Native Cardiac Output Research Articles

Control strategy for rotary blood pumps.

The control strategy for ventricular support with a centrifugal blood pump was examined in this study. The control parameter was the pump rpm that determines pump flow. Optimum control of pump rpm that reflects the body's demand is important for long-term, effective, and safe circulatory support. Moreover, continuous, reliable monitoring of ventricular function will help successfully wean the patients from the ventricular assist device (VAD). The control strategy in this study includes determination of the target pump rpm that can provide the flow required by the body, fine-rpm-tuning to minimize deleterious effects such as suction in the ventricle, and assessment of ventricular function for successful weaning from VADs. To determine the target pump rpm, we proposed to use the relation between the native heart rate and cardiac output, and the relation between the pump rpm and centrifugal pump output. For fine-tuning of the pump rpm, the motor current waveform was used. We computed the power spectral density of the motor current waveform and calculated the ratio of the fundamental to the higher order components. When this ratio was larger than approximately 0.2, we assumed there would be a suction effect in the ventricle. As for assessment of ventricular function, we used the amplitude of the motor current waveform. The control system implemented using a DSP functioned properly in the mock circulatory loop as well as in acute animal experiments. The motor current also showed a good correlation with the ventricular pressure in acute animal experiments.

PULMONARY REPLACEMENT WITH A TOTAL ARTIFICIAL LUNG

Purpose: An oxygenator with low resistance may be perfused by the native cardiac output (CO) and serve as a Total Artificial Lung (TAL). However, the absence of compliance may increase pump load in a pulsatile system. Impedance is the resistance to pulsatile flow, and is expressed as a function of harmonic. While the zero harmonic impedance (Z9) represents the resistance to steady flow, the first harmonic (Z1) dominates the load to pulsatile flow. The tension-time index (TTI) is a correlate of myocardial oxygen consumption. We sought to characterize right ventricular (RV) load and adaptation in the setting of pulmonary replacement with a TAL. Methods: Five adult male sheep were instumented with micromanometers in the RV, proximal pulmonary artery (PA), and left atrium (LA), and an ultrasonic flow probe was placed around the proximal PA. Conduits were anastamosed to the PA and LA, and connected to a TAL. Data were obtained under two conditions: 1) conduits clamped, allowing native pulmonary circulation, and 2) distal PA occluded, diverting the total RV output through the TAL. Epicardial and intracardiac echocardiographs were obtained. The pulsatility index (PI) was calculated by dividing the amplitude of flow in the first harmonic by the CO. Results: Although CO, mean PA pressure (mPAP) and resistance were unchanged (CO: 6.2 ± 1.5 vs 5.6 ± 0.9 L/min, p=NS; mPAP: 18.5 ± 3.6 vs 18.1 ± 3.2 mmHg, p=NS; Z0: 3.1 ± 0.6 vs 3.3 ± 0.5 Woods Units, p=NS), there was a substantial increase in Z1 (0.4 ± 0.1 vs 5.8 ± 1.4 Woods Units, p<0.01). This elevated impedance was associated with an increase in the TTI from 843.5 ± 113.3 to 1032.1 ± 224.3 mmHg-s/min, p=0.04. Blood flow ejected from the right ventricle became less pulsatile, reflected by a reduction in the PI from 0.64 ± 0.12 to 0.23 ± 0.09, p<0.01. Echocardiographs suggested incomplete closure of the pulmonic valve with forward diastolic flow. In the setting of pulmonary replacement with a TAL, high impedance with unchanged resistance alters cardiac function, and may have long-term effects upon the RV. Addition of a compliance chamber can reduce Z1, and may restore normal ventricular function.

EARLY EXPERIENCE WITH DRIVING PRIVILEGES FOR LVAD PATIENTS

With increasing durations of left ventricular assist device (LVAD) support, patients are pursuing greater levels of independence. To improve quality of life, we have introduced a selective approach to allow patients to drive. We have developed a protocol to test a patient's ability to enact emergency procedures within two minutes following LVAD malfunction. After instituting anticoagulation, LVAD output is decreased to a minimal rate to avoid blood stasis yet not significantly augment native cardiac output. With the TCI Heartmate® VE LVAD (Thermo Cardiosystems, Woburn, MA) this involves instituting hand pumping once every ten seconds. The patient is evaluated for signs and symptoms of inadequate cardiac output; blood pressure, heart rate and rhythm are monitored in the supine, sitting, and standing positions. Patients with a history of hemodynamically compromising arrhythmias are excluded. Five patients have been tested, all supported by TCI Heartmate® VE LVADs for an average duration of 154 days (38–251 days). Four of five have been cleared to drive. The current driving experience has been uneventful. Based on our early experiences, employing careful precautions, we have found driving to be safe and emotionally beneficial for LVAD patients.

Resting left ventricular size predicts improved native cardiac output during exercise and the ability to exercise with the device weaned in patients with the TCI heartmate left ventricular assist device: S. Maybaum, A. Beniaminovitz, M. Oz, M. Di Tullio, S. Savin, D. Mancini. Columbia-Presbyterian Medical Center, New York, New York

Resting left ventricular size predicts improved native cardiac output during exercise and the ability to exercise with the device weaned in patients with the TCI heartmate left ventricular assist device: S. Maybaum, A. Beniaminovitz, M. Oz, M. Di Tullio, S. Savin, D. Mancini. Columbia-Presbyterian Medical Center, New York, New York

Estimation of the native cardiac output from a rotary blood pump flow: in vitro study.

The rotary blood pump will be an implantable left ventricular assist device (LVAD) in the near future. However, the best control method and the interrelationship between the rotary blood pump and native heart functions are unclear. An estimation was made of the native heart cardiac output from the change of an LVAD's outflow waveform. The mock circulation loop was composed of an aortic compliance chamber, left arterial chamber, total artificial heart as a native heart, and a rotary blood pump that was placed as an LVAD with left ventricular drainage. The fast Fourier transform (FFT) technique was utilized to analyze the LVAD's outflow waveform and calculate the pulse power index (PPI) to examine a relation between the PPI and total artificial heart (TAH) output. The PPI increased with the increase of the TAH output; there was a positive correlation, and there was an inverse correlation between the PPI and the assist ratio. From this viewpoint, an estimation of the pulsatility change of the LVAD's outflow wave may indicate the native cardiac output.

Effect of pressure-flow relationship of centrifugal pump on in vivo hemodynamics: a consideration for design.

We have been developing centrifugal pumps for an implantable left ventricular assist device. We manufactured 2 prototype centrifugal pumps (PI, PII). These two have similar designs except for the PII having a volute casing and a large output port. To determine the differences in the hydraulic characteristics between the PI and PII, we carried out in vitro and in vivo experiments. In vitro study showed that the PII had a shallower H-Q curve than that of the PI, and the PII required a pump speed faster than the PI for the same flow rate and pressure head. On the other hand, in vivo study showed that the PII demonstrated a flow pulsatility greater than that of the PI at 1,900 rpm and 8 L/min although no significant change was observed at low pump speeds (< or = 1,500 rpm). This greater pulsatility consisted of a large discharge according to the small differential pressure during the systolic phase and a small discharge according to the large differential pressure during the diastolic phase. In contrast, the PI, having the steeper H-Q curve, showed a small discharge in the systolic phase and a large discharge in the diastolic phase. These results showed that pulsatility synchronized with the native heart beating depended on the slope of the H-Q curve. As a result, the slope of the H-Q curve is important to determine the component of pulsatility synchronized with native cardiac output. Regarding the slope of the H-Q curve, a pump having a volute casing and a large outlet port demonstrates a shallow slope in the H-Q curve. In conclusion, we suggest that a centrifugal pump for use in left ventricular aortic bypass should be designed considering the effect on the native heart pulsatility.

End-Tidal Carbon Dioxide Tension as a Monitor of Native Blood Flow During Resuscitation by Extracorporeal Circulation

In a porcine model of cardiac arrest, we investigated end-tidal carbon dioxide tension as a monitor of native blood flow during resuscitation by extracorporeal circulation. After 15 minutes of cardiac arrest and after precordial compression and transthoracic countershocks had failed, extracorporeal circulation consistently restored spontaneous circulation. Native end-tidal carbon dioxide tension, which averaged 29.8 +/- 1.0 mm Hg before arrest, was only 5.2 +/- 0.8 mm Hg during precordial compression. After the start of extracorporeal circulation, native end-tidal carbon dioxide tension was measured during 15-second interruptions of pump flow. End-tidal carbon dioxide tension progressively increased with a corresponding increase in native cardiac index. The correlation coefficients between end-tidal carbon dioxide tension and native cardiac index averaged 0.92 +/- 0.03 (mean +/- standard error of the mean). When end-tidal carbon dioxide tension exceeded 15 mm Hg, mean aortic pressure in each instance was 60 mm Hg or greater, and the animal was successfully weaned from extracorporeal support. We conclude that end-tidal carbon dioxide tension serves as a reliable monitor of blood flow through the lung and therefore of native cardiac output during weaning from extracorporeal circulation. It therefore indicates when native cardiac output is likely to be adequate to sustain spontaneous circulation.

End-tidal carbon dioxide tension as a monitor of native blood flow during resuscitation by extracorporeal circulation

In a porcine model of cardiac arrest, we investigated end-tidal carbon dioxide tension as a monitor of native blood flow during resuscitation by extracorporeal circulation. After 15 minutes of cardiac arrest and after precordial compression and transthoracic countershocks had failed, extracorporeal circulation consistently restored spontaneous circulation. Native end-tidal carbon dioxide tension, which averaged 29.8 +/- 1.0 mm Hg before arrest, was only 5.2 +/- 0.8 mm Hg during precordial compression. After the start of extracorporeal circulation, native end-tidal carbon dioxide tension was measured during 15-second interruptions of pump flow. End-tidal carbon dioxide tension progressively increased with a corresponding increase in native cardiac index. The correlation coefficients between end-tidal carbon dioxide tension and native cardiac index averaged 0.92 +/- 0.03 (mean +/- standard error of the mean). When end-tidal carbon dioxide tension exceeded 15 mm Hg, mean aortic pressure in each instance was 60 mm Hg or greater, and the animal was successfully weaned from extracorporeal support. We conclude that end-tidal carbon dioxide tension serves as a reliable monitor of blood flow through the lung and therefore of native cardiac output during weaning from extracorporeal circulation. It therefore indicates when native cardiac output is likely to be adequate to sustain spontaneous circulation.