Arrhythmia as a result of poor intercellular coupling in the sinus node: a simulation study.

The effects of reduced intercellular coupling in the sinus node were investigated by means of simulations. Coupling was reduced both uniformly, and by introducing localized interaction blocks. In either case, model sinus node element networks typically splitted into frequency domains. These were defined as groups of neighbour elements which all attained the same mean firing frequency. In systems, simulating the vicinity of an impulse outlet to the atrium, the sinus node elements often splitted into two domains, one slowly firing just inside the outlet, and one normally firing large domain in the sinus node interior. This two-domain situation was analysed using a two-element system. Wenckebach conduction and advanced (m:1) exit blocks were seen, together with more odd block patterns and slow chaotic rhythms. The two-domain situation appeared also when two discrete outlets were considered. The slow domains around each outlet synchronized via the atrium. However, if there were some degree of exit block through one of the outlets only, brady-tachy like rhythms could be simulated due to a re-entrant circuit including both sinus node and atrial tissue. In conclusion, poor coupling in the sinus node seems to be sufficient to produce most arrhythmias in the sick sinus syndrome

Simulated sinoatrial exit blocks explained by circle map analysis.

In an accompanying study, it was seen that most cardiac arrhythmias that were simulated during poor intercellular coupling in the sinus node, were the same as those obtained in a two-element system in which one element suffered from a strong leakage current. This element corresponds to the sinus node periphery and is thus the one which feeds the atrium. In this paper, the interior element was replaced by a periodic stimulator. The dynamics of the peripheral element is then determined by its phase response curve. Phase response curves for sinus node elements subject to leakage were simulated for many different amplitudes of depolarizing stimuli. Simulations with circle maps based on these curves produced the same sequence of progressing levels of exit block as stimulus strength decreased, as did the two-element system when coupling strength was reduced. The bifurcations of the circle maps leading to the observed rhythms were identified. We found that the essential qualities of the phase response curves were determined by generally accepted properties of membrane currents. This suggests that the observed rhythms and bifurcations are generic.

The gastrocnemius muscle is an antagonist of the anterior cruciate ligament.

Since the proximal tendon of the gastrocnemius muscle wraps around the posterior aspect of the tibia, its contraction could potentially strain the anterior cruciate ligament (ACL) by pushing the tibia anteriorly. However, the relationship between contraction of the gastrocnemius muscle and ACL strain has not been studied in vivo. The objectives of this study were to evaluate the ACL strain response due to isolated contractions of the gastrocnemius muscle and to determine how these strains are affected by cocontraction with the hamstrings and quadriceps muscles. Six subjects with normal ACLs participated in the study; they underwent spinal anesthesia to ensure that their leg musculature was relaxed. Transcutaneous electrical muscle stimulation (TEMS) was used to induce contractions of the gastrocnemius, quadriceps and hamstrings muscles while the strains in the anteromedial bundle of the ACL were measured using a differential variable reluctance transducer. The ACL strain values produced by contraction of the gastrocnemius muscle were dependent on the magnitude of the ankle torque and knee flexion angle. Strains of 2.8% and 3.5% were produced at 5 degrees and 15 degrees of knee flexion, respectively. The ACL was not strained at 30 degrees and 45 degrees. Changes in ankle angle did not significantly affect these strain values. Co-contraction of the gastrocnemius and quadriceps muscles produced ACL strain values that were greater than those produced by isolated activation of either muscle group when the knee was at 15 degrees and 30 degrees. Co-contraction of the gastrocnemius and hamstrings muscles produced strains that were higher than those produced by the isolated contraction of the hamstrings muscles. At 15 degrees and 30 degrees of knee flexion. the co-contraction strain values were less than those produced by stimulation of the gastrocnemius muscle alone. This study verified that the gastrocnemius muscle is an antagonist of the ACL. Since the gastrocnemius is a flexor of the knee, this finding may have important clinical ramifications in ACL rehabilitation since flexor torques are generally thought to be protective of a healing ACL graft.

Postoperative mental impairment in hip fracture patients. A randomized study of reorientation measures in 223 patients.

