Increase In Pump Speed Research Articles

Detecting performance degradation in a dead-ended hydrogen-oxygen proton exchange membrane fuel cell used for an unmanned underwater vehicle

Hydrogen-oxygen proton exchange membrane fuel cells (HO-PEMFCs) play a crucial role in the unmanned underwater vehicle (UUV) transportation field due to their distinct advantages of zero emission and low infrared radiation intensity. Effective water management is essential for the stable operation and performance output of HO-PEMFC. However, the vital role of increasing gas velocity to mitigate performance issues and catalyst degradation during prolonged fuel cell operation is frequently overlooked. The objective of this study is to systematically enhance both catalyst durability and overall system efficiency in dead-ended HO-PEMFCs by manipulating cathode flow velocities. The research findings demonstrate that a recirculation pump speed of 200 rpm (corresponding to a gas flow rate of approximately 0.4 m/s) can enhance cell performance by 58.39% at a current density of 1200 mA cm−2 compared to the dead-ended mode. With the gradual increase in pump speed, the flow pattern of liquid water at the cathode outlet transitions from film flow to droplet flow. Moreover, a weight coefficient is proposed to characterize the Pt oxidation states. Electrochemical and morphological characterization show that the increased gas flow rate mitigates water accumulation at the fuel cell outlet. In addition, the degradation of the catalyst due to dehydration of the membrane in the reactant gas inlet area is also mitigated by exhaust gas recirculation.

Longitudinal analysis of pump parameters over long-term support with the HeartMate 3 left ventricular assist device.

Recurrence of heart failure emerged as the main cause of long-term mortality in patients implanted with the HeartMate 3 (HM3) left ventricular assist device (LVAD). We aimed at deriving a possible mechanistic rationale of clinical outcomes and analyzed longitudinal changes in pump parameters over prolonged HM3 support to investigate long-term effects of pump settings on left ventricular mechanics. Data on pump parameters (i.e. pump speed, estimated flow, and pulsatility index) were prospectively recorded in consecutive HM3 patients following postoperative rehabilitation (baseline) and then at 6, 12, 24, 36, 48, and 60 months of support. Data of 43 consecutive patients were analyzed. Pump parameters were set according to regular patients' follow-up, including clinical and echocardiographic assessment. We recorded a significant progressive increase in pump speed over the course of support: from 5200 (5050-5300) rpm at baseline to 5400 (5300-5600) rpm at 60 months of support ( P = 0.0007). Consistently with the increase in pump speed, a significant increase in pump flow ( P = 0.007) and a decrease in pulsatility index ( P = 0.005) were also recorded. Our results reveal unique features of the HM3 on left ventricular activity. The need for progressive increase in pump support suggests indeed a lack of recovery and worsening of left ventricular function, which emerge as a possible mechanistic rationale of heart failure related mortality in HM3 patients. New algorithms to optimize pump settings should be envisioned to further improve LVAD-LV interaction and, ultimately, clinical outcomes in the HM3 population. https://clinicaltrials.gov/ct2/show/NCT03255928. NCT03255928.

Right Ventricular Failure and Left Ventricular Transmural Pressure as Predictors of Successful Left Ventricular Unloading in Patients with HM 3 CF-LVAD

<h3>Purpose</h3> To investigate whether right ventricular failure was associated with impaired left ventricular (LV) unloading during Left Ventricular Assist Device (LVAD) speed optimization. <h3>Methods</h3> Patients with Heartmate (HM) 3 LVADs implanted between January 2016 and April 2021 who had undergone speed optimization using right heart catheterization within one year from the implant were identified. We included patients with inadequate LV unloading as demonstrated by a pulmonary capillary wedge pressure (PCWP) above 12mmHg. We grouped patients based on the presence of right ventricular (RV) failure, defined as a right atrial pressure (RAP) to PCWP ratio >0.8. We also studied the impact of left ventricular transmural pressure (LVTMP) as a surrogate for true LV preload. Mean changes with increased pump speed (per 100 RPM) in PCWP were determined by the generalized linear mixed model. Line plots were used to present the slope of PCWP. All the analyses were performed on Stata version 17.0 (StataCorp LLC, College Station, TX, USA). A p-value of <0.05 was considered statistically significant. <h3>Results</h3> Thirty-six patients were included in this study. The median age was 52.5±11.8, 86.1% were male, 77.8% were implanted as destination therapy, and the median number of RPM changes was 3.5. Patients with RA/PCWP>0.8 had a trend towards lower decrease in PCWP with pump speed increase compared to RA/PCWP ≤0.8 (-0.57 vs -0.72, P=0.06, Figure 1A). Reduced LVTMP, defined as PCWP-RAP ≤4 mmHg, was significantly associated with a decreased slope of PCWP reduction with LVAD speed (-0.44 vs -0.96, p=0.02, Figure 1B). <h3>Conclusion</h3> In patients with RV failure the unloading of the LV is impaired, likely due to reduced LVTMP which is a surrogate for true LV preload. The presence of RV failure should be taken into account in the optimization of the LV pressures with speed optimization in HM3 patients.

