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Biomedical subjects

G Christensen

Publications and source records attributed to G Christensen.

At least 55 records · Page 3Linked to original sources
PubMed1997

Statistical methods in computational anatomy.

This paper reviews recent developments by the Washington/Brown groups for the study of anatomical shape in the emerging new discipline of computational anatomy. Parametric representations of anatomical variation for computational anatomy are reviewed, restricted to the assumption of small deformations. The generation of covariance operators for probabilistic measures of anatomical variation on coordinatized submanifolds is formulated as an empirical procedure. Populations of brains are mapped to common coordinate systems, from which template coordinate systems are constructed which are closest to the population of anatomies in a minimum distance sense. Variation of several one-, two- and three-dimensional manifolds, i.e. sulci, surfaces and brain volumes are examined via Gaussian measures with mean and covariances estimated directly from maps of templates to targets. Methods are presented for estimating the covariances of vector fields from a family of empirically generated maps, posed as generalized spectrum estimation indexed over the submanifolds. Covariance estimation is made parametric, analogous to autoregressive modelling, by introducing small deformation linear operators for constraining the spectrum of the fields.

Algorithms↗
PubMed1997

The significance of positive cultures from orthopedic fixation devices in the absence of clinical infection.

Microbiologic samples for culture were obtained from 21 patients during elective removal of fracture-fixation hardware. The hardware was being removed for pain, displacement, nonunion or malunion correction, or patient preference. None of the patients had evidence of infection as determined by medical history, physical examination, white blood count, and sedimentation rate performed on the day of hardware removal. Two sets of culture samples were obtained from each patient: swabs of the wound and of the hardware were obtained and processed by the hospital laboratory. Eleven hardware cultures and nine wound cultures were positive for microbiologic growth. None of the patients was treated with antibiotics, and none developed any problems with wound infection or healing nor any evidence of osteomyelitis. We concluded that positive cultures obtained during hardware removal in the absence of clinical signs of infection are not meaningful.

Adolescent↗
PubMed1996

Atrial contractile dysfunction after short-term atrial fibrillation is reduced by verapamil but increased by BAY K8644.

BACKGROUND: Reduced atrial contractility occurs after cessation of atrial fibrillation. Its mechanism is unknown, and no pharmacological treatment exists. It has been hypothesized that this atrial contractile dysfunction results from intracellular calcium overload due to rapid depolarizations during fibrillation. Accordingly, we examined the effects of drugs that reduce or increase transsarcolemmal calcium influx on postfibrillation atrial dysfunction. Furthermore, we examined whether the dysfunction could be attributed to atrial ischemia. METHODS AND RESULTS: Atrial contractility after atrial fibrillation was examined in open-chest pigs paced with a constant ventricular rate after complete AV block. Atrial contractility was computed as systolic shortening of left atrial diameter divided by atrial preload. Three groups of six pigs each were subjected to two 5-minute periods of atrial fibrillation separated by 1 hour of AV pacing. Verapamil or the calcium channel agonist BAY K8644 was administered intravenously before the second fibrillation period. The degree and duration of postfibrillation atrial contractile dysfunction were reduced with verapamil but increased with BAY K8644. In a control group, parallel changes occurred after the first and second fibrillation periods. Atrial tissue content of creatine phosphate declined slightly during fibrillation, whereas the tissue content of ATP and lactate remained unchanged. CONCLUSIONS: Atrial contractile dysfunction after short-term atrial fibrillation is reduced by the calcium antagonist verapamil, which suggests that transsarcolemmal calcium influx contributed to this dysfunction. The calcium agonist BAY K8644 increased postfibrillation atrial contractile dysfunction. Atrial ischemia was not observed during fibrillation.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1996

Removal of surface bacteria by irrigation.

