Swine as an in vivo model for electrophysiologic evaluation of cardiac pacing parameters

Pre-clinical studies of cardiac pacemakers and new electrodes, materials, and designs are for the most part conducted in dogs. Dogs, however, have electrophysiological differences which may preclude accurate translation to clinical trials. The purpose of this study was to develop normal electrophysiological parameters for an animal whose cardiovascular system more closely resembles that of man than any nonprimate animal. The threshold (voltage and current) strength-duration curves of the pig showed the same inverse relationship between the pulse duration and threshold requirements as other species. At 0.5 ms the atrium had 3.5-5.5 times greater energy requirements, over twice the current (2.04 mA vs 0.72 mA) and twice the voltage (0.75 mV vs 0.32 mV) requirements when compared to the ventricles. The pig's S-A nodal P-wave was superior in amplitude (7.80 +/- 1.80 mV vs 4.28 +/- 2.27 mV) and the slew rate was faster (1.30 +/- 0.56 mV/ms vs 0.44 +/- 0.50 mV/ms) compared to that of the atrial appendage. The pig's left ventricular myocardial R-wave had significantly greater amplitude (19.00 +/- 6.44 mV vs 10.70 +/- 4.34 mV) and faster slew rate (1.60 +/- 0.62 mV/ms vs 0.90 +/- 0.30 mV/ms) compared to the right ventricular endocardial R-wave. The electrophysiological parameters of the pig were more similar to those of man than the dog; therefore, the pig is a useful animal model for electrophysiological studies and the testing of pacemaker equipment.

Brainstem afferents to the lateral geniculate nucleus of the cat

The ascending connections from the brainstem to the dorsal division of the lateral geniculate nucleus were examined using retrograde transport of horseradish peroxidase. Labelled cells were identified in a variety of structures, including the nucleus of the optic tract (NOT), the posterior pretectal nucleus (NPP), the superior colliculus (SC), the parabigeminal nucleus (PBN), the midbrain reticular formation (MRF), locus coeruleus and nucleus sub-coeruleus, the substantia nigra (SN), and parts of the raphe complex. The projections from NOT, NPP, MRF, LC and PBN were all bilateral in origin. The most intense labelling was observed in the nucleus of the optic tract and the superior colliculus. Colliculo-geniculate cells were located primarily in the superficial gray (lamina II1 and II2 of Kaneseki and Sprague (1974), but sparse labelling was also observed in lamina II3 and in statum opticum (lamina III). Consistent with the report of Harrell et al. (1982), these cells represent a morphologically diverse population, which includes stellate cells, granule cells, and both vertical and horizontal fusiform cells. A similarly diverse population of cell types contributes to the geniculate projection arising from NOT. These results confirm and extend earlier descriptions of the brainstem projections to the cat LGNd, and serve to emphasize the diversity of brainstem influences over the geniculate.

Fabrication method for endotracheal tubes for sheep, goats, and calves

A reliable, inexpensive, and reusable endotracheal tube for sheep, goats, and calves was designed and constructed. The tube consisted of reinforced polyvinylchloride tubing with a segmented polyether polyurethane coated latex cuff. These materials were used to make a durable tube which allowed easy intubation.

Evaluation of a cubicle containment system in preventing gaseous and particulate airborne cross-contamination

The effectiveness of a cubicle containment system in preventing gaseous and particulate cross-contamination in animal facilities was evaluated using several techniques. Using a nitrous oxide dilution technique, no airborne cross-contamination was found between cubicles as long as all cubicle doors were kept closed. If the doors to the cubicle in which the gas was released were partially or completely opened, low concentrations of nitrous oxide could be detected in adjacent cubicles. These concentrations increased when the air exchange rates in the cubicle were decreased. Similar results were obtained when particulate transfer was measured using aerosolized Staphlococcus epidermidis and a slit to agar sampling technique. Air flows and point air velocities within the cubicle and the animal room were also studied. A trial of Sendai virus transmission between cubicles revealed no intercubicle transmission after 3 weeks of exposure. Overall, the cubicle containment system appeared to be an effective means of achieving limited biohazard containment, applicable to many research housing needs.