Optimal Mechanical Unloading in Left Ventricular Assist Device Recipients Relates to Progressive Up-Titration in Pump Speed

Left ventricular (LV) assist devices (LVADs) are known to elicit reverse remodeling by mechanically unloading the left ventricle. Current guidelines target a reduction in LV end-diastolic diameter (LVEDD) of15% compared with pre-LVAD dimensions; however, there is significant heterogeneity in the degree of unloading achieved. We sought to investigate factors associated with mechanical unloading at 6months of LVAD support. Data were retrospectively collected for 75 LVAD recipients at five time points: pre-LVAD, within 14days post-LVAD, and at 1, 3, and 6months post-LVAD. The percentage change in LVEDD between the pre-LVAD and 6months post-LVAD time points was termed ΔLVEDD. Optimal LV unloading was defined as ΔLVEDD of ≥15% at 6months. Patients who achieved optimal unloading (group A, n=30) were compared with patients who did not (group B, n=45). At 6months, optimally unloaded patients (group A) demonstrated higher fractional shortening (15%±10% vs 10%±7%, P=.007), lower rates of moderate or severe mitral regurgitation (10% vs 33%, P=.02), and lower pulmonary capillary wedge pressure (9±4 vs 16±7mm Hg, P=.02). Right ventricular dysfunction was more prevalent at 6months in poorly unloaded (group B) patients (73% vs 43%, P=.008). Between hospital discharge and 6months, the percentage increase in pump speed (Δ revolutions per minute) washigher in group A patients (4.4%±3.7% vs 0.1%±2.6%, P<.001). In a multivariate analysis, Δ revolutions per minute and tricuspid annular systolic velocity (S') at 6months were independently associated with 6-month ΔLVEDD. Recipients of LVADs who undergo progressive pump speed up-titration during outpatient follow-up are more likely to sustain optimal LV unloading. Progressive LVAD-related right ventricular failure is prevalent in suboptimally unloaded patients.

Individual drive of internal combustion engine lubrication system based on switched reluctance motor

The article analyses the modern lubrication systems for internal combustion engines. Systems with mechanical drive components that contain mechanical and electronic components have been found to have a number of disadvantages. In particular, when the internal combustion engine is started cold, when the viscosity of the oil is high, the hydrodynamic resistance characteristic rises sharply, which leads to high pressure at low speeds and the drive requires low pump speeds. Again, the increase in oil temperature causes a decrease in viscosity, the hydrodynamic resistance characteristic becomes flatter. This, in turn, reduces the pressure in the lubrication system and requires an increase in pump speed in order to keep the pressure constant. Based on the analysis, the requirements for lubrication systems are formulated and a separate lubrication system with forced oil supply is proposed in this paper. For the drive of pump lubrication system of the internal combustion engine, a switched reluctance motor is proposed and calculated. Such motor by its qualities is one of the most useful in this type of systems.

Investigation on dynamic characteristics of a plate-type discharge valve in a diaphragm pump for SCR system by two-way FSI model

A plate-type port valve with rubber valve plate in a miniature diaphragm pump is presented. To investigate the dynamic characteristics of this type of discharge valve, a two-way fluid structure interaction model is proposed. The interaction between the dynamic behavior of the fluid and rubber is considered in the fluid structure interaction model. Based on the fluid structure interaction model, the internal flow of the pump, the deflection of the diaphragm and discharge valve plate is calculated. To verify the validity of the numerical model, a prototype pump is fabricated and tested. The experimental pressures in the working chamber of the pump show the same overall trends with the numerical results. The deviations between the numerical and experimental flowrates are less than 7.2%. The experimental results prove that the numerical model is effective in predicting a complete discharge process of the pump. There is a big difference between the deflection of the center of the valve plate and the edge of the valve plate. The oscillation period of the pressure in the working chamber of the pump is approximately double that of the discharge valve plate. When the pump speed is lower than 2500 r/min, it has little influence on the lag angles of the discharge valve under rated pressure. The lag angles at rated pump speed increase when the backpressures increase. The stress of the discharge valve plate reaches a peak when the valve plate impact on the valve limiter or valve seat.

