess the pressure drop and heat transfer characteristics, particularly emphasizing the impacts of nucleation frequency and compressible buffer volume. The combined influence of nucleation frequency with channel height, inlet velocity and heat flux is also studied. The increased nucleation frequency reduces the pressure drop amplitude and enhances the heat transfer coefficient. As the buffer volume is increased, the pressure drop and heat transfer coefficient are reduced due to the predominant occurrence of reverse flow in the channel. The amplitude and duration of reverse flow are identified as two important key parameters. As the reverse flow strongly affects the thermal-hydraulics of the channel, a sensitivity analysis is performed to assess the variation of these two key parameters over a range of operating conditions. The variation of amplitude and duration of reverse flow strongly depends on the trends of channel pressure drop, which is the consequence of the competing effects of bubble growth rate and reverse flow. Lastly, the effect of variable vapor properties on pressure drop and heat transfer characteristics of the channel has been discussed.

Safety and feasibility of a novel transcervical access neuroprotection system for carotid artery stenting in the PROOF Study

Randomized controlled trials have shown that periprocedural rates of stroke and death are higher with carotid artery stenting (CAS) than with carotid endarterectomy (CEA) in the treatment of carotid artery stenosis. Diffusion-weighted magnetic resonance imaging (DW-MRI) has shown higher rates of clinically silent new ischemic brain lesions when CAS is performed as compared with CEA. The Silk Road Medical Embolic PROtectiOn System: First-In-Man (PROOF) Study is a single-arm first-in-man study using the MICHI Neuroprotection System (Silk Road Medical Inc, Sunnyvale, Calif), a novel transcervical access and cerebral embolic protection system. This system enables stent implantation under controlled blood flow reversal of the carotid artery, also known as Flow Altered Short Transcervical Carotid Artery Stenting (FAST-CAS). Between March 2009 and February 2010, a total of 44 subjects were enrolled into the study. The primary composite endpoint was major stroke, myocardial infarction, or death within 30 days. Forty-three patients (97.7%) completed the study through the 30-day endpoint. One patient was lost to follow-up. In a subgroup of consecutive subjects, DW-MRI examinations were performed preprocedure and within 24 to 48 hours after the stent implantation. Blinded independent neuroradiologists reviewed all DW-MRI studies and confirmed the absence or presence of new ischemic brain lesions. All enrolled patients were successfully treated, and no major adverse events were seen through the follow-up period. Thirty-one subjects had DW-MRI examinations. Of these, five patients (16%) had evidence of new ischemic brain lesions but no clinical sequelae. Transient intolerance to reverse flow was reported in 9% of cases, but in all cases, a stent was successfully placed, and the intolerance was managed by minimizing the duration of reverse flow during the procedure. In this first-in-man experience, FAST-CAS using the MICHI Neuroprotection System was shown to be a safe and feasible method for carotid revascularization. DW-MRI findings suggest controlled reverse flow provides cerebral embolic protection similar to that seen with CEA.

Duplex ultrasonography for assessment of venous valvular function of the lower limb.

Both legs of 29 patients with venous disease and those of 15 controls without venous disease were assessed by duplex ultrasonography. The duration of reverse flow after release of manual calf compression was measured in the common femoral, long saphenous, popliteal and short saphenous veins. Before undertaking the study, the reproducibility of the technique was evaluated in six subjects by repeating the examination over 3 consecutive days; the coefficient of variation of the test was 7.3 per cent. The 95 per cent confidence interval (c.i.) of the median (0.16 s) of all measurements in the normal limbs was 0.12-0.18 s. The 95 per cent c.i. for the 95th percentile of all measurements in normal limbs was 0.32-0.52 s. In limbs with clinical evidence of venous disease at least one of the sites examined was found to have reverse flow lasting longer than 0.5 s. These data suggest that the measurement of reverse flow after release of manual calf compression is a reproducible technique. While the method records some reverse flow in normal veins, its duration is unlikely to exceed 0.5 s; significant reflux is therefore defined as reverse flow exceeding 0.5 s.