We examined the efficacy of various irrigation solutions delivered through a power irrigator to remove bacteria from three different surfaces. Titanium, stainless-steel, and cortical bone surfaces were coated with three different bacterial species: Staphylococcus aureus, Pseudomonas aeruginosa, and Staphylococcus epidermidis. They were then irrigated with 1 L of fluid delivered by jet lavage. The fluids tested were normal saline and solutions of bacitracin, neomycin, and soap. One set of specimens was not irrigated, as a control. After irrigation, the specimens were sonicated to remove residual bacteria, and the sonicate was quantitatively cultured to allow evaluation of the amount of residual bacteria on the surface. The results showed that removal of bacteria reflects an interaction between bacterial species, surface characteristics, and irrigation solution. Fewer bacteria were present in all the irrigation groups than in the control. Soap solution was as good as or better than any other solution at removing all three types of bacteria from all three surfaces, although not all of the pairwise comparisons were statistically significant. There was a significant advantage to soap solution over antibiotic irrigant or saline alone in removing Staphylococcus epidermidis from metallic surfaces. The use of soap solution for irrigation seems to improve the removal of some bacteria from some surfaces in this experimental model and may represent a better type of irrigation additive.

Anti-Bacterial Agents↗
PubMed1996

Lack of inhibitory effect of atrial natriuretic factor on renin release induced by renal hypotension.

To examine the effect of atrial natriuretic factor (ANF) on renin release induced by renal hypotension, experiments were performed in seven barbiturateanaesthetized dogs with denervated kidneys. Renin release induced by renal arterial constriction to 55 mmHg was measured before and during intrarenal infusion of ANF (200 ng min-1 kg-1 body weight). Before ANF infusion, renal arterial constriction increased renin release from 0.2 +/- 0.1 to 21.8 +/- 3.3 micrograms angiotensin I min-1 (p < 0.05). During ANF infusion renal arterial constriction increased renin release as much as before from 0.8 +/- 0.6 to 23.7 +/- 4.6 micrograms angiotensin I min-1 (p < 0.05). Although ANF increased glomerular filtration rate from 33.9 +/- 4.2 to 43.4 +/- 5.6 ml min-1 (p < 0.05) and sodium excretion from 72 +/- 22 to 567 +/- 112 mumol min-1 (p < 0.05) at normal renal perfusion pressure, ANF was without effect on glomerular filtration rate and sodium excretion during renal arterial constriction. The present study shows that ANF is not an inhibitor of renin release induced by renal arterial constriction in anaesthetized dogs with denervated kidneys. Our findings indicate that ANF does not influence renin release induced by the haemodynamic mechanism.

Angiotensin I↗
PubMed1996

Disinfecting agents for removing adherent bacteria from orthopaedic hardware.

This investigation seeks to determine whether surfactants or detergents can be used to clean and disinfect orthopaedic wounds with implanted hardware. Thus, a stepwise investigation of biocompatible surfactants and detergents was performed to identify an irrigation agent for disinfecting orthopaedic wounds. Bacterial adhesions assays, irrigation studies, and bactericidal assays determined that benzalkonium chloride showed the greatest efficacy. Testing involved stainless steel screws colonized with a preformed biofilm of Staphylococcus epidermidis, Staphylococcus aureus, or Pseudomonas aeruginosa, which were immersed in benzalkonium chloride solutions for various time intervals under static conditions. After 10 minutes, benzalkonium chloride achieved a minimum 4 log kill (10,000-fold) for all 3 strains of bacteria. Additional studies demonstrated that the high mechanical energy of jet irrigation improved the disinfecting properties of this agent. With jet lavage, both 1:1000 and 1:5000 concentrations of benzalkonium chloride achieved a minimum 2 log kill (100-fold) for all 3 bacteria. The results or this study suggest that at tissue compatible concentrations, benzalkonium chloride has significant disinfection properties for in vitro colonized orthopaedic devices, and these properties may be enhanced via jet lavage.

Anti-Infective Agents, Local↗
PubMed1995

Causes of mortality among sows in Danish pig herds.

