Biomedical subjects

K B Campbell

Publications and source records attributed to K B Campbell.

At least 19 recordsLinked to original sources

PubMed1992

Single perturbed beat vs. steady-state beats for assessing systolic function in the isolated heart.

Single-beat and steady-state techniques for evaluating end-systolic pressure-volume relationship (ESPVR) and Frank-Starling mechanism (FSM) in the crystalloid-perfused isolated rabbit heart were compared. In the single-beat technique, a train of stable isovolumic beats was interrupted with a single perturbed beat that either ejected against various levels of imposed isobaric load (ESPVR protocol) or beat isovolumically against various levels of end-diastolic volume (V(ED); FSM protocol). In steady-state technique, sustained beating was established, isobarically, at each of various loads (ESPVR protocol) or, isovolumically, at each of various V(ED) values (FSM protocol). ESPVR from steady-state technique lay above and to the left of that from single-beat technique. Contractile state was not uniform within steady-state technique, whereas it was uniform within single-beat technique. In the FSM protocol, single-beat technique exhibited the following features relative to steady-state technique: 1) greater range of developed pressures, 2) steeper ascending limb and more sharply defined maxima, 3) higher maximal developed pressure (Pdmax), and 4) greater volume at Pdmax(Vmax). Again. a common contractile state existed within single-beat technique but not within steady-state technique. It was concluded that single-beat technique was preferable to steady-state technique for evaluating ESPVR and FSM because 1) single-beat technique required less time for obtaining data, 2) single-beat technique allowed identification of uncomplicated values of Pdmax and Vmax, and 3) single-beat technique provided a common contractile-state reference for all data, whereas steady-state technique did not.

Animals↗

PubMed1991

Effects of performance feedback on P300 and reaction time in closed head-injured outpatients.

In an earlier study, we have demonstrated that the choice reaction times (RTs) of closed head-injured patients may be reduced significantly by instructions which emphasize speed, thus indicating that this portion of the RT delay was not due to a motor deficit. Despite the reduction in patient RTs, however, they remained significantly longer than those of controls. Furthermore, the RTs of patients were 56 msec longer than their P300 latencies, whereas those of controls occurred at about the same time as P300. The delay which remained in patient RTs was thus not entirely due to longer stimulus evaluation times. The goal of the present study was to further reduce patient RTs using feedback (FB) on response speed and time windows within which subjects were required to respond. When FB on speed and a 'narrow' time window were used, patient RTs occurred at about the same time as P300 latency (as was also the case with controls) indicating that the longer RTs of these patients in our earlier study, and in the other conditions of the present study, could not have been exaggerated beyond P300 latency due to a motor deficit. It was suggested that CHI patients may not be able to internally monitor their responses to the same degree as controls. The provision of external cues may have compensated for this deficit.

Adult↗

PubMed1991

Decision-making following closed-head injury: can response speed be retrained?

The present study was designed to assess the extent to which the effects of feed-back (FB) and time windows, on the RTs of closed-head-injured outpatients, might carry-over when these external cues were removed. RT and evoked potentials were recorded while 12 closed-head-injured outpatients and 12 matched controls performed an auditory discrimination task. The FB and time windows were provided on three trials, then gradually removed. Following the removal of the FB and windows, patient RTs remained significantly faster and were approximately equal to the RTs of controls prior to the presentation of the cues. As in controls, the RTs of patients following removal of the cues occurred at approximately the same time as their P300s (the latency of this event-related potential component is thought to index stimulus evaluation time). To this extent, patient RTs did not appear to be exaggerated beyond the time needed for stimulus evaluation. The results obtained, therefore, provided tentative evidence that the experimental manipulation might have a lasting effect on CHI patient RTs.

Adult↗

PubMed1991

Method for studying arterial wave transmission effects on left ventricular function.

A technique for studying the real-time effects of arterial wave reflections on the performance of the isolated left ventricle was investigated. Real-time arterial loading of an isolated ferret heart with an asymmetric T-tube wave transmission model was obtained with the use of a multiprocessor computer control system and a volume control linear motor pump. The multiprocessor computer system was programmed to compute the instantaneous aortic flow from the instantaneous ventricular pressure. The time integral of the flow was used as a command to the linear motor pump to control the instantaneous ventricular volume. This loading system allowed the imposition of a wide variety of vascular impedances on the ventricle by changing the parameters of the asymmetric T-tube model.

Animals↗

PubMed1991

Two arterial effective reflecting sites may appear as one to the heart.