A new atrial lead with improved stability and P-wave detection

A new segmented polyether polyurethane atrial pacemaker lead has been developed and tested acutely and chronically in dogs. This lead was constructed so that its tip could quickly and accurately be positioned in close proximity to the S-A node and provide long-term stability without the use of active fixation devices such as tines or screws. The acute intrinsic P-wave potentials seen at the S-A node area (9.7 +/- 4.7 mV) were superior to those detected in the appendage (4.7 +/- 2.8 mV) and coronary sinus (6.8 +/- 4.1 mV). There were no significant differences in pacing thresholds between sites. In long-term studies, transvenous (jugular) leads maintained their position for over three months without dislodgment in active unrestrained dogs with only a 25% decrease in intrinsic P-wave potentials. The chronic pacing thresholds for this lead were similar, or superior, to those reported for other pacemaker lead placement sites. The polyurethane coating material produces no adverse tissue reaction or thrombosis.

Evaluation of a chemical scrubber for the removal of airborne bacteria from recycled air

The effectiveness of a chemical scrubber utilizing a recycled aqueous media was compared to high efficiency particulate air filtration in its ability to remove bacteria from recycled air. Two bacteria, Staphylococcus epidermidis and Bacillus subtilis, which have different biologic and physical properties were used in the study. The scrubber, using an aqueous chlorine-dioxide solution, and high efficiency particulate air filtration were equally effective in removing both types of bacteria from recycled air. Viable bacteria could not be recovered from the processed air following either treatment. When the scrubber was used with the chlorine dioxide solution, sufficient chlorine dioxide residues remained in the scrubber processed air to affect bacterial counts in the isolator to which the processed air was returned. The bacteria differed in their susceptibility to low levels of chlorine dioxide in the return air which influenced the number of viable organisms recovered under similar conditions. The use of water without a chemical additive eliminated bacteria in recycled air immediately following the start of the unit but allowed their eventual build-up in recycled air to concentrations of approximately half that of untreated air. Overall, the use of a chemical scrubber for the removal of bacteria from recycled air appeared to be no more effective than high efficiency particulate air filtration especially if the scrubbing media is recycled.

Evaluation of a chemical scrubber for the removal of gaseous contaminants from recycled air

A chemical scrubber was evaluated for its ability to remove two gases with differing water solubilities from recycled air. Operating parameters of the scrubber were also evaluated and included effect on air temperature and humidity. Ammonia, which has a high water solubility, was completely removed from the recycled air. Even when ammonia was continuously infused into the isolator at the rate of 1.0 liter/minute, the concentration in the recycled air never rose above 1 ppm despite concentrations of ammonia in the isolator of 4,000 ppm. Sulfur dioxide, which has a much lower water solubility, was not as rapidly removed. Continuous infusion of sulfur dioxide at the same flow rates used for ammonia resulted in increased gas pressure within the isolator. During operation of the scrubber, air temperature and humidity of recycled air rose significantly unless the water temperature to the unit was maintained at 15 degrees C. Overall, the chemical scrubber provided an effective adjunct to existing methods of purifying recycled air, but its application must be carefully selected in the laboratory animal facility.

Psorergatic mange (Acari:Psorergatidae) of the stumptail macaque (Macaca arctoides)

Psorergates (Psorobia) sp were recovered from dermal cysts on the face, chest, and abdomen of three stumptail macaques (Macaca arctoides) in a breeding colony of research animals. The lesions were multiple and appeared as white, crusted structures measuring 2 to 10 mm in diameter. These lesions were not associated with pruritus or other clinical symptoms. The mites were embedded in the epidermis and associated with mild hyperkeratosis. A few mononuclear leukocytes were present in the dermal and subcutaneous tissues.