The likely causes of sow mortality in Danish pig herds were investigated in a sample of 598 of the breeding animals delivered to a large rendering plant in the winter seasons of 1992 and 1993. In 263 cases information about the circumstances of the death or euthanasia and the herd characteristics were available, including the size of the herd, its health status, the age at weaning, the method of feeding and the use of straw for bedding. For these animals the distribution of likely causes of death or euthanasia was: leg weakness, 28.5 per cent; problems related to farrowing and late pregnancy, 20.9 per cent; disorders of the digestive system, 17.1 per cent; disorders of the urinary system, 13.1 per cent; physical injuries, 10.7 per cent; and other disorders, 9.5 per cent. For the other 335 sows the distribution of likely causes of death was: leg weakness, 16.1 per cent; problems related to farrowing and late pregnancy, 10.7 per cent; disorders of the digestive system, 21.2 per cent; disorders of the urinary system, 15.2 per cent; other disorders, 15.0 per cent; and unknown causes of death, 21.8 per cent. According to the official statistics from Danish rendering plants, more than 60,000 carcases of breeding pigs were processed during 1992, corresponding to a mortality rate of 5 to 6 per cent in the sow herds. The mortality rate appeared to increase with increasing herd size, and in herds with more than 100 sows the mortality rate was three times the mortality in herds with fewer than 50 sows. Compared with previous reports, the proportion of disorders involving the gastrointestinal system has increased during the past 20 years. Gastric dilation is particularly common, probably as a result of the intensification of pig production and the associated changes in management practises. The use of straw bedding was marginally significant (P = 0.06) and associated with a low frequency of gastrointestinal disorders.

Animals↗
PubMed1995

Increased in vivo expression and production of endothelin-1 by porcine cardiomyocytes subjected to ischemia.

Circulating levels of the endothelium-derived vasoconstrictor peptide endothelin-1 (ET-1) are increased in association with myocardial ischemia and infarction. The present study investigates whether ET-1 is synthesized and produced locally in the ischemic heart. Sixteen pigs were divided into three groups. In the first group, the left anterior descending coronary artery was occluded for 90 minutes, followed by 150 minutes of reperfusion (group A, n = 8). Two additional groups were subjected to 90 minutes (group B, n = 4) or 240 minutes (group C, n = 4) of ischemia without reperfusion. Biopsies from the nonischemic and ischemic myocardium were rapidly obtained from the beating heart and subsequently examined by Northern blot, in situ hybridization, and immunohistochemistry. Arterial plasma ET-1 was measured before ischemia and at the end of the experiments. Northern blot revealed a twofold increase in ET-1 mRNA in the ischemic myocardium compared with the nonischemic myocardium. In situ hybridization showed a considerable increase in ET-1 mRNA over the ischemic cardiomyocytes. Substantial ET-1-like immunoreactivity (ET-1-ir) was detected in cardiomyocytes in the ischemic region. In contrast, little or no ET-1-ir or mRNA was detected in nonischemic cardiomyocytes. Both in the ischemic and nonischemic regions, little ET-1-ir was detected in vascular endothelial cells or vascular smooth muscle cells. There was no difference in the intensity and distribution of ET-1 mRNA expression or ET-1-ir among experimental groups A, B, and C. Arterial plasma ET-1 was increased only in group A, the reperfused group.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

[Endothelin--an important factor in coronary heart disease].

The peptide hormone endothelin was initially discovered in endothelial cells. However, it has recently been shown that endothelin is produced by cardiomyocytes too. Endothelin is the most potent of all known vasoconstrictors. The coronary circulation is highly sensitive to endothelin, and intracoronary infusion may lead to lethal coronary spasm. Patients with variant angina pectoris have a higher plasma concentration of endothelin. Moreover, exposure to cold leads to a further increase in endothelin. Therefore increased production of endothelin may be an important mediator of vasospasm. Acute myocardial infarction is followed by an increase in circulating endothelin. Remaining high endothelin levels after myocardial infarction are associated with poor prognosis. Cardiomyocytes increase their production of endothelin during ischemia, and this increase may be harmful, either through an effect on the coronary circulation or by acting directly on the cardiomyocytes. Blockade of endothelin receptors has been shown to reduce the size of an infarction. Thus, intense research is directed towards discovering therapeutical agents that either inhibit production or block the effects of endothelin. Recently, an orally active endothelin receptor antagonist was discovered, and clinically useful therapeutical agents will most probably be available in the near future.

Coronary Circulation↗
PubMed1994

Functional comparisons between plant plasma membrane H(+)-ATPase isoforms expressed in yeast.