The relation between reflected waves and features of ascending aortic pressure waveforms and impedance patterns was investigated with a modified T-tube model of the systemic arterial circulation. Ascending aortic pressure and flow and descending aortic flow were measured in 10 dogs under basal conditions and under the effect of an agent (methoxamine) that caused vasoconstriction and an increase of mean aortic pressure. A broad range of aortic pressure amplitudes and features was obtained. These waveshapes were classified into four groups. Under basal conditions, cases for which a prominent diastolic fluctuation was present (n = 8) were grouped in A. Cases for which this fluctuation was absent (n = 2) were grouped in B. Groups C (n = 4) and D (n = 3) included cases that, under vasoconstricted conditions, did or did not display, respectively, a diastolic fluctuation in pressure. Arterial T-tube model parameters were estimated by simultaneously fitting the model to both ascending and descending aortic flow with aortic pressure as input. A good fit was obtained in any case considered. After parameter estimation, forward and reflected waves and impedance patterns at the entrance of head circulation (head and upper limbs) and body circulation (trunk and lower limbs) as well as their merger in the ascending aorta were determined. T-tube input impedance compared well with impedance data points obtained from the ratio of corresponding harmonics of ascending aortic pressure and flow. In some cases (group A), modulus and phase spectra displayed two distinct minima, in the range from 0 to 10 Hz. In some other circumstances, these minima were less distinct (groups B and C) and could even appear as one (group D). Whether one or two minima appeared in the ascending aortic impedance spectra at low frequency and whether a prominent diastolic fluctuation did or did not appear in aortic pressure, pressure and flow waveshapes proximal to the heart were explained by the presence of two effective reflecting sites in the systemic circulation. In group B, a diastolic fluctuation in pressure was absent despite the fact that head-end and body-end reflected waves were distinct. This happened because body-end reflected waves peaked corresponding to a minimum of the head-end reflected wave. In group D, a diastolic fluctuation in aortic pressure was absent because the body-end reflected wave moved into systole and superimposed on the head-end reflected wave. This superimposition was due to increased pulse wave velocity in the body transmission path as a result of decreased arterial distensibility.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1991

Short-time-scale left ventricular systolic dynamics. Evidence for a common mechanism in both left ventricular chamber and heart muscle mechanics.

Based on the premise that short-time-scale, small-amplitude pressure/volume/outflow behavior of the left ventricular chamber was dominated by dynamic processes originating in cardiac myofilaments, a prototype model was built to predict pressure responses to volume perturbations. In the model, chamber pressure was taken to be the product of the number of generators in a pressure-bearing state and their average volumetric distortion, as in the muscle theory of A.F. Huxley, in which force was equal to the number of attached crossbridges and their average lineal distortion. Further, as in the muscle theory, pressure generators were assumed to cycle between two states, the pressure-bearing state and the non-pressure-bearing state. Experiments were performed in the isolated ferret heart, where variable volume decrements (0.01-0.12 ml) were removed at two commanded flow rates (flow clamps, -7 and -14 ml/sec). Pressure responses to volume removals were analyzed. Although the prototype model accounted for most features of the pressure responses, subtle but systematic discrepancies were observed. The presence or absence of flow and the magnitude of flow affected estimates of model parameters. However, estimates of parameters did not differ when the model was fitted to flow clamps with similar magnitudes of flows but different volume changes. Thus, prototype model inadequacies were attributed to misrepresentations of flow-related effects but not of volume-related effects. Based on these discrepancies, an improved model was built that added to the simple two-state cycling scheme, a pathway to a third state. This path was followed only in response to volume change. The improved model eliminated the deficiencies of the prototype model and was adequate in accounting for all observations. Since the template for the improved model was taken from the cycling crossbridge theory of muscle contraction, it was concluded that, in spite of the complexities of geometry, architecture, and regional heterogeneity of function and structure, crossbridge mechanisms dominated the short-time-scale dynamics of left ventricular chamber behavior.

Animals↗

PubMed1991

Left ventricular function depends on previous beat ejection but not previous beat pressure load.