Mental impairment is a common and serious complication in geriatric surgery. We studied 223 hip fracture patients. They were over 64 years of age (mean 81), with no history of mental deterioration and acutely admitted to hospital from independent living conditions. They were randomized into two groups. One of these was subjected to reorientation measures during the perioperative phase, i.e., presurgery admission to the orthopedic ward, accompanied home visits during the hospital stay and access to reorientation devices--they received a large clock, calendar, radio, TV-set, telephone and were encouraged to wear their own clothing. Otherwise, there were no differences in the treatment given to the two groups. We used monitoring of cognitive function with the Short Portable Mental Status Questionnaire (SPMSQ) and a feedback program for evaluation of the treatment results. There was a low incidence of postoperative cognitive deterioration in both groups, compared with historical controls. However, no difference in mental status was noted when we compared the two groups. The conclusion is that attributes were less important than the psychological environment for postoperative mental deterioration. The mean total continuous hospitalization (transfers between departments and hospitals included) in the reorientation group was 22 (95% CI: 17-43) days, the corresponding figures for the controls were 30 (14-29) days.

Properties of spiral waves in a piece of isotropic myocardium.

Tachyarrhythmias of the heart can be due to the presence of one or more spiral waves of electrical activity. Spiral waves were simulated using a previously described ionic model of cardiac action potentials in a 75 x 75 network of compartments. The compartments were connected by means of resistors and made isotropic in order to catch basic properties of spiral waves. The cross-field stimulation technique was used to generate single or double spiral waves. The analysis showed that a spiral wave was created when the second excitation front became critically curved, in the wake of the preceding wave, so that decremental propagation occurred. A spiral wave could also be generated from a wave circulating around an obstacle when the obstacle size was suddenly reduced. The spiral waves steadily circled around an area with excitable but unexcited cells. An undisturbed spiral wave in the isotropic medium circled around in a stable pathway, but drifted along the borders of cells made non-excitable. An excitation within an existing spiral wave could generate new spiral waves that interacted with each other and formed complex excitation patterns. A sudden prolongation of the refractory period reduced the central area with unexcited cells in the spiral pathway but only slightly prolonged the revolution time. A further prolongation of the refractory period extinguished the spiral wave when the tip of the spiral wave invaded refractory areas. The described ionic compartment model could accurately produce spiral waves with properties in line with experimental results reported by others.

Continuity, assessment and feedback in orthopaedic nursing care practice is cost-effective.

Hip fracture treatment and rehabilitation are often considered as separate issues and generally performed in different locations. The rapidly increasing proportion of patients that is old and very old now calls for a new approach. This is a prospective study of 909 consecutive hip fracture patients, over 64 years old, admitted to hospital from an independent living situation. In a specialized hip fracture unit, 256 patients were given an early and intense rehabilitation program. It focused on continuity, feedback, reorientation and continuous monitoring of cognitive function and activities of daily living (ADL). Eighty-five percent of the patients could return directly to their own home after the initial treatment period. The mean total utilization of health care resources during the first 4 months after the injury was lower (28 institutional days) than in 286 historical controls given conventional rehabilitation (39 days, p < 0.05). At 4 months, 81% of the patients still lived in their own home vs. 72% of the controls and the 1-year mortality was 14% (21% in the controls, p < 0.05). After the project had been completed 297 patients formed an additional control group. In this group the proportion discharged to their own home decreased to 44% and the number of bed-days during the first 4 months returned to the pre-study level (39 days). It thus appears to be cost-effective to allocate sufficient rehabilitation resources early in the treatment of elderly patients with hip fractures. Annual savings in the catchment area due to the intervention were calculated to approximately 5,000,000 SEK. Continuous assessment and adequate continuity gives the staff emotional feedback, which promotes their personal commitment.

Prospective payment systems and hip fracture treatment costs.

We studied the health care consumption and costs after a hip fracture in 1,060 and 1,178 elderly patients admitted from their own home before and after the implementation of a prospective payment system in Stockholm. The total number of bed-days was estimated by merging the inpatient database and the municipal records of living accommodations for the elderly. By using a detailed patient-related accounting system and separating cost for surgery and "hotel" cost, we could compare costs in different types of rehabilitation. After the change in reimbursement system, the orthopedic stay was almost halved from 20 to 12 days. This was achieved by earlier and increased discharge to geriatric wards, where bed-day consumption doubled (107%), so that the total cost actually increased by 12%. This is not readily apparent from the official heath care statistics, which depict a more favorable cost development, as diagnosis-related registration for a large part of the geriatric care is no longer included. In contrast, a rehabilitation program in one of the acute hospitals, emphasizing continuity in the postoperative phase, reduced the total cost for treatment and rehabilitation by 12%. A prospective reimbursement aiming at reducing the costs of acute care does not necessarily result in overall savings.