The Effect of Hemodialysis on the Echocardiographic Indexes of Left Ventricular Diastolic Function in Chronic Renal Failure

Background and Objectives:The assessment of left ventricular (LV) diastolic function is important in chronic renal failure because abnormal LV diastolic function has been frequently described in patients on maintenance hemodialysis both during the dialysis and in the dialysis-free interval despite the normal LV systolic function. But the echocardiographic indexes of LV diastolic function is known to be affected by several factors such as loading condition, LV compliance and heart rate. The purpose of this study is to investigate the effect of hemodialysis on the echocardiographic indexes of left ventricular diastolic function in chronic renal failure. Materials and Methods:We examined transmitral flow velocity, pulmonary venous flow velocity, and mitral annulus velocity in 20 patients (15 men and 5 women, average 50±14, range 19-69 years) of chronic renal failure with normal LV systolic function by echocardiography before and after hemodialysis. Results: 1) According to the body weight change (from 59.5±8.3 to 57.2±8.1 kg, p=0.0001), after hemodialysis, inferior vena cava dimension (from 18±4 to 13±5 cm, p=0.0001), left ventricular end-diastolic dimension (from 57±6 to 53±7 cm, p=0.0001), and left ventricular outflow tract (LVOT)-time velocity integral (TVI, from 26±5 to 23±5 cm, p=0.004), which reflect intravascular blood volume, decreased significantly. 2) The peak velocity of early transmitral flow (E, from 0.79±0.14 to 0.64±0.11 m/s, p=0.0001), the peak velocity of late transmitral flow (A, from 0.84±0.21 to 0.78±0.21 m/s, p=0.011), and E/A ratio (from 0.99±0.25 to 0.87±0.27, P=0.007) decreased significantly, and deceleration time (DT, from 241±48 to 267±59 ms, p= 0.055) showed tendency of prolongation after hemodialysis. 3) Peak systolic velocity of pulmonary venous flow decreased significantly after hemodialysis (from 0.65±0.11 to 0.59±0.12 m/s, p=0.042). 4) The difference between duration of reversal flow of pulmonary vein and duration of transmitral flow during atrial contraction (ADD) did not change significantly after hemodialysis (from 5±31 to 1±29 ms, p=0.502), and did not correlate with the change of peak velocity of early transmitral flow during hemodialysis (DMVE, r= 0.390, p=0.089). 5) The peak early diastolic velocity (Ean, from 0.07±0.02 to 0.06±0.02 m/s, p=0.002)and Ean/the peak late diastolic velocity (Aan) ratio (from 0.78±0.27 to 0.62±0.19, p=0.003) of medial annulus 논문접수일:1998년 12월 19일 심사완료일:1999년 4월 2일 교신저자:박성훈, 158-050 서울 양천구 목동 911-1 이화여자대학교 의과대학 내과학교실 전화:(02) 650-5018·전송:(02) 650-2076 E-mail:PSH7935@chollian.net

Retrograde flow in the deep veins of subjects with normal venous function

Duplex ultrasonography has become the optimal non-invasive method of assessing the deep venous system of the leg. The stimuli used to promote reverse flow, subject positioning, and the veins scanned have varied between studies. Reverse flow of less than 0.5 s is accepted as 'normal', however, all published studies have demonstrated 'normal' subjects with deep vein reverse flow exceeding this time. The aim of this study was to establish the duration of reverse flow in all deep vein segments, and to determine the optimal method of eliciting reverse flow in individuals with proven normal deep vein function. Duplex scanning was used to detect the presence and duration of reverse flow in the superficial femoral, popliteal and posterior tibial veins of 61 legs with normal venous function. The veins were scanned with each subject inclined at 10 degrees, 45 degrees and standing erect. The abrupt release of distal tourniquet compression and the Valsalva manoeuvre were used as stimuli for reverse flow. There was significantly less reverse flow in distal veins than in proximal veins when the subject was scanned at 10 degrees and 45 degrees. Median (range) reverse flow in the femoral vein at 10 degrees was 0.21 (0-1.18) s, posterior tibial vein 0.08 (0-0.67) s, (P < 0.0001); in the femoral vein at 45 degrees was 0.14 (0.04-2.14) s, posterior tibial vein 0.07 (0-0.56) s, (P < 0.0001). The duration of reverse flow was significantly less in the superficial femoral and popliteal veins when the subjects were erect than when inclined at 10 degrees and 45 degrees. Median (range) reverse flow in the femoral vein at 10 degrees was 0.21 (0-1.18), and when standing 0.1 (0-1.36) (P < 0.0001); below-knee popliteal vein at 10 degrees was 0.15 (0.04-1.95) s, and when standing 0.10 (0-1.03) (P = 0.0004). Reverse flow of more than 0.5 s was never elicited in the posterior tibial veins if the subject stood, nor did it occur in the popliteal vein during a Valsalva manoeuvre. The optimal sites to assess deep vein reverse flow are the posterior tibial vein following removal of distal compression with the patient standing, and the popliteal vein during a Valsalva manoeuvre, as retrograde flow more than 0.5 s does not occur in subjects with normal venous function.