Previous beat contraction history, in which the performance of the left ventricle on any one beat is influenced by the mechanical events of the previous beat, may be important in the beat-to-beat regulation of left ventricular performance in the intact cardiovascular system. Prior studies of this phenomenon have established that mechanical events of the previous beat influence the function of the current beat, but it is not known whether the important mechanical influence is exerted by previous beat ejection or previous beat pressure. In addition, the magnitude of the effect of previous beat contraction history on left ventricular performance is unknown. To make these determinations, we performed experiments in six isolated rabbit left ventricle preparations buffer perfused at 30 degrees C. Left ventricular pressure and volume were controlled precisely with a servo-controlled linear motor system. After steady-state ejecting conditions were established by clamping left ventricular ejection pressure at 60% of peak isovolumic pressure, single test beats, which were pressure clamped at 40%, 60%, 80%, and 100% of peak isovolumic pressure, were introduced and followed by an isovolumic reference beat. As the level of pressure clamp decreased from 100% to 40%, developed pressure on the isovolumic beat following the single test beats increased from 139 +/- 15 (mean +/- SD) to 151 +/- 13 mm Hg. Similarly, peak positive left ventricular dP/dt increased from 1,718 +/- 209 to 1,864 +/- 181 mm Hg.sec-1 (both p less than 0.01). Multiple regression analysis showed that this increase in left ventricular function was related to previous beat ejection but not to previous beat pressure load or relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Electrophysiological assessment of cognitive disorder in closed head-injured outpatients.

Severe closed head-injury results in a multitude of long-lasting cognitive deficits. ERPs can effectively complement the more traditional behavioural measures to provide information that is not available through any other means. It is now fairly clear that a late positive wave, P3, associated with contextual updating is attenuated and prolonged in a variety of conditions in the head-injured. It is also possible that measures of selective attention such as the processing negativity may be abnormal in this group. A more definitive statement will, however, have to wait the results of further investigations. A number of investigators have now indicated that cognitive processing is slowed in the head-injured. Again, ERPs have been instrumental in explaining why it is slowed. Decision-making time as measured by RT is generally longer than P3 latency. Because P3 latency is delayed in the head-injured, the time required for evaluation of the stimulus (recognition and classification) is slowed in the head-injured. The additional delay in RT must, however, be explained by other processes, most probably a response bias that perhaps emphasizes accuracy at a cost of speed. Such a strategy can be manipulated if the patient is provided with cues about their speed of responding. Finally, ERPs have been instrumental in explaining possible reasons for cognitive slowing. A powerful CNV technique may permit the categorization of the head-injured into at least 2 distinctive groups: those that tend to underprocess information (perhaps as a result of apathy or a lack of motivation) and those that tend to overprocess (perhaps as a result of an inefficient and needless processing of irrelevant information resulting in fatigue). The extent to which the different modes of information processing are related to the site of brain injury and possible personality change remains an issue of speculation.

Brain Injuries↗

PubMed1990

Event-related potentials and recognition memory for pictures and words: the effects of intentional and incidental learning.

Event-related potentials were recorded under conditions of intentional or incidental learning of pictures and words, and during the subsequent recognition memory test for these stimuli. Intentionally learned pictures were remembered better than incidentally learned pictures and intentionally learned words, which, in turn, were remembered better than incidentally learned words. In comparison to pictures that were ignored, the pictures that were attended were characterized by greater positive amplitude frontally at 250 ms and centro-parietally at 350 ms and by greater negativity at 450 ms at parietal and occipital sites. There were no effects of attention on the waveforms elicited by words. These results support the view that processing becomes automatic for words, whereas the processing of pictures involves additional effort or allocation of attentional resources. The N450 amplitude was greater for words than for pictures during both acquisition (intentional items) and recognition phases (hit and correct rejection categories for intentional items, hit category for incidental items). Because pictures are better remembered than words, the greater late positive wave (600 ms) elicited by the pictures than the words during the acquisition phase is also consistent with the association between P300 and better memory that has been reported.

Adult↗

PubMed1990

Time-domain formulation of asymmetric T-tube model of arterial system.

An asymmetric T-tube model of the arterial system with complex terminal loads was formulated in the time domain. The model was formulated to allow it to be fitted to the aortic pressure waveform, the aortic flow waveform, or simultaneously to both the aortic and descending aortic flow waveforms. Pressure and flow measurements were taken in anesthetized open-chest dogs under basal, vasoconstricted, and vasodilated states. It was found that the T-tube model fitted the data well in all formulations and in all vasoactive states. However, all parameters were estimated accurately in all vasoactive states only with the formulation that fitted to both aortic and descending aortic flow simultaneously. The T-tube model was compared with the three-element windkessel model with regard to the respective models' ability to recreate specific aspects of the pressure waveform and with regard to the estimates of global arterial parameters. The T-tube model recremated those features of the pressure waveform, such as diastolic waves, that the windkessel model could not. Also, the T-tube model systematically estimated lower global arterial compliance and higher characteristic impedance than the windkessel. It was argued that the T-tube model accurately represented important wave transmission features of the arterial loading system. The model is recommended for use in characterizing the arterial load and for merging with representations of the left ventricle in studies of left ventricle-systemic arterial interaction.