Skull fractures in infant squirrel monkeys (Saimiri sciureus)

Skull fractures resulting from trauma and leading to hemorrhage in the subdural area and brain parenchymal have been a common cause of neonatal mortality in a breeding colony of squirrel monkeys. The skull fractures may have been caused by abnormal carrying behavior by the dam or by the dam being frightened while cradling the infant during nursing.

The effects of electrode position on the detection of the transvenous cardiac electrogram

Adequate sensing of the cardiac electrogram is a necessary requisite for all demand cardiac pacemakers. In order to define better the effects of position of the sensing electrode on the ability of the pacer to detect the electrogram, a specially designed lead system with a movable, large surface area (50 mm2) proximal ring electrode and a small surface area distal tip electrode was placed transvenously in 12 mongrel dogs. As the sensing electrode was moved away from the right ventricular apex, there were slight but insignificant decreases in R wave amplitude. When the electrode was moved retrograde through the tricuspid valve into the atria there was a marked decrease in R wave sensing; however, this was accompanied by a significant increase in P wave amplitude. Therefore, with a large surface area electrode, direct contact with the myocardium is not needed for the detection of adequate P and R waves. In addition, a pacer lead system utilizing this sliding sensing electrode design could be used to optimize sensing position, either for atrial or ventricular sensing pacemakers.

Experimental toxemia in the pregnant guinea pig (Cavia porcellus)

In two groups of nonpregnant guinea pigs, uterine ischemia was produced by banding the uterine arteries and transecting the ovarian arteries. Since this procedure prevented the increase of uterine blood supply that normally occurs during pregnancy, uteroplacental ischemia resulted when the animals became pregnant. Intraarterial blood pressures were recorded immediately after surgery and again near term. Hypertension, proteinuria and elevated creatinine levels (changes similar to those of human preeclampsia) were consistently found near term in all of the banded animals that became pregnant. These findings support the view that it is possible to develop a toxemia model in the guinea pig.

Endotracheal intubation: an easy way to establish a patent airway in rabbits

Endotracheal intubation of rabbits was accomplished by inserting a polyethylene catheter into the trachea through a guide needle which had been inserted into the trachea caudal to the larynx. The catheter was advanced cranially until it emerged from the mouth. The catheter then was used as a guide on which the tracheal tube was advanced. Adverse effects were not observed in intubated rabbits.

The induction of myocardial damage by open-chest low-energy countershock

Anesthetized rabbits were used to study the effects of internal electrical defibrillation on the heart. Single electrical shocks of 0.5, 1.0, and 3.0 Wsec/kg or five consecutive shocks of 0.5 Wsec/kg spaced at either 1 shock per min, per 2 min, or per 3 min, were used. Half of the rabbits given a single shock of 1 Wsec/kg or multiple shocks at a spacing of 1 shock per min were given intravenous injection of dexamethasone sodium phosphate (6 mg/kg) 5 min before receiving the defibrillatory shocks. Damage to the hearts was assessed by electrocardiography and histopathology. Myocardial damage was evident at the lowest energy levels used. All indices showed that the degree of damage was increased by increasing the energy level of single shocks or by decreasing the time interval between multiple shocks. The steroid pretreatment did not appear to protect the hearts from damage caused by the shocks.

True pregnancy toxemia (preeclampsia) in the guinea pig (Cavia porcellus)

In this study, normal nonpregnant, normal full-term pregnant, fasting ketotic and spontaneous pregnancy toxemic guinea pigs were compared to define the mechanism of this disease. In addition to conventional clinical, laboratory and pathologic studies, arterial blood pressure (thoracic and abdominal aortic) measurements and angiography were used. The results showed that in spontaneous cases of pregnancy toxemia, there is an aortic compression just caudal to the renal arteries. This compression reduced the aortic diameter by 22% of prerenal level as compared to 10% for fasting ketotic and normal pregnant guinea pigs. The aortic compression also resulted in a 30% postcompression reduction in blood pressure. No pressure differences were seen in the other groups. The postulated etiology for true toxemia of pregnancy in guinea pigs is, therefore, similar to that of man where aortic compression produces uterine ischemia and the resultant syndrome.