Feasibility of obtaining pulmonary venous flow velocity in cardiac patients using transthoracic pulsed wave Doppler technique

The purpose of this study was to determine, in an adult population, the percentage of patients in whom high quality pulmonary venous flow velocity recordings can be obtained using current transthoracic pulsed wave Doppler techniques. Pulmonary venous and mitral flow velocity variables obtained with a pulsed wave Doppler method were used for the indirect assessment of left ventricular (LV) diastolic function and LV filling pressures. The general clinical use of these methods, however, remains uncertain because the transthoracic success rate of obtaining all components of pulmonary venous flow velocity has been variable, and sometimes reported to be as low as 30% to 60%. Mitral and pulmonary venous flow velocity variables were obtained using pulsed wave Doppler signals in 200 consecutive adult patients (mean age 68.2 +/- 11.4 years) in normal sinus rhythm who were referred for echocardiographic study. Six cardiac sonographers and five ultrasound systems were used. The success rate for obtaining pulmonary venous systolic and diastolic flow velocity was 95%, reverse flow velocity at atrial contraction was 90%, and the duration of reverse flow at atrial contraction was 89%. In the 5% to 11% of patients in whom pulmonary flow velocities could not be adequately recorded, the most common reasons were depth limitations of the pulsed wave Doppler machine, marked cardiac enlargement, or left atrial wall motion artifact. The success rate also was influenced by the ultrasound equipment used, individual variation among sonographers, and even the type (impaired, pseudonormal, restricted) of associated mitral filling pattern. Given current machine technology, sonographer education, and daily practice, high quality, complete recordings of pulmonary venous flow velocity can be obtained in approximately 90% of adult patients using the precordial transthoracic Doppler technique. These results suggest that using these variables as an aid for evaluating LV diastolic function and filling pressures may have broader clinical applicability than previously appreciated.

Abnormalities of left ventricular filling in valvular aortic stenosis. Usefulness of combined evaluation of pulmonary veins and mitral flow by means of transthoracic Doppler echocardiography

It has been recently demonstrated that indexes obtained from the study of pulmonary venous flow by Doppler echocardiography are related to left ventricular (LV) pressures during diastole and may improve the assessment of LV filling derived from analysis of mitral flow velocities. In this study we evaluated the pattern of pulmonary venous flow and transmitral flow by means of transthoracic pulsed Doppler echocardiography in 31 adult patients (11 females, 20 males, mean age 72 ± 10 years) with valvular aortic stenosis (Doppler valve area: 0.77 ± 0.17 cm 2) and in 15 age-matched normal subjects (five females, 10 males, mean age 68 ± 6 years). Doppler indexes of mitral flow were similar between the two groups; on pulmonary venous flow, peak systolic velocity was lower (46 ± 13 vs. 63 ± 17 cm/s, P < 0.01) and both duration of reversal flow during atrial systole and difference between pulmonary atrial reversal flow and mitral A wave duration were longer in aortic stenosis patients compared to normals (148 ± 21 vs. 111 ± 16 ms and 6 ± 27 vs. −26 ± 21 ms, respectively, P < 0.001). Twenty-two aortic stenosis patients showed an early to late mitral flow ratio (E/A) ≤ 1 and the remaining nine patients had a E/A > 1. Patients with E/A ≤ 1 showed, compared to those with E/A > 1, a significant prolongation of deceleration time of early filling (242 ± 76 vs. 127 ± 32 ms, P < 0.001) and, on pulmonary venous flow, a higher peak systolic and lower peak diastolic velocity (52 ± 9 vs. 33 ± 12 cm/s and 37 ± 8 vs. 56 ± 10 cm/s, respectively, P < 0.001), determining a significantly lower diastolic to systolic velocity ratio (D/S: 0.7 ± 0.14 vs. 1.9 ± 0.9, P < 0.001). Also, the duration difference between pulmonary atrial reversal flow and mitral atrial systole wave was significantly different between the two groups (−6 ± 21 ms in patients with E/A < 1 vs. 35 ± 15 ms in those with E/A > 1, P < 0.001); furthermore, this difference was negative in 15 and positive in seven patients with E/A < 1, while it was always positive in patients with E/A > 1. All patients with E/A > 1 had symptoms of congestive heart failure, often severe and associated to a reduction of LV systolic function. In the other group, nine out of 22 patients had heart failure; it was usually mild and associated to a preserved LV systolic function. In conclusion, in patients with valvular aortic stenosis the combined evaluation of pulmonary venous and mitral flow may allow the recognition of different patterns of LV filling; they probably reflect different values of LV filling pressure and may contribute to the noninvasive identification of the hemodynamic profile in the single patient.