Animals↗

PubMed1990

Late-systolic pumping properties of the left ventricle. Deviation from elastance-resistance behavior.

Elastance-resistance [E(t)-R] representations of the left ventricle (LV) were evaluated for their ability to reproduce instantaneous pressure [P(t)] and outflow [Q(t)]. Experiments were performed in open-chest rats. P(t) and Q(t) were measured during steady-state ejecting beats and during a beat in which the aorta was suddenly clamped. The degree of clamping varied from partial to total occlusion. The total occlusion beat was considered an isovolumic beat that generated an isovolumic pressure [Piso(t)] with a characteristic time to maximal Piso(t) [Tpisomax]. In ejecting beats, 34% of stroke volume was delivered after Tpisomax. P(t) and Q(t) from the steady-state ejecting beats and Piso(t) from the clamped beat were then used to estimate parameters of an E(t)-R model. Components of P(t) and Q(t) not accounted for by E(t)-R were identified and termed extra-pressure [Pext(t)] and extra-outflow [Qext(t)]. Pext(t) and Qext(t) were near-zero valued until Tpisomax; then they became systematically positive and finally negative valued after end ejection. During partial aortic occlusion, P(t) was elevated and Q(t) was reduced. However, the time of ejection was extended, and the fraction of stroke volume delivered after Tpisomax increased as P(t) was made higher. Partial occlusion also prolonged the positive phase of Pext(t) and Qext(t). Elements possessing "active" and "deactive" properties were added to the E(t)-R model in an attempt to account for Pext(t) and Qext(t) during partial occlusion. Optional forms of these elements were considered. These expanded E(t)-R models were fitted to basal ejecting data and then asked to predict data from a partial occlusion beat. All expanded models failed to adequately predict the partial occlusion pressure and/or outflow. It was concluded that 1) late ejection was quantitatively important to LV pumping, 2) behavior during late ejection was inconsistent with E(t)-R, and 3) ad hoc modification of E(t)-R models was not likely to yield LV pumping models that could satisfactorily reproduce instantaneous P(t) and Q(t) behavior over the entire ejection period.

Animals↗

PubMed1989

Modified asymmetric T-tube model to infer arterial wave reflection at the aortic root.

A modified version of the T-tube model was constructed to represent the systemic arterial loading system as "seen" by the left ventricle (LV). This model consisted of two uniform tubes connected in parallel. It differs from the original T-tube model in that the transmission paths have no frictional losses and are terminated with complex impedances, rather than simple resistors. To estimate model parameters (load and tube compliances, tube inertances, characteristic impedances, and peripheral resistances) we measured ascending aortic pressure and flow in a group of five open-chest, anesthetized dogs. Parameter estimates were obtained by fitting experimental pressure to the pressure predicted by the model from experimental flow. To check the reliability of the model, an additional experiment was performed where flow in the upper descending thoracic aorta was measured in addition to ascending aorta pressure and flow. The fit between the experiment and model predicted ascending aortic pressure was satisfactory in all six dogs. This pressure was always characterized by the presence of a prominent diastolic oscillation. Our model showed that this oscillation is due to reflections from the lower body, the effective reflection site being most probably located at the level of middle to low abdominal aorta. The effective reflection site located in the upper body is closer to the heart. The related reflected wave affects pressure in late systole.

Analog-Digital Conversion↗

PubMed1989

Taurine: effect on myocardial relaxation.

1. The effects of taurine on cardiac inotropic state and relaxation were studied in the isolated rabbit heart using solutions containing three concentrations of calcium: 0.7, 1.8 and 3.0 mmol/l. 2. Increases in Ca2+ caused increases in inotropic indices, but had no effect on rate constant of relaxation. 3. Taurine had a positive inotropic effect at Ca2+ = 0.7 mmol/l, an equivocal effect at Ca2+ = 1.8 mmol/l and a negative inotropic effect at Ca2+ = 3.0 mmol/l. 4. Taurine had no effect on the rate constant of relaxation at any Ca2+ concentration. 5. Taurine modulates the positive inotropic effect of Ca2+ but neither taurine nor Ca2+ affect the rate constant of relaxation.

Animals↗

PubMed1989

Pulse reflection sites and effective length of the arterial system.

The concept of effective length (L) of the arterial system implies that it may be represented by a single viscoelastic tube terminated by an impedance, creating a single reflection site. Although the concept is straightforward, investigators for years have been unable to agree on the value of L. Proposed values range from a few millimeters to a few meters, confounding the identification of arterial reflection sites. This report shows analytically and illustrates with experimental data that the determination of the effective length leaves room for an infinite number of exact solutions for L and the corresponding terminal impedance if the input impedance of the tube is to match the measured input impedance of an arterial system. None of the possible values of L needs to bear any relationship to actual reflection sites.

Animals↗

PubMed1989

Pressure response to quick volume changes in tetanized isolated ferret hearts.

Observed pressure responses to quick volume changes in the isolated tetanized heart of ferrets were compared with previously reported tension responses to quick length changes in isolated cardiac muscle. Hearts were isolated from ferrets, perfused with ryanodine solution, and stimulated rapidly (50 ms between stimulations) to produce repeated 4-s intervals of tetanus. During each tetanus interval, volume increments of different amplitudes were rapidly removed and then reinfused into the left ventricular chamber. The pressure responses to these volume changes were evaluated for differences between withdrawals and infusions and for dependence on the amplitude of the volume change. It was found for both withdrawal and infusion that the response could be divided into three phases: 1) an immediate phase coincident with volume change, 2) a fast-recovery phase, and 3) a slow-recovery phase. The amplitude of the immediate phase was linearly dependent on the volume change so that a single regression line fit all the data (withdrawal and infusion). The fast recovery phase was 2.5 times faster for infusion than for withdrawal and generated a rebound effect with the pressure going below the initial pressure in the response to infusion. The pressure never went above the initial pressure in the response to withdrawal. The slow-recovery phases in infusion and withdrawal did not differ. These responses in the isolated heart bear striking similarities to tension responses to quick length changes in isolated constantly activated cardiac muscle. We concluded that muscle fiber dynamics were being faithfully transformed to left ventricular (LV) chamber dynamics without appreciable distortion because of the many intervening factors between the wall muscle fiber and the LV chamber.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1988

A peak detector program for event-related potentials.

A computer program, developed for peak detection of averaged evoked potential waveforms, is described. The program is able to automatically score up to 200 data files in a single session. The user is asked to define intervals (windows) in which a maximum peak is to be detected. The program can optionally search each EEG channel or a single 'reference' channel from which all other channels will be compared. The mean amplitude of several data points can also be computed within a particular window. When evoked potentials are particularly noisy, it is possible to estimate the 'signal' in the higher frequency background 'noise'. Because the program is written in BASIC, it can be readily transferred to many different computer systems.

Arousal↗

PubMed1988

Recognition memory for words and event-related potentials: a comparison of normal and disabled readers.

Visual event-related potentials elicited during a word-recognition memory task were examined for groups of normal and disabled readers (RD). The strong association of reading ability with recognition memory performance endorsed the appropriateness of this signal detection paradigm as a reading-related task. Enhanced amplitude of the P200 component for the RD group was evident during both the acquisition and recognition series and it is indicative of differences at an early sensory stage of item encoding and retrieval. Normal readers displayed greater N400 amplitude than the RD group during both the acquisition and recognition series, an effect which is consistent with more extensive semantic evaluation or memory search that is attributed to that component. In the absence of any remarkable differences in P300 amplitude between groups, the poorer recognition memory performance for the RD group may not be attributable to attentional deficits.

Child↗

PubMed1988

Area of the aortic pressure-flow loop measures energy storage by the proximal arteries.

We show that power measured by the area of the pressure-flow loop (APQ) is directly proportional to reactive power [Im(W)], which measures pulsatile energy storage in the proximal arteries. Analytically, Im(W) differs from APQ by K.2 pi, where K is a postulated constant of proportionality. To determine the value of K, 338 heartbeats obtained from 8 dogs under a wide range of hemodynamic conditions were analyzed. The relationship between APQ and Im(W) remained highly linear (r = 0.98), and the pooled value of K was -1.19 +/- 0.01. For the individual dogs, values of K differed at most by +/- 9% from its average value. There was also a small offset in the relationship between APQ and Im(W), probably caused by truncating the calculation of Im(W) at the 10th harmonic. Ignoring the small differences in the values of K and the offset, we found that APQ still proved to be an excellent predictor of Im(W). We also found that APQ was sensitive to changes in heart rate, left ventricular stroke output, and peripheral resistance. This reflects the dependence of pulsatile energy storage on these variables and indicates that APQ cannot serve as an index for changes in just the reactive properties of the proximal arteries.

Animals↗