Ischemic injury to the conducting system of the heart. Involvement of myocardial lysosomes

The conducting system was studied in an in situ perfused swine heart preparation with reduced coronary flow (ischemia) using perfusate containing high and low levels of glucose (26.6 versus 8.6mM) with and without insulin. Coronary flow was maintained at normal levels for 60 minutes in control hearts. In ischemic hearts flow was reduced to about 50 percent of control levels for 30 minutes. Ultrastructural studies documented only subtle modifications of Purkinje fibers in ischemic hearts. Glycogen depletion and disruption of cell junctions were observed in some fibers. One consistent finding was the activation of the lysosomal system. The outer membranes of primary lysosomes appeared herniated and in some cases disrupted, and small vesicles containing hydrolytic enzymes were seen in association with the Golgi apparatus and larger primary lysosomes. Specimens prepared for the demonstration of acid phosphatase indicated a redistribution of hydrolytic enzymes in Purkinje fibers with a depostion of acid hydrolases in smaller lysosomal vesicles, the transverse and side-to-side junctions between cells, and occasionally in the sarcoplasmic reticulum. Enriched perfusate containing high levels of glucose with insulin appeared to have no therapeutic effects in terms of the structure of the Purkinje fibers. The results suggest that alterations in the lysosomal system may be one of the earliest structural changes which occur in oxygen-deficient hearts.

Effects of chronic parenteral carbohydrate administration on hepatic drug metabolism in the rat

Effects of chronic parenteral carbohydrate administration on hepatic microsomal enzyme activity were studied in the rat. Intraperitoneal injections of either glucose or fructose (2.88 g daily for 7 days) significantly decreased hepatic cytochrome P-450 content and ethylmorphine N-demethylase and aniline hydroxylase activities. By the 5th day, cytochrome P-450 content decreased to 70-76% and ethylmorphine N-demethylase activity to 66-69% of control values. Aniline hydroxylase activity was not significantly altered until the 7th day, by which time it was 77-79% of control values. In vivo assessment of hepatic drug-metabolizing capacity using antipyrine as a test drug confirmed these decreases observed in vitro. Two major conclusions of these experiments are that such variables as time and dose of carbohydrate administration can affect the magnitude of the changes produced and that each parameter measured exhibited a distinctive pattern of change with time. Chronic carbohydrate administration produced hepatic fatty infiltration and glycogen depletion. Since all groups received identical amounts of specific nutrients, fatty infiltration was probably due to increased lipogenesis with decreased hepatic oxidative metabolism of fat. During these experiments neither hypoinsulinemia nor increased levels of cyclic AMP were observed. The molecular mechanisms responsible for hepatic glycogen depletion and decreased MFO activities remain to be established.

Myocardial stimulation impedance: the effects of electrode, physiological, and stimulus variables

With exposed metal at the electrode tissue interface (8 mm2, 28 mm2, 57 mm2), myocardial threshold stimulation impedance increased as pulse duration was lengthened, with left ventricular intramyocardial stimulation, and with the smaller surface area electrode. An 0.5 mm2 differential-current-density electrode, which eliminated direct metal-to-tissue contact at the electrode-myocardial interface, was associated with notably higher impedances than each of the three metal tip electrodes and did not show increasing impedance levels with changes in pulse duration, confirming the minimization of polarization energy losses with this device. The majority of electrode, electrode tissue interface, and myocardial variables that are characterized by high threshold stimulation impedance are associated with low threshold energy requirements for pacing and reduced pacemaker power source drain. No accurate information about sensing impedances can be derived from current knowledge of pacing impedance.