Comprehensive effects of left ventricular assist device speed changes on alveolar gas exchange, sleep ventilatory pattern, and exercise performance.

Increasing left ventricular assist device (LVAD) pump speed according to the patient's activity is a fascinating hypothesis. This study analyzed the short-term effects of LVAD speed increase on cardiopulmonary exercise test (CPET) performance, muscle oxygenation (near-infrared spectroscopy), diffusion capacity of the lung for carbon monoxide (Dlco) and nitric oxide (Dlno), and sleep quality. We analyzed CPET, Dlco and Dlno, and sleep in 33 patients supported with the Jarvik 2000 (Jarvik Heart Inc., New York, NY). After a maximal CPET (n = 28), patients underwent 2 maximal CPETs with LVAD speed randomly set at 3 or increased from 3 to 5 during effort (n = 15). Then, at LVAD speed randomly set at 2 or 4, we performed (1) constant workload CPETs assessing O2 kinetics, cardiac output (CO), and muscle oxygenation (n = 15); (2) resting Dlco and Dlno (n = 18); and (3) nocturnal cardiorespiratory monitoring (n = 29). The progressive pump speed increase raised peak volume of oxygen consumption (12.5 ± 2.5 ml/min/kg vs 11.7 ± 2.8 ml/min/kg at speed 3; p = 0.001). During constant workload, from speed 2 to 4, CO increased (at rest: 3.18 ± 0.76 liters/min vs 3.69 ± 0.75 liters/min, p = 0.015; during exercise: 5.91 ± 1.31 liters/min vs 6.69 ± 0.99 liters/min, p = 0.014), and system efficiency (τ = 65.8 ± 15.1 seconds vs 49.9 ± 14.8 seconds, p = 0.002) and muscle oxygenation improved. At speed 4, Dlco decreased, and obstructive apneas increased despite a significant apnea/hypopnea index and a reduction of central apneas. Short-term LVAD speed increase improves exercise performance, CO, O2 kinetics, and muscle oxygenation. However, it deteriorates lung diffusion and increases obstructive apneas, likely due to an increase of intrathoracic fluids. Self-adjusting LVAD speed is a fascinating but possibly unsafe option, probably requiring a monitoring of intrathoracic fluids.

LVAD Pump Speed Increase is Associated with Improvement of Cardiac Output, O2 Kinetic and O2 Muscle Extraction

LVAD Pump Speed Increase is Associated with Improvement of Cardiac Output, O2 Kinetic and O2 Muscle Extraction

LVAD Pump Speed Increase is Associated With Increased Peak VO2 and Lower Cardiogenic Exercise Limitation

In patients affected by hearth failure (HF) one of the major determinants of quality of life is functional limitation during effort. Oxygen uptake at peak exercise (VO2 peak) assessed through cardiopulmonary test (CPET) is nowadays a strong prognostic predictor in HF and it is also commonly used to monitor patients’ functional status. As per present literature, a change of speed in left ventricular assistance device (LVAD) at rest causes a variation in CO, but is not related to better exercise capacity. In the present analysis, we tested the hypothesis that the change of pump speed during exercise in patients wearing LVAD might be associated with a better exercise performance.

Lvad pump speed increase is associated with increased peak exercise cardiac output and vo2, postponed anaerobic threshold and improved ventilatory efficiency