To examine the functional properties of the three major isoforms of plasma membrane H(+)-ATPase expressed in Arabidopsis thaliana (AHA1, AHA2, and AHA3), we employed a system for the heterologous expression of functional plant plasma membrane H(+)-ATPase in yeast (Villalba, J. M., Palmgren, M. G., Berberian, G. E., Ferguson, C., and Serrano, R. (1992) J. Biol. Chem. 267, 12341-12349). Each isoform was expressed efficiently but appeared to be retained in the endoplasmic reticulum of yeast. All isoforms displayed qualitatively similar enzymatic properties, but quantitative differences were found. When compared with AHA3, AHA1 and AHA2 had an apparent higher turnover rate for ATP hydrolysis, exhibited a 10-fold higher apparent affinity for ATP, and a 3-fold higher sensitivity toward vanadate. In addition, AHA2 had a slightly lower apparent affinity for H+ and seemed to be more susceptible to activation by lysophosphatidylcholine than did AHA1 and AHA3. This study represents the first comparison of the functional properties of isoforms of the plant plasma membrane H(+)-ATPase.

Arabidopsis↗
PubMed1994

The efficacy of various irrigation solutions in removing slime-producing Staphylococcus.

To determine which type of irrigation solution and which method of irrigation most effectively removes slime-producing Staphylococcus from implant surfaces, we performed experimental washings of bacteria-coated stainless steel screws with various solutions delivered by bulb syringe or by jet lavage. The quantity of bacteria remaining on the screw surface was determined after irrigation with 1 L of saline, 1 L of antibiotic solutions, or 1 L of saline containing a liquid soap. Antibiotic solutions tested included bacitracin, neomycin, and polymyxin/neomycin. We found that the use of power irrigation increased the removal of bacteria by a factor of at least 100 over bulb syringe irrigation of the same volume, no matter which solution was used. This effect ranged from a 100-fold improvement for neomycin, to a 285-fold effect for the polymyxin solution. The addition of antibiotic drugs to the irrigation solution had no significant effect on bacterial removal, and none of the antibiotic solutions were statistically different from saline alone in the amount of bacteria removed from the screws. The addition of a liquid soap solution dramatically increased the amount of bacteria removed by irrigation, reducing the residual bacteria per screw from a colony count of 3.5 x 10(4) for polymyxin (the best of the antibiotic solutions), to 4.38 x 10(3). This difference was statistically significant as judged by Student's t test, with p = 0.01. We have concluded that the use of power irrigation improves the ability to clean this pathogenic bacteria from metallic surfaces, and that the addition of antibiotic drugs to the irrigation solution does not.(ABSTRACT TRUNCATED AT 250 WORDS)

Anti-Bacterial Agents↗
PubMed1994

Cardiac contractile function following repetitive brief ischemia: effects of nucleoside transport inhibition.

The effects of nucleoside transport inhibition on cardiac contractile function were examined in anesthetized pigs subjected to five 6-min left anterior descending coronary artery (LAD) occlusions, separated by 20-min reperfusion, and followed by 150-min reperfusion. In group 1 (n = 8), saline was infused. In group 2 (n = 9), endogenous myocardial accumulation of adenosine was increased by intracoronary infusion of the specific nucleoside transport inhibitor R-75 231. Left ventricular segment lengths were recorded by ultrasonic crystals in the inner one-third of the myocardium. Percent systolic segment length shortening (SS) (normalized to percent of preischemic value) was significantly better maintained in the R-75 231 group compared with the saline group after each occlusion. SS in the saline group reached a nadir of 30% (22-40) at 30-min reperfusion after the last occlusion compared with 66% (54-73) in the R-75 231 group. In the R-75 231 group, but not in the saline group, maximal postischemic decline in SS and decline at 20-min reperfusion were significantly reduced following the last occlusion. We conclude that R-75 231, which inhibits nucleoside transport, attenuates contractile dysfunction following repetitive brief ischemia and results in a preconditioning-like effect against stunning in the pig. On the basis of the well-documented biochemical effects of R-75 231, increased accumulation of endogenous adenosine most likely explains these findings.

Animals↗