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Biomedical subjects

P Tompkins

Publications and source records attributed to P Tompkins.

At least 19 recordsLinked to original sources
PubMed2001

The importance of valve alignment in determining the pressure/flow characteristics of differential pressure shunt valves with anti-gravity devices.

OBJECT: The proper functioning of shunt valves in vivo is dependent on many factors, including the valve itself, the anti-siphon device or ASD (if included), patency of inlet and outlet tubing, and location of the valve. One important, but sometimes overlooked, consideration in valve function is the valve location relative to the tip of the ventricular inlet catheter. As with any pressure measurement, the zero or reference position is an important concept. In the case of shunt valves, the position of the proximal inlet catheter tip is fixed and therefore serves as the reference point for all pressure measurements. This study was conducted to document the importance of this relationship for the pressure/flow characteristics of the shunt valve. METHODS: We bench-tested differential pressure valves (with integral anti-gravity devices; AGDs) from three manufacturers. Valves were connected to an "infinite" reservoir, and the starting head pressure for each was determined from product inserts. The inlet catheter tip was fixed at this position, and the valve body was moved in relation to the inlet catheter tip. Outflow rates were determined gravimetrically for positions varying between 4 cm above and 8 cm below the inlet catheter tip. CONCLUSIONS: All differential pressure valves utilized in this study that contained AGDs showed significant increases in outflow rate as the valve body was moved incrementally below the level of the inlet catheter tip. To allow functioning as a zero-hydrostatic pressure differential pressure valve, the AGD and the inlet catheter tip should be aligned at the same horizontal level.

Cerebrospinal Fluid Pressure↗
PubMed2001

The effect of alignment on the pressure/flow characteristics of flow control shunt valves.

OBJECT: The proper functioning of shunt valves in vivo is dependent on many factors, including the valve itself, the antisiphon device (if included), patency of inlet and outlet tubing and location of the valve. Two general categories of shunt valves are available today, the differential-pressure valve (with or without antisiphon device) and the flow-control valve. We have previously shown that the relationship between the position of the valve body and the inlet catheter tip can have profound effects on the outflow rate of differential pressure valves with antigravity devices. The current study was conducted to evaluate the importance of this relationship for the pressure/flow characteristics of the flow-control shunt valve. METHODS: We bench-tested flow-control valves from two manufacturers in the system we devised for testing differential-pressure valves. Valves were connected to an "infinite" reservoir, and the starting head pressure was determined from product inserts. The inlet catheter tip was fixed at this position and the valve body was moved in relation to the inlet catheter tip. Outflow rates were determined gravimetrically for positions +4 to -8 cm relative to the inlet catheter tip. CONCLUSIONS: All flow-control valves utilized in this study showed nearly constant outflow rate as the valve body was moved incrementally with respect to the level of the inlet catheter tip. As previously tested, differential-pressure valves exhibit significant increases in outflow rate as the valve body is moved below the inlet catheter tip. The outflow rate for the flow-control shunt valves does not change over the range of effective head pressures used in this study.

Cerebrospinal Fluid Shunts↗
PubMed2000

Neuroprotective effect of postischemic administration of progesterone in spontaneously hypertensive rats with focal cerebral ischemia.

OBJECT: Exogenous progesterone has been shown to reduce brain edema and ischemia-induced cell damage and to improve physiological and neurological function during the early stage of focal cerebral ischemia. In the present study, the authors assessed the neuroprotective potential of progesterone during the late stage of ischemia in a transient middle cerebral artery (MCA) occlusion model in the rat. METHODS: Forty-eight male spontaneously hypertensive rats were randomly assigned to six groups. Progesterone was dissolved in dimethyl sulfoxide (DMSO). In four groups of rats, the dissolved progesterone (4 mg/kg or 8 mg/kg) was administered for 2 or 7 days after ischemia. In two control groups DMSO was administered for 2 or 7 days after ischemia. Occlusion of the MCA was induced by insertion of an intraluminal suture, and reperfusion was accomplished by withdrawal of the suture. Treatment was initiated on reperfusion, which followed 2 hours of MCA occlusion, and continued once a day. Lesion volume, neurological deficit, and body weight loss were measured 2 or 7 days after ischemia, depending on the animal group. Treatment with a high dose of progesterone (8 mg/kg) resulted in reductions in lesion size, neurological deficits, and body weight, compared with control rats. CONCLUSIONS: Administration of progesterone to male rats 2 hours after MCA occlusion reduces ischemic brain damage and improves neurological deficit even 7 days after ischemia.

Analysis of Variance↗
PubMed1999

The importance of shunt valve position in flow characteristics of the Medtronic PS Medical Delta valve.

This study was conducted to document the extent to which flow depends on valve position in relation to head-pressure reference. Medtronic PS Medical Delta valves (contour model, performance levels 0.5, 1.0, 1.5, and 2.0) were studied in a bench test designed to evaluate flow rates with respect to valve position in relation to the head-pressure reference postion. The valves were connected to an "infinite" reservoir by the standard inlet catheter. An initial head (proximal) pressure was selected for each valve based on package insert data. The position of the inlet catheter tip was fixed at this starting head pressure, thus making the inlet catheter tip position the reference for relative head pressures on the valve assembly. When the valve body is positioned above this level, the effective head pressure is lowered, and when the valve body is positioned below this level, the effective head pressure is raised. Flow was established with the siphon control portion of the valve body located on the same horizontal level as the inlet catheter tip (the reference head pressure or "0" position). A standard silastic catheter was attached to the outlet of the valve, and its length was fixed at 50 cm for all valves (-50 cm H(2)0). The distal end of the outlet catheter was connected to a fraction collector, and 1-minute samples (five replicates) were collected for gravimetric determination of flow rate. The valve assembly was then moved in 1-cm increments through the range of 4 cm above to 8 cm below the head-pressure reference position. Samples were collected from each position (4 cm to -8 cm) relative to the inlet catheter tip. Flow rate, in milliliters/hour, was plotted against both relative position (4 cm to -8 cm) and absolute head pressure (in centimeters of water). Each of the valves tested was shown to have a linear relationship between flow and position relative to the inlet catheter tip (or absolute head pressure). The average increase in flow per centimeter of displacement of valve from catheter tip was 16.5 ml/hr/cm (range 14.4-17.6 ml/hr/cm). Once the inlet catheter tip is fixed in position, it serves as a pressure reference. Movement of the valve above this level results in a net decrease in effective head pressure, and movement below this position results in a net increase in effective head pressure. Thus, the positioning of shunt valves in locations different from this pressure reference position should be performed only with the knowledge that significant increases in outflow rate may occur when the valve body is positioned lower than the inlet catheter tip. This increase in outflow rate is not the result of siphoning or a defect in the antisiphon device but instead the result of a net increase in effective head pressure.

Journal Article↗
PubMed1998

ICP-CBF trauma bolt, laboratory evaluation.

Thermal diffusion flowmetry is a continuous quantitative technique of measuring regional cerebral blood flow utilizing a silastic strip probe placed through a craniotomy or craniectomy in the operating room. A new bolt like application of this technology is now available for commercial use and is especially designed for bedside placement in trauma patients. This new trauma bolt is tested in juvenile pigs who are subjected to episodes of hypercapnea to increase cerebral blood flow, and records significant changes in blood flow. Another feature of the trauma bolt is a second port for the placement of an intracranial pressure (ICP) monitor. Placement of the probe and ICP monitor were easier than with the silastic probe and had relatively little complication.

Animals↗
PubMed1994

Attenuation of monochromatic X-rays by normal and abnormal breast tissues.

RATIONALE AND OBJECTIVES: A prior study indicated that differences in the x-ray linear attenuation coefficients of cancerous and normal breast tissues tend to increase as the energy of the incident beam decreases. The authors investigated x-ray energies down to 20 keV. In the current study, the linear attenuation coefficients for normal and selected cancerous breast tissues within the energy range of 14 to 18 keV were determined. METHODS: Fifty breast biopsy specimens consisting of a mixture of breast malignancies, normal tissues, fat specimens, and tumors grown in rats were used. X-ray linear attenuation coefficients were measured for each sample within the energy range of 14.15 to 18 keV, using monoenergetic x-rays from beamline X-19A at the National Synchrotron Light Source at Brookhaven National Laboratory. Each sample was measured at 130 different energies starting at 14.15 keV with step sizes of 0.030 keV. Correlation of the measured attenuation coefficients for cellular makeup was performed. RESULTS: The mean of linear attenuation coefficients for samples classified as "cancers" was 10.9% higher than the mean of samples classified as "normal" breast tissues and was 66.5% higher than the mean of samples classified as normal breast fat. CONCLUSIONS: Differences in the linear attenuation coefficients of monochromatic x-rays between 14.15 and 18 keV do exist between normal and cancerous tissues, but there is some degree of overlap.

Biopsy↗
PubMed1991

The effect of apnea on brain compliance and intracranial pressure.

The effect of 2 minutes of apnea during endotracheal intubation on intracranial pressure (ICP), compliance, and cerebral blood volume (CBV) was studied in 19 adult dogs during normo-, hypo-, and hypercapnia. The compliance was measured from the cisterna magna in response to an intrathecal bolus injection (pressure-volume index). CBV was monitored by radiolabeled red blood cell activity. These measurements were made before and after 2 minutes of apnea. At normocapnia (pCO2 of 35-40 mm Hg), a period of apnea resulted in an increase in ICP from 9.6 to 26.3 mm Hg, a decrease in compliance from 0.051 to 0.020 ml/mm Hg (60%), and an increase in CBV of 0.26 ml (9.6%). When the animals were hypocapnic (pCO2 of 24-28 mm Hg), ICP increased from 12.8 to 19.6 mm Hg, compliance fell from 0.041 to 0.029 ml/mm Hg(29%), and CBV increased 0.07 ml (3.1%). Hypercapnia (pCO2 of 50-58 mm Hg) before apnea resulted in an increase in ICP from 21.5 to 47.1 mm Hg, a decrease in compliance from 0.032 to 0.015 ml/mm Hg (52%), and an increase in CBV of 0.41 ml (13.4%). These results suggest that hyperventilation (hypocapnia) before intubation limits the adverse decrease in brain compliance and increase in ICP by reducing changes in cerebral blood volume.

Animals↗
PubMed1985

Evaluation of cardiac output, total peripheral vascular resistance, and plasma concentrations of vasopressin in the conscious, unrestrained rat during endotoxemia.

To eliminate the influence of anesthesia while investigating the role of vasoactive hormones during shock, we have developed an unanesthetized rat model that provides information on key cardiovascular parameters pertinent to shock. Enfluane anesthesia was used while the animals were being catheterized. After recovery from anesthesia, endotoxin (40 mg/kg) was given, and after 4 hr of measurements, animals (11) were killed. Cardiac index (CI) fell significantly 5 min after endotoxin and remained depressed for 4 hr. Mean blood pressure (MBP) decreased from 121 +/- 5.7 mmHg to 79 +/- 5.6 at 5 and 15 min, and increased to 113 +/- 3.0 at 180 min. Central venous pressure (CVP) was decreased from 30 min to the end of the study. Heart rate (HR) increased from 357 +/- 13 to a high of 448 +/- 14 at 5 min and remained at well over 400 for the 4-hr monitoring period. Total peripheral vascular resistance (TPVR) was twice that of the control at 30 min and remained elevated. Stroke volume decreased to 37% of that of the control at 15 and 30 min and remained at 50% of that of the control until sacrifice. Plasma concentrations of vasopressin measured by radioimmunoassay increased from 14 pg/ml (+/- 2.2, SEM) to 144 (+/- 56) and 144 (+/- 66) at 15 and 240 min after endotoxin. Pathological examination at the end of the study revealed extensive hemorrhage in all areas of the small intestine. We conclude that in the conscious endotoxic rat decreases in CI, MBP, and CVP are accompanied by compensatory increases in HR and TPVR; plasma vasopressin concentrations are greatly elevated; and severe small intestinal hemorrhage occurs. The control animals (12) were stable throughout the entire study period, establishing the conscious, instrumented, unrestrained rat presented here as an attractive model for the study of shock without the bias of anesthesia.

Animals↗
PubMed1984

Laryngoscopic endotracheal intubation of rats for inhalation anesthesia.

Endotracheal intubation of the rat under direct vision is described together with the details of procedures and apparatus for conducting inhalation anesthesia in this species. Our intubation method requires no special manufacture of equipment, because it employs the human laryngoscope equipped with an infant blade (size 0). Using inhalation anesthetics such as enflurane or halothane for induction, clear laryngoscopic visualization of the glottis is reliably obtained, allowing rapid and routine intubation of the rat in a highly predictable amount of time. In contrast, the injected anesthetics such as ketamine or pentobarbital sodium seem unsuited to laryngoscopic intubation as a result of problems of variable induction times, copious oral secretions, and strong pharyngeal-laryngeal reflexes.

Anesthesia, Inhalation↗
PubMed1984

Anesthetic-induced changes in cardiovascular and small intestinal responses to endotoxin in the rat.

The influence of anesthesia on endotoxin shock patterns in the rat was examined. Blood pressure, heart rate, arterial blood gases, glucose, lactate, and pathology of the small intestine were measured before and after endotoxin (40 mg/kg) challenge in male Sprague-Dawley rats. The three groups studied were: (1) enflurane anesthetized (N = 10), (2) ketamine anesthetized (N = 10), and (3) awake and unrestrained (N = 13). Measurements were made during 30 min prior to endotoxin injection (IV) and for 240 min afterward. Reflex tachycardia, which occurred in the awake group simultaneously with the initial endotoxin-induced hypotension, was not present during either enflurane or ketamine anesthesia. Significantly less gross intestinal pathology was found at 4 hr after endotoxin in the anesthetized groups compared to the awake group. These results suggest that the response of the sympathoadrenal system to the hypotension following endotoxin may be blunted in the anesthetized animal.

Anesthesia↗
PubMed1982

Survival characteristics during septic shock in 39 baboons.

In septic shock nonsurvival is characterized by failure of multiple organ systems. The design of therapeutic measures to increase survival would be enhanced if critical responses could be identified early. Escherichia coli LD100 was given to 39 baboons by IV infusion over two hours followed by different therapy regimens in 31 [2--4]. There were 18 permanent survivors (seven days or more), all receiving antibiotic/steroid combination therapy. Responses of survivors and nonsurvivors were measured and compared during the first 12 hours from onset of infusion. Changes in blood pressure and acid-base parameters were not significantly different between groups. Five responses indicative of permanent survival were lower heart rates, less elevation of blood urea nitrogen, normal blood glucose at eight hours, hyperglycemia with normal insulin at 12 hours, and lower plasma lactate concentrations beginning at four hours.

Animals↗
PubMed1981

Elevated plasma vasopressin concentrations during endotoxin and E. coli shock.

Plasma vasopressin concentrations, measured by radioimmunoassay, were remarkably elevated during endotoxin and E. coli shock. The concentrations were often above 500 pg/ml in dogs and above 300 pg/ml in baboons; they reached 1200 pg/ml in two dogs and 1800 pg/ml in another. Plasma concentration in a quiet, hydrated subject is 4 pg/ml; osmoregulation is maximally effective at 20 pg/ml. Elevation of plasma vasopressin occurred by 15 minutes after beginning infusion of endotoxin or E. coli and reached concentrations of 200-350 pg/ml with a decrease in cardiac output but before hypotension, which suggests decreased thoracic blood volume and decompression of left atrial stretch receptors. Even higher vasopressin levels were associated with a reduction of arterial blood pressure. The typical pattern was an early peak elevation followed by a sustained plateau of plasma vasopressin concentration in dogs and baboons with endotoxin and/or E. coli-induced circulatory shock.

Animals↗
PubMed1981

Vasopressin release during sepsis and septic shock in baboons and dogs.

Plasma vasopressin concentration was measured by radioimmunoassay in lightly anesthetized baboons and dogs before and during experimental Escherichia coli septic shock. Since vasopressin is a potent vasoconstrictor, and activator of clotting factors and a myocardial depressant, we postulated that, if found in substantial amounts in the plasma, vasopressin may contribute to the physiopathology of the septic shock syndrome. Quite high plasma vasopressin concentrations were found in both baboons and dogs. In the baboons, increased plasma vasopressin concentrations occurred, while mean arterial blood pressure was still within normal limits and remained elevated for as long as 12 hours during septic shock. Plasma vasopressin concentrations of this magnitude have been previously reported only with direct hypothalamic stimulation or after hypotensive shock secondary to hemorrhage.

Animals↗
PubMed1980

Reduced left ventricular hypertrophy following long-term water deprivation in the young spontaneously hypertensive rat.

Biochemical and physical parameters of cardiac hypertrophy accompanying hypertension were studied in water deprived versus non-deprived immature spontaneously hypertensive rats (SHR) and their normotensive progenitor strain, Wistar Kyoto (WKY). A 23.5 hour/day water deprivation schedule was maintained from 5 to 13 weeks of age in 23 SHR and 8 WKY rats to compare the non-deprived animals (16 SHR and 8 WKY controls). Water deprived SHR had lower left ventricular weight, lower total protein and hydroxyproline and the same total DNA as the non-deprived SHR. DNA concentration was higher in the deprived SHR than in the non-deprived SHR. No differences were found among the four groups in right ventricular weight or DNA concentration. Left to right ventricular weight ratio was significantly lower and left to right ventricular DNA concentration ratio significantly higher in the deprived SHR relative to non-deprived SHR. These data indicate that the water deprived SHR, which was less hypertensive than the non-deprived SHR, had less hypertrophy of their left ventricles. While water deprivation lowered mean arterial pressure in the WKY, also, there was no effect on left ventricular weight or biochemical indices of left ventricular cell size and cell number.

Animals↗