Basic principles in selecting animal species for research projects

One of the most important facets of biomedical research is selecting the species of laboratory animal best suited for a particular purpose. The four major categories of animal models are experimental, spontaneous, negative, and orphan. The experimental model is the one most frequently used, but each category has a place in medical research. Once the investigator has decided which type of model the project calls for, numerous other factors must be considered: ease of handling and surgical manipulation; size, docility, and body conformation; cost of the animal and its related supplies (food, bedding, and caging); the accessibility of superficial blood vessels; and the investigator's familiarity with the various species which are available. Among the biologic factors which should be considered are resistance to natural diseases, nutritional requirements, genotype, anatomy, physiology, and biochemistry.

The organization of reticulo-olivo-cerebellar circuits in the North American opossum

By employing the autoradiographic method we have determined that the inferior olivary nucleus receives input from the reticular formation of the midbrain, pons and, probably, the medulla. The remarkable thing about such connections is that they are not diffuse, but targeted in large part on restricted portions of the caudal acessory nuclei. From our previous studies it is clear that some of the olivary regions receiving reticular input also receive projections from the cerebral cortex, the spinal cord and the dorsal column nuclei. In addition, the HRP technique reveals that these same olivary regions relay to parts of the spinal cerebellum, i.e. to restricted zones of the anterior lobe (present study). Taken together these observations suggest that some reticulo-olivo-cerebellar connections provide indirect routes through which cortical and spinal information gains access to the spinal cerebellum. Such circuits have also been suggested by the physiological literature. Of particular interest, however, was our finding that several reticular areas of the midbrain and pons project to the portion of the medial acessory nucleus which relays in turn to auditory-visual areas of the cerebellar vermis (declive, folium and tuber). It would appear that at least some reticulo-olivo-cerebellar circuits are significant in the organization of motor responses to visual and/or auditory stimuli.

Anatomical and neurobehavioral investigations concerning the thalamo-cortical organization of the rat's visual system

The organization of thalamic afferents to the rat's visual cortex was investigated autoradiographically and through the retrograde transport of horseradish peroxidase (HRP) following infections into striate and peristriate cortex. The results revealed that Nucleus lateralis posterior (NLP) projects to a large peristriate cortical field that includes areas 18A, 7, and the anterior portion of area 18, and to a circumscribed temporal area corresponding to Krieg's ('46a,b) area 20. The dorsal lateral geniculate nucleus (LGNd) was shown to project to two spatially discontinuous cortical areas. The largest geniculate receiving area is partially coextensive with Krieg's area 17, but an extension of this projection posterior and medial to the striate cortex was found. In addition, a geniculate projection to a restricted field located in the lateral peristriate cortex was identified. Concurrent investigations were designed to assess the pattern discrimination abilities of rats prepared with striate cortical ablations, lesions in NLP and combined striate-cortical and thalamic ablations. Comparison of these animals with normal control subjects revealed that the striate cortex in the rat (as in the cat [Doty, '71; Sprague et al., '77] and the tree shrew [Killackey and Diamond, '71; Ware et al., '74]) is not necessary for successful pattern discrimination, and that the geniculo-striate and NLP-extra-striate projection systems are both involved in mediating the visual discriminative abilities of the rat. The results add species generality to the concept that the central connections to the visual cortex are characterized by parallel-conducting thalamic channels and contribute to the growing number of demonstrations that the extra-striate cortex and associated thalamic cell groups contribute significantly to the process of visual-pattern recognition.

Effect of perfusate temperature on myocardial protection from ischemia

During reperfusion, functional and metabolic recovery of the isolated working rat heart from one hour of ischemia was best in hearts selectively cooled at the onset of the ischemic interval by perfusion with 5 to 10 ml. of 10 degrees C. or 15 degrees C. Krebs-Henseleit buffer. Hearts similarly perfused at 4 degrees C., 20 degrees C. recovered significantly less well or not at all. Immediately after the hour of ischemia and prior to reperfusion, the absolute levels of glycogen and high-energy phosphates were best in the hearts perfused at 4 degrees C. However, metabolic function was best preserved in those perfused at 10 degrees C. and 15 degrees C., as evidenced by rapid recovery of high-energy phosphates and glycogen to control levels compared to metabolic deterioration in the 4 degrees C. group.

