Although impedance cardiography provides safe and reliable noninvasive estimates of stroke volume in humans, its usefulness is limited by the necessity for subjects to be apneic and motionless. In an effort to circumvent this restriction we studied the validity of ensemble-averaging of impedance data in exercising normal subjects and in intensive-care patients. The correlation coefficient (r value) between 128 ensemble-averaged and standard hand-digitized determinations of stroke volume index from the same records taken during rest and exercise in six normal male subjects was +0.97 (P less than 0.001). The r value for ensemble-averaged stroke volume indices during free breathing and breath hold in the same subjects was +0.92 (P less than 0.001), suggesting that breath hold did not significantly affect the stroke volume estimation. In 14 freely breathing hospital intensive-care patients the r value between simultaneous thermodilution cardiac output readings and ensemble-averaged impedance determinations was +0.87 (P less than 0.01). The results indicate that ensemble-averaging of transthoracic impedance data provides waveforms from which reliable estimates of cardiac output can be made during normal respiration in healthy human subjects at rest and exercise and in critically ill patients.

Although cerebral hemorrhage is a widely occurring neurologic disorder thought to be caused by fluctuating blood flow, the response to flow in the neonatal cerebrovasculature has not been characterized. In the present study, we examined the effect of changing flow on middle cerebral artery diameter and pathways by which flow modulates cerebrovascular tone. Arteries from 2-14-d-old piglets were mounted on cannulas and bathed in and perfused with physiologic saline solution. An electronic system controlled pressure and a syringe pump provided constant flow. The transmural pressure was held constant at 20 mm Hg, and changes in vessel diameter were measured as flow was increased in steps from 0 to 1.60 mL/min (flow/diameter curves). Increasing flow at constant pressure resulted in constriction at flows from 0.077 to 0.152 mL/min and dilation at flows from 0.212 to 1.60 mL/min. The flow/diameter curves were repeated in arteries bathed in Na(+)-reduced or Ca(2+)-free physiologic saline solution; denervated with 6-hydroxydopamine; or treated with indomethacin, N-nitro-L-arginine methyl ester, N omega-nitro-L-arginine (NLA), and L-arginine), ryanodine, or glutaraldehyde. In Na(+)-reduced and in Ca(2+)-free physiologic saline solution, flow constriction was eliminated. Neither indomethacin nor 6-hydroxydopamine affected the biphasic response. N-Nitro-L-arginineL, NLA, and ryanodine blocked dilation, whereas L-arginine restored dilation in NLA-treated arteries. These data suggest that neither prostaglandins nor adrenergic nerve endings participate in flow-induced responses in piglet cerebral arteries. Elimination of flow-constriction by Na+ reduction or Ca2+ removal is consistent with findings in other artery types. The elimination of dilation by N-nitro-L-arginine methyl ester, NLA, and ryanodine suggests that dilation is mediated by nitric oxide and intracellular Ca2+. Whereas the contractile and dilatory responses to agonists remained intact after glutaraldehyde perfusion, both flow-induced constriction and dilation were eliminated, indicating that both types of flow responses result from endothelial cell deformation.

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A novel tri-axial plantar pressure sensor has been developed. This sensor simultaneously measures vertical plantar pressure and anterior-posterior and medial-lateral shear plantar pressures utilizing a central post, four parallel plates, and a commercial miniature pressure transducer. As a subject walks over the sensor, the central post is deflected and the shear pressures are measured utilizing capacitive sensing technology. The miniature pressure transducer (MPT) is simultaneously loaded to measure the vertical pressure. Each individual tri-axial plantar pressure sensor has the capability of measuring shear forces ranging from 0 to 15 N and vertical pressures ranging from 0 to 28 MPa. The shear component of the tri-axial pressure sensor has a sensitivity of 1.3 mV/g, a non-linearity of 8.3 %, and hysteresis of 7.3 %. The commercial vertical MPT has a sensitivity of 220 nv/V/psi, a non-linearity of 0.094%, and a hysteresis of 0.567%. An array of individual tri-axial plantar pressure sensors in the form of a platform will be developed to measure plantar pressure in patients. This pressure platform is placed on the surface of a walkway and is suitable for barefoot walking trials.