Influence of heart rate and vasoactive drugs on blood flow patterns at the canine ilio-femoral bifurcation

The aim was to study the effects of altered heart rate and vasoactive drugs on the blood velocity patterns in the region of an arterial bifurcation. Blood velocity profiles were measured in an exposed iliofemoral bifurcation of paced dogs using a pulsed Doppler ultrasound velocimeter with high temporal and spatial resolution. Decrease of the heart rate from 120 beats.min-1 (2 Hz) to 60 beats.min-1 (1 Hz) increased the peak forward velocity (30%), the peak reverse velocity (20%), and the duration of reverse flow (25%). Each drug caused qualitatively similar changes in velocity patterns at both heart rates. The systemic administration of angiotensin II reduced peak forward velocity (-26% at 2 Hz and -33% at 1 Hz) and forward flow duration (-15% at 1 Hz), the peak reverse velocity (-30% at 1 Hz), and reverse flow duration (-20% at 2 Hz and -28% at 1 Hz). Glyceryl trinitrate also reduced the peak forward velocity (-19% at both 2 and 1 Hz) but prolonged forward flow duration (28% at 2 Hz and 17% at 1 Hz) and that of reverse flow (45% at 2 Hz and 24% at 1 Hz), and also decreased the degree of oscillation (-16% at 2 Hz). Barnidipine hydrochloride (a calcium channel antagonist) also increased the duration of forward flow (48% at 1 Hz) and of reverse flow (31% at 2 Hz) but reduced the peak reverse velocity (-29% at 1 Hz) and flow oscillation (-22% at 2 Hz and 20% at 1 Hz). These dramatic changes in the pattern of blood flow, including alterations in the amplitudes and durations of the different phases of the flow cycle, are expected to have important consequences on the shear dependent responses of endothelial cells in the region of the bifurcation.

Abnormal arterial flow pattern in untreated essential hypertension: Possible link with the development of atherosclerosis

1. We compared the velocity waveforms in the superficial femoral artery measured by multichannel Doppler ultrasound in 45 subjects: 21 patients with untreated essential hypertension and 24 normal subjects of similar age and sex. 2. The pattern of arterial flow was abnormal in hypertensive patients, with the acceleration time, the duration of reverse flow and the time to maximum flow reversal being abbreviated. The internal arterial diameter, calculated from the velocity profile, was reduced despite raised pressure, suggesting altered arterial wall mechanics in essential hypertension. 3. These abnormalities will influence the wall shear stress, a major determinant of arterial function. The abnormal arterial wall mechanics and abnormal blood flow pattern may contribute to the increased risk of arterial disease in patients with untreated hypertension.

Oscillatory flow patterns in patients with aortic valve disease

The directional Doppler velocity detector was used in 10 normal adult subjects and 42 patients with aortic valve disease to define the flow velocity patterns associated with each of the valvular lesions and to determine how they are altered by repair or replacement of the aortic valve. The patients with aortic stenosis and aortic stenosis plus insufficiency were found to have a much greater time interval from onset of systole to peak ejection than control subjects. The average value in patients with aortic insufficiency and those with mitral valve disease plus aortic insufficiency was slightly less than normal. This time interval decreased after surgery in all patients with aortic stenosis and aortic stenosis plus aortic insufficiency, whereas there was no consistent change in those with aortic insufficiency or with mitral valve disease plus aortic insufficiency. Reverse flow was present in the femoral arterial tracing of all control subjects, and the simultaneous occurrence of forward and reverse flow was consistently demonstrated. The degree of flow reversal was less in the brachial artery and absent in all but 1 of the radial arterial tracings of these normal individuals. The magnitude and duration of retrograde flow was greatest in the patients with aortic regurgitation, and in many of these cases it was marked in the radial artery. Reversal of flow was not present in the radial artery of patients with pure aortic stenosis. In the patients with aortic stenosis, the duration and magnitude of reverse flow tended to increase after surgery. Postoperatively there was usually a decrease in the duration of reverse flow in the group with aortic insufficiency and the group with aortic stenosis plus aortic insufficiency. The average peak reverse flow decreased after correction of aortic insufficiency but was essentially unchanged in the group with aortic stenosis plus aortic insufficiency.