An inexpensive microphotometer system for measuring silver grain densities in autoradiographs

An inexpensive microphotometric system is described which will facilitate reading grain-dense areas in autoradiographic material. It is reliable, easy to operate and adaptable to most light microscopes with a dark field condenser. In addition to the microscope, the components include a photodiode, an operational amplifier and a digital mV meter. Data are presented to demonstrate the sensitivity of the system and to illustrate the differential variability between the microphotometer and the counting of grains by eye.

Rechargeable silver-modified mercuric oxide-zinc cell for cardiac pacemakers

Tests were conducted on rechargeable mercury-zinc pacemaker batteries under simulated and actual biologic conditions, using a variety of discharge rates and charging schedules. In tests on 96 cells at a 6.4 milliampere (ma) discharge, recharging once every 15 months of simulated pacing at a 25 microampere (mua) drain, the earliest cell failure occurred after an equivalent of 50 years of pacing. The mean pacing equivalent for all 96 cells was more than 140 years. In 6.4 ma discharge tests on 24 cells, recharging once every 8 days of simulated pacing, only 1 cell in 24 failed after an equivalent of more than 500 years of pacing (actual time 2 years). In tests on 13 cells pacing at a 200 mua drain without recharging, the simulated mean duration of pacing before total discharge was 4.8 years. Seven other cells at a 200 mua drain with periodic recharging continue to function normally after more than 7 years of actual time, simulating 56 years of pacing at a 25 mua drain. Cardiac pacemakers using the rechargeable mercury-zinc cell have been implanted in animals for more than 2 1/2 years and in patients for more than 1 year with all units continuing to function satisfactorily. It has been demonstrated unequivocally that a rechargeable mercury-zinc pacemaker will function continuously for more than 4 years without recharging and that periodic recharging will extend pacing life far beyond that predicted for lithium and nuclear primary power sources.

Effects of hyperthermic stress on myocardial function during experimental coronary ischemia

We evaluated hyperthermic influences on ischemic hearts by comparing two groups of intact working swine hearts (n = 20) made globally ischemic. Heart muscle temperature was selectively increased from 37.5 +/- 0.3 degrees C to 39.7 +/- 0.3 degrees C in one group (n = 11) by warming the coronary perfusate. Ischemia in normothermic hearts significantly (P less than 0.05) decreased mechanical function (as reflected by increases in left ventricular end-diastolic pressure [LVEDP]), myocardial oxygen consumption (MVO2), glucose uptake, glycolytic flux, free fatty acid (FFA) uptake and oxidation, and tissue stores of high energy phosphates. Hearing in ischemic hearts further depressed mechanical function at similar reductions in coronary flow and MVO2. Glucose uptake was terminally increased over normothermic values (329 vs. 221 mumol/hr per g) as was glycolytic metabolism, FFA uptake (26 vs. 17 mumol/hr per g), and FFA oxidation (21 vs. 11 mumol/hr per g). However, these changes were not translated into increased energy stores of tissue creatine phosphate and ATP. Thus, in ischemic hearts, hyperthermia neither prevented the development of mechanical deterioration nor improved oxidative phosphorylation despite increases in metabolic substrate utilization. These data suggest that in experimental global ischemia heat is an added energy drain in already burdened myocardium.

Chronic testing of a pacemaker that needs recharging only once every four years

Since 1967, three series of rechargeable single-cell silver-mercuric oxide-zinc pacemakers have been implanted in dogs with complete heart block. The five nonhermetic units in series 1 failed after less than or equal to 18 months, primarily due to prototype cell deficiencies, although one cell functioned for eight years. The six units in series II contained improved cells, but failed due to gradual transepoxy fluid absorption after less than or equal to 31 months. All rechargeable cells were salvaged and dried, and, seven years after their manufacture, they continue to power pacing circuits. Series III now totals 20 doubly hermetically sealed units, tested for up to three years (total more than 300 months or 26 years), with no pacemaker failures. Accelerated tests indicate a minimum life of more than 50 years. A clinical trial is in progress.