The cost of a hip fracture. Estimates for 1,709 patients in Sweden.

We calculated the costs related to hip fractures and estimated the potential cost savings from preventing hip fractures. Subjects for this retrospective study were 1,709 hip fracture patients admitted for a primary hip fracture during 1992 in Stockholm, Sweden. Direct costs were compiled for the services of hospital orthopedics, hospital geriatrics, nursing homes, home for the elderly, group living, other acute hospital care, and municipal home help. The direct costs per patient during 1 year after a fracture amounted to about USD 40,000. The county council was responsible for 59% of the direct costs during 1 year after a hip fracture, while the remaining 41% were referred to the municipality. In the morbidity group, the potential cost savings per patient from preventing hip fractures was about USD 22,000.

The appearance and disappearance of cognitive impairment in elderly patients during treatment for hip fracture.

We studied the natural course of cognitive state in 256 consecutive hip fracture patients who were admitted from an independent living situation. We employed a treatment programme that focused on preventing postoperative cognitive impairment. Cognitive function was assessed with the SPSMQ screening test. The incidence of postoperative cognitive impairment among those lucid on admission was 13%, which generally was reversed before discharge. Thirty-seven percent were cognitively impaired on admission; of those, 51% reached normal test scores while in hospital. Those who recovered within the first week had as good a prognosis during the first year as those who remained lucid throughout the hospital stay. Cognitive impairment was associated with an increased complication rate, e.g. a three-fold increase of early fracture displacement and a four-fold increase of wound infection. This increased risk was present even in patients with mild/moderate cognitive impairment and could not entirely be explained by age. Our results suggest that it is possible to decrease postoperative cognitive impairment by routine monitoring of cognitive status, a high level of continuity and a reorientation programme. The routine assessment of the cognitive function is recommended in geriatric patients who are admitted for surgery.

Simulation of re-entry in a piece of myocardial tissue: strong sensitivity to spatial and temporal conditions.

A simple model for the simulation of re-entrant excitation was created. The model consists of a matrix of 15x15 compartments, where each compartment has its own action potential that depends dynamically on four ion currents (INa, ICa, Ik and Ib) having time and voltage-dependent activation and inactivation kinetics. The compartments were combined with resistors to simulate electrotonic interaction. At short excitation intervals the action potential was shortened in duration, and at even shorter coupling intervals decremental propagation occurred. Re-entry around an obstacle could be elicited in response to a properly timed extra stimulus. A time dependent unidirectional block was made by making some of the action potentials longer in duration. An obstacle was not a necessary substrate for re-entry, but the timing of the extra stimulus was critical. In the presence of an obstacle, the induction of re-entry was critically dependent on the shape of the obstacle. The most important result of the simulations is that the system is highly sensitive to the initial spatial and temporal conditions. These sensitivities are generic features of dynamic systems that are described by non-linear differential equations and are typical for chaotic systems. The system studied shows features associated with deterministic chaos.

Prediction of the outcome after hip fracture in elderly patients.

We report a prospective study of 232 consecutive patients with hip fractures. All were over 64 years of age and living independently before admission to a geriatric orthopaedic ward. We assessed the value, at admission, of predicting factors for independent living at one year after injury. The most important factors were: (1) preinjury function in activities of daily living (grade A or B on the Katz et al (1963) scale); (2) absence of other medical conditions which would impair rehabilitation; and (3) cognitive function better than 7 on the Pfeiffer (1975) mental questionnaire. The odds ratios (95% CI) for these three predictors were 3.5 (1.3 to 9.1), 2.9 (1.3 to 6.1) and 2.4 (1.9 to 4.9), respectively. When all predictors were positive at admission, 92% were living independently at one year; with one, two or three negative predictors, the percentages living independently were 76, 61 and 27, respectively. The median values of the total number of days in hospital, irrespective of diagnosis, during the first year were 12, 24, 29 and 149 days for the four groups. The mortality at one year was predictable on admission only by the number of medical conditions: with no other diagnosis than the fracture the mortality was 0%; with one or two additional conditions the mortality was 14%; and with three or more additional diagnoses it was 24%. These simple and robust predictors can be used to optimise resources for rehabilitation.

Spinal sagittal configuration and mobility related to low-back pain in the female gymnast.

The purpose of this study was to correlate low-back complaints in 64 female gymnasts (mean age, 12 years) to spinal sagittal configuration and mobility as measured by two noninvasive methods. Measurements by Debrunner's kyphometer and Myrin's inclinometer were highly correlated. Average thoracic kyphosis was 32 degrees, lumbar lordosis 35 degrees. The mean range of sagittal motion was 57 degrees in the thoracic spine and 113 degrees in the lumbar spine. On average, 1 degree of the total sagittal lumbar mobility was lost for every 1 degree of increased lordosis. Low-back pain (LBP) was reported by 20% of the girls, and these girls had a significantly larger lordosis (41 degrees) than girls with no history of LBP (35 degrees).

Measurement of spinal sagittal configuration and mobility with Debrunner's kyphometer.

A considerable number of instruments have been described for noninvasive measurements of the sagittal configuration and mobility of the thoracolumbar spine. With Debrunner's kyphometer, it is possible to obtain this information in approximately 1 minute in a clinical setting. A minor modification of the kyphometer to increase its range of measurement of lumbar extension is described. Repeatability of measurements with the instrument was studied on 31 healthy individuals in a randomized cross-over model. In another study, ten individuals were examined ten times each by the same investigator within 3 weeks. These studies showed good reproducibility for measurements with the kyphometer. The importance of standardized instructions for measurement procedures is emphasized. The statistical approach to repeatability of measurements is discussed.

The sagittal configuration and mobility of the spine in idiopathic scoliosis.

The aim of this study is to see how the spinal sagittal configuration and mobility in 127 patients with idiopathic scoliosis are influenced by increasing scoliotic deformity and to determine when this deformity gets clinically significant compared to controls (n = 92). In patients with thoracic curves the degrees of thoracic kyphosis and lumbar lordosis were significantly less than those of the controls. Neither the kyphosis nor the lordosis were correlated to the Cobb angles. Even patients with small curves have straight spines in the sagittal plane; there is no tendency for the kyphosis and lordosis to decrease when the scoliotic deformity increases. This indicates that it is especially individuals with straight spines in the sagittal plane who are prone to develop scoliosis. It is also suggested that the limitation in spinal function for curves with Cobb angles below 50 degrees may be neglected.

Harrington instrumentation for thoracic and lumbar vertebral fractures.

Thirty-seven patients with fractures of the thoracic and lumbar spine treated with Harrington instrumentation were reviewed. Twenty-seven patients with a follow-up time of more than 2 years were summoned for a clinical and radiographic examination. This report presents the results related to reduction, stabilization, return of neural function, spinal posture and mobility, and residual disability. It is concluded that Harrington instrumentation can be performed without a substantial number of complications. Its major advantages are early mobilization and ambulation. The operative technique is discussed with special reference to the preservation of the normal configuration of the back. The value of computerized tomography in the preoperative assessment is stressed.

The effect of Harrington instrumentation on the sagittal configuration and mobility of the spine in scoliosis.

The spinal mobility and sagittal configuration in 96 scoliotic patients treated according to Harrington were investigated with a Debrunner kyphometer and a Myrin inclinometer. The more distal the fusion limit in the lumbar spine, the more reduced was the lumbar lordosis and flexion. The patients' own estimation of lumbar rigidity and pain, scored on visual analogue scales, increased as the fusion limit increased distally. The results for the thoracic spine's sagittal configuration and mobility were analogous but the trend was less pronounced. The patients' estimation of thoracic rigidity had little correlation with the number of segments fused. This study shows that fusion of the lumbar spine below L3 is to be avoided, except when specifically indicated. Furthermore, the study shows that the present technique does not adequately preserve the sagittal configuration of the thoracic and lumbar spine.