BackgroundsPeak exercise cardiac output (CO) increase is associated with an increase of peak oxygen uptake (VO2), provided that arteriovenous O2 difference [Δ(Ca−Cv)O2] does not decrease. At anaerobic threshold, VO2, is related to CO.We tested the hypothesis that, in heart failure (HF) patients with left ventricular assistance device (LVAD), an acute increase of CO obtained through changes in LVAD pump speed is associated with peak exercise and anaerobic threshold VO2 increase. MethodsFifteen of 20 patients bearing LVAD (Jarvik 2000) enrolled in the study successfully performed peak exercise evaluation. All patients had severe HF as shown by clinical evaluation, laboratory tests, echocardiography, spirometry with alveolar-capillary diffusion, and maximal cardiopulmonary exercise testing (CPET). CPETs with non-invasive CO measurements at rest and peak exercise were done on 2days at LVAD pump speed set randomly at 2 and 4. ResultsIncreasing LVAD pump speed from 2 to 4 increased CO from 3.4±0.9 to 3.8±1.0L/min (ΔCO 0.4±0.6L/min, p=0.04) and from 5.3±1.3 to 5.9±1.4L/min (ΔCO 0.6±0.7L/min, p<0.01) at rest and peak exercise, respectively. Similarly, VO2 increased from 788±169 to 841±152mL/min (ΔVO2 52±76mL/min, p=0.01) and from 568±116 to 619±124mL/min (ΔVO2 69±96mL/min, p=0.02) at peak exercise and at anaerobic threshold, respectively. Δ(Ca−Cv)O2 did not change significantly, while ventilatory efficiency improved (VE/VCO2 slope from 39.9±5.4 to 34.9±8.3, ΔVE/VCO2 −5.0±6.4, p<0.01). ConclusionsIn HF, an increase in CO with a higher LVAD pump speed is associated with increased peak VO2, postponed anaerobic threshold, and improved ventilatory efficiency.

Pump speed modulations and sub-maximal exercise tolerance in left ventricular assist device recipients: A double-blind, randomized trial

The effect of pump speed increase on sub-maximal exercise tolerance, corresponding to activities of daily living (ADLs), is unknown. The aim of this study was to determine the effects of increasing pump speed during exercise at a sub-maximal level below anaerobic threshold (AT). Patients each completed 3 exercise sessions on an ergometer cycle. On Day 1 workload at AT was defined. On Day 2 of the study, 2 sub-maximal tests at a workload below AT were undertaken: one at fixed baseline pump speed (Speedbase) and the other with baseline pump speed + 800 rpm (Speedinc). The sequence of the 2 sub-maximal tests was determined by randomization. Both patient and physician were blinded to the sequence. Exercise duration, oxygen consumption (VO2) and rate of perceived exertion (RPE), using the Borg scale (score 6 to 20), were recorded. Nineteen patients (all with a HeartMate II ventricular assist device) completed 57 exercise tests. Baseline pump speed was 9,326 ± 378 rpm. At AT, workload was 63 ± 26 W (25 to 115 W) and VO2 was 79 ± 14% of maximum. Exercise duration improved by 106 ± 217 seconds (~13%) in Speedinc compared with Speedbase (837 ± 358 vs 942 ± 359 seconds; p = 0.048). The RPE was 13.2 ± 2.5 in Speedbase vs 12.7 ± 2.4 in Speedinc (p = 0.2). Increasing pump speed by 800 rpm during sustained, low-intensity physical activity is safe and prolongs exercise duration in patients supported with a HeartMate II device. Automated pump speed increase during light exercise may contribute to improved quality of life by facilitating ADLs.

Effect of exercise and pump speed modulation on invasive hemodynamics in patients with centrifugal continuous-flow left ventricular assist devices

Continuous-flow left ventricular assist devices (CF-LVADs) improve functional capacity in patients with end-stage heart failure. Pump output can be increased by increased pump speed as well as changes in loading conditions. The effect of exercise on invasive hemodynamics was studied in two study protocols. The first examined exercise at fixed pump speed (n = 8) and the second with progressive pump speed increase (n = 11). Patients underwent simultaneous right-heart catheterization, mixed venous saturation, echocardiography and mean arterial pressure monitoring. Before exercise, a ramp speed study was performed in all patients. Patients then undertook symptom-limited supine bicycle exercise. Upward titration of pump speed at rest (by 11.6 ± 8.6% from baseline) increased pump flow from 5.3 ± 1.0 to 6.3 ± 1.0 liters/min (18.9% increase, p < 0.001) and decreased pulmonary capillary wedge pressure (PCWP; 13.6 ± 5.4 to 8.9 ± 4.1 mm Hg, p < 0.001). Exercise increased pump flow to a similar extent as pump speed change alone (to 6.2 ± 1.0 liters/min, p < 0.001), but resulted inincreased right- and left-heart filling pressures (right atrial pressure [RAP]: 16.6 ± 7.5 mm Hg, p < 0.001; PCWP 24.8 ± 6.7 mm Hg, p < 0.001). Concomitant pump speed increase with exercise enhanced the pump flow increase (to 7.0 ± 1.4 liters/min, p < 0.001) in Protocol 2, but did not alleviate the increase in pre-load (RAP: 20.5 ± 8.0 mm Hg, p = 0.07; PCWP: 26.8 ± 12.7 mm Hg; p = 0.47). Serum lactate and NT-proBNP levels increased significantly with exercise. Pump flow increases with up-titration of pump speed and with exercise. Although increased pump speed decreases filling pressures at rest, the benefit is not seen with exercise despite concurrent up-titration of pump speed.

Systemic perfusion at peak incremental exercise in left ventricular assist device recipients: Partitioning pump and native left ventricle relative contribution

BackgroundIn continuous-flow left ventricular assist device (LVAD) recipients, little is known about the relative pump- and left ventricle-generated blood flow (PBF and LVBF, respectively) contribution to peak systemic perfusion during incremental exercise and about how PBF/LVBF interplay and exercise capacity may be affected by pump speed increase. MethodsTwenty-two LVAD recipients underwent ramp cardiopulmonary exercise tests at fixed and increasing pump speed (+ 1.5% of baseline speed/10 W workload increase), echocardiography and NT-proBNP dosage. Peak systemic perfusion was peak VO2/estimated peak arterio-venous O2 difference, and LVBF was systemic perfusion minus PBF provided by LVAD controller. A change of peak percentage of predicted VO2max (Δpeak%VO2) ≥ 3 in increasing- vs. fixed-speed test was considered significant. ResultsTricuspid annular plane systolic excursion (TAPSE) and NT-proBNP were significantly lower and higher, respectively, in Δpeak%VO2 < 3 than ≥ 3. A LVBF contribution to systemic perfusion significantly larger than that of PBF was observed in Δpeak%VO2 ≥ 3 vs. < 3 in fixed-speed test, which was further amplified in increasing-speed test (2.4 ± 1.7 l/min vs. 2.0 ± 1.5 l/min and 0.8 ± 2.2 l/min vs. 1.3 ± 2.3 l/min, respectively, p for trend < 0.0005). Among several clinical-instrumental parameters, logistic regression selected only TAPSE > 13 mm as a predictor of Δpeak%VO2 ≥ 3. ConclusionsA significant LVBF contribution to peak systemic perfusion and pump speed increase-induced peak VO2 improvement was detectable only in patients with a more preserved right ventricular systolic function and stable hemodynamic picture. These findings should be taken into consideration when designing LVAD controllers aiming to increase pump speed according to increasing exercise demands.

Acoustic analysis of a mechanical circulatory support.

Mechanical circulatory support technology is continually improving. However, adverse complications do occur with devastating consequences, for example, pump thrombosis that may develop in several parts of the pump system. The aim of this study was to design an experimental clot/thrombosis model to register and analyze acoustic signals from the left ventricular assist device (LVAD) HeartMate II (HMII) (Thoratec Corporation, Inc., Pleasanton, CA, USA) and detect changes in sound signals correlating to clots in the inflow, outflow, and pump housing. Using modern telecom techniques, it was possible to register and analyze the HMII pump-specific acoustic fingerprint in an experimental model of LVAD support using a mock loop. Increase in pump speed significantly (P < 0.005) changed the acoustic fingerprint at certain frequency (0–23 000 Hz) intervals (regions: R1–3 and peaks: P1,3–4). When the ball valves connected to the tubing were narrowed sequentially by ∼50% of the inner diameter (to mimic clot in the out- and inflow tubing), the frequency spectrum changed significantly (P < 0.005) in P1 and P2 and R1 when the outflow tubing was narrowed. This change was not seen to the same extent when the lumen of the ball valve connected to the inflow tube was narrowed by ∼50%. More significant (P < 0.005) acoustic changes were detected in P1 and P2 and R1 and R3, with the largest dB figs. in the lower frequency ranges in R1 and P2, when artificial clots and blood clots passed through the pump system. At higher frequencies, a significant change in dB figs. in R3 and P4 was detected when clots passed through the pump system. Acoustic monitoring of pump sounds may become a valuable tool in LVAD surveillance.

731 Acoustic Analysis of a Mechanical Circulatory Support

Mechanical circulatory support technology is continually improving. However, adverse complications do occur with devastating consequences, for example, pump thrombosis that may develop in several parts of the pump system. The aim of this study was to design an experimental clot/thrombosis model to register and analyze acoustic signals from the left ventricular assist device (LVAD) HeartMate II (HMII) (Thoratec Corporation, Inc., Pleasanton, CA, USA) and detect changes in sound signals correlating to clots in the inflow, outflow, and pump housing. Using modern telecom techniques, it was possible to register and analyze the HMII pump-specific acoustic fingerprint in an experimental model of LVAD support using a mock loop. Increase in pump speed significantly (P<0.005) changed the acoustic fingerprint at certain frequency (0-23,000 Hz) intervals (regions: R1-3 and peaks: P1,3-4). When the ball valves connected to the tubing were narrowed sequentially by ∼50% of the inner diameter (to mimic clot in the out- and inflow tubing), the frequency spectrum changed significantly (P<0.005) in P1 and P2 and R1 when the outflow tubing was narrowed. This change was not seen to the same extent when the lumen of the ball valve connected to the inflow tube was narrowed by ∼50%. More significant (P<0.005) acoustic changes were detected in P1 and P2 and R1 and R3, with the largest dB figs. in the lower frequency ranges in R1 and P2, when artificial clots and blood clots passed through the pump system. At higher frequencies, a significant change in dB figs. in R3 and P4 was detected when clots passed through the pump system. Acoustic monitoring of pump sounds may become a valuable tool in LVAD surveillance.

A Dynamic Method for Setting Roller Pumps Nonocclusively Reduces Hemolysis and Predicts Retrograde Flow

In general, roller pumps are set almost occlusively despite evidence that nonocclusive settings cause less hemolysis. Almost-occlusive settings are used because of the concern that forward flow would not be accurately known if retrograde flow were allowed to occur through a nonocclusive gap. This article presents a dynamic method for setting roller pumps nonocclusively that overcomes the many difficulties of the "drop method" for setting occlusion. Studies were conducted to determine the effect of nonocclusive settings on pump flow and hemolysis generated; the results suggest that roller pumps can and should be set more nonocclusively than is the currently accepted standard to reduce pump related hemolysis without greatly affecting pump performance. The dynamic method allows retrograde flow to be easily predicted and corrected with an increase in pump speed.

ベーンポンプの摩擦トルク特性

A vane pump is being widely used in hydraulic systems. The vane pump for automotive power steering should be compact in size, low in cost and to have low frictional torque for savings in energy. The power steering pump is used at a wide range of pump speeds, pressures and oil temperatures. Although such working conditions affect the friction torque, its characteristics have yet to be revealed.Influences of working conditions such as pump speed, pressure and oil temperature regarding the friction torque characteristics of the vane pump have been investigated in this research. As a result, it became clear that friction torque in the high pressure range reduces with an increase in pump speed when the pump speed is less than 1000 rpm. Also, the friction torque at high temperatures is higher than that at low temperatures when the pressure is high. This tendency is quite the opposite when the pressure is low. On the basis of these experimental results, a mathematical model for the friction torque could be constructed. The agreement between the calculated values and the experimental ones was excellent.In addition, it could be verified that the larger the vane stroke and the thinner the vane thickness, the less the friction torque.

Reversibility of diaphragm fatigue by mechanical hyperperfusion.

Diaphragm function is thought to depend on the balance between diaphragm blood flow and metabolic demand. If blood flow is inadequate, fatigue should ensue. Likewise, in the presence of fatigue, function should improve when blood flow is increased. To test this hypothesis, we evaluated the effect of increasing blood flow to the fatigued diaphragm. Studies were performed on anesthetized, mechanically ventilated dogs in which strips of costal diaphragm were developed in situ. Strip tension was measured with an isometric tension transducer. The inferior phrenic artery supplying the strip was cannulated and pump perfused at a pressure of 92 +/- 3 mm Hg; phrenic artery flow and pressure were continuously monitored with in-line doppler flow probes and pressure transducers, respectively. Fatigue was produced by electrically stimulating strips to contract 15 times/min (initial tension 80% of maximum, duty cycle 50%). Rhythmic contraction resulted in a downward shift in the diaphragm force-frequency relationship in all strips. In eight strips, stepwise increments in phrenic artery perfusion pressure to 161 +/- 8 and 281 +/- 17 mm Hg were produced by increasing pump speed at 6 and 8 min into rhythmic stimulation; in four strips, phrenic artery perfusion pressure was increased to 152 +/- 20 and 257 +/- 32 mm Hg at 20 and 25 min into rhythmic stimulation, respectively. Each increase in phrenic artery pressure resulted in increases in phrenic artery flow and diaphragm tension and produced an upshift in the diaphragm force-frequency relationship.(ABSTRACT TRUNCATED AT 250 WORDS)