Effects of treatment with pyruvate and tromethamine in experimental myocardial ischemia

Failure of glycolysis to increase sufficiently to supply optimal levels of energy production in ischemic heart muscle is due in part to the cummulative restrainst of acidosis on rate-limiting enzymes, particularly glyceraldehyde-3-phosphate dehydrogenase. In an effort to modify this inhibition and salvage jeopardized myocardium, treatment with excess levels of pyruvate and tromethamine (Tris), designed to buffer intracellular hydrogen ion accumulations and improve the oxidation-reduction ratio, NAD+/NADH, was tested in 59 swine hearts in two separate preparations of global and regional ischemia. Global ischemia, per se, caused hemodynamic deterioration and shortened survival time (44.3 +/- 3.1 minutes). Myocardial oxygen consumption, fatty acid oxidation, and glucose uptake were all significantly (P less than 0.001) reduced as were estimates of glycolysis and tissue stores of creatine phosphate and ATP (P less than 0.01). Although treatment with Tris alone was inconclusive, administrations of pyruvate (40-50 mM) buffered with Tris (added directly into the coronary perfusate) effected an improvement in mechanical function and a significant prolongation in survival time (56.9 +/- 2.6 minutes. P less than 0.01). Glycogenolysis was enhanced and levels of key glycolytic intermediates were reduced, suggesting an acceleration of glycolytic flux. Excess levels of pyruvate (1.52 +/- 0.48 mumol/ml of coronary perfusate) provided added substrate for oxidation and led to a greater than 5-fold incrase in rates of pyruvate decarboxylation as compared to untreated ischemic hearts...

Antenatal sex determination in Macaca arctoides

Transabdominal amniocentesis was performed on 11 Macaca arctoides, between the 10th and 20th weeks of gestation, for the purpose of antenatal sex determination. The technique employing Y-chromatin fluorescence was unsuccessful in predicting sex, but the sexes were differentiated with complete accuracy by examining amniotic cells stained by a modified Papanicolaou technique. Of the cells from male fetuses, fewer than 10% contained X-chromatin (Barr bodies); of those from females, more than 45% contained X-chromatin.

Two to three years of failure-free testing of a rechargeable pacemaker in experimental complete heart block

Six hermetically sealed single cell rechargeable mercury-zinc pacemakers (B.T.) that will run continuously for over 4 years between rechargings have paced dogs with complete heart block for from 2 to 3 years. To maintain full cell capacity (over 1,000 mA hours) requires recharging for from 2-3 min/day to 60 to 80 hr once every 4 years, with any variation between these extremes being acceptable. Six realtime bench tests continue after over 7 years and accelerated tests have simulated a minimum of 50 years continuous pacing. Battery voltage is assessed by direct telemetry, eliminating the risk of patient intrinsic rhythm-pacemaker competition which is present with all current indirect (stimulation rate change) battery assessment techniques. The B.T. is an excellent 4-5 year primary pacemaker, fully rechargeable after several 4 year periods of battery rundown. A clinical test series has been initiated.

Effects of excess glucose and insulin on glycolytic metabolism during experimental myocardial ischemia

The selective metabolic effects of glucose and insulin were tested in an intact working swine heart preparation. Supplements of glucose (26.6 millimolar [mM] and insulin (0.025 units/ml) were provided to 18 hearts, 9 control hearts (coronary flow 151 ml/min) and 9 hearts rendered globally ischemic (coronary flow reduced from 167 to 85 ml/min). These hearts were compared with 14 additional hearts (6 control and 8 ischemic) given no supplements (glucose 8.6 mM, no excess insulin). In hearts without supplements, ischemic significantly decreased mechanical performance, myocardial oxygen consumption, fatty acid oxidation and tissue high energy phosphate stores. Glucose consumption was reduced from 133 micromoles (mumol)/hr per g (before ischemia) to 58 mumol/hr per g (P less than 0.05), presumably from inhibition at glyceraldehyde-3-phosphate dehydrogenase. Data for control hearts with excess glucose and insulin were similar to data in control hearts without supplements except that glucose consumption and glycolytic flux were increased. Ischemia in treated hearts, as compared with untreated ischemic hearts, effected similar significant decreases in myocardial oxygen consumption, fatty acid oxidation and high energy phosphate stores and resulted in greater reductions in mechanical performance and in 10 minutes' less average survival time. Glucose consumption was reduced from 483 (before ischemia) to 242 mumol/hr per g (P less than 0.005) and inhibition at glyceraldehyde-3-phosphate dehydrogenase was again noted. Thus, excess carbohydrate and insulin hormone, when infused directly into the ischemic myocardium, did not provide an efficacious increase in either glycolytic flux or energy production. These findings suggest that an alternative explanation for the reported efficacy of glucose-insulin-potassium infusions must be sought.

Failure of demand pacing with small surface area electrodes

As pacemaker electrode myocardial contact area is reduced, the energy required to stimulate the heart decreases; but the effect of surface area on an electrode's ability to transmit R-wave potentials has not been well documented. Endocardial and intramyocardial R-wave potentials were measured in ten dogs with seven commercially available pacemaker electrodes of different surface area. With a load impedance of 1000 ohms, there was a direct correlation between surface area and the R-wave potentials measured. The amplitude of the R-wave that resulted from conduction from the heart to the sensor via the electrode system decreased with decreasing surface area. A similar direct correlation was seen between the R-wave potentials measured and the threshold current and energy requirements. With the present trend toward utilization of small surface area electrodes to reduce pacemaker cell drain, care must be taken to optimize pacemaker circuit impedance if sensing problems are to be avoided.

Residual effects when chronic propranolol therapy is discontinued within 48 hours of cardiopulmonary bypass

Recommendations regarding the safe waiting period between discontinuing chronic oral propranolol therapy and beginning cardiopulmonary bypass have varied from a few hours to 2 weeks. In the present study, utilizing adult dogs, propranolol was discontinued 8 or 48 hours prior to surgery. A reduction in cardiac output and elevations of left ventricular end-diastolic pressure, peak systolic pressure, and systemic resistance were noted when cardiac function was evaluated following the induction of anesthesia and prior to undertaking cardiopulmonary bypass. The magnitude of these differences was directly related to the degree of volume loading and inversely related to the interval between the last dose of propranol and the determination of cardiac function. Reduction of heart rate was the most evanescent of propranolol's hemodynamic effects as the marked bradycardia which persisted throughout the course of propranolol therapy was no longer evident 8 hours after the last oral dose of the drug. Following total cardiopulmonary bypass of 1 hour's duration, undertaken 8 hours after the last oral dose of propranolol, cardiac output and left ventricular end-diastolic pressure had returned to normal but peak systolic pressure and systemic resistance remained significantly elevated, When 48 hours had elapsed between discontinuing propranolol and beginning cardiopulmonary bypass, postbypass cardiac function was essentially normal with only slight persistent elevations of peak systolic pressure and systemic resistance detected. When the combined effects of ischemic heart disease and propranolol therapy, the altered metabolic and hemodynamic effects of different routes of drug administration, and the varying durations of cardiopulmonary bypass are taken into consideration, some of the discrepancies between previously reported clinical and experimental findings regarding the duration of persistent propranolol effects can be understood. The clinical course is usually benign in patients who have received propranolol to within a few hours of surgery without specific indication. However, it is often complicated when the drug is continued until just prior to surgery in patients dependant on propranolol for pain or arrhythmia control. In patients demonstrating propranolol dependence, control of symptoms with intra-aortic balloon counterpulsation is recommended followed by the gradual withdrawal of propranolol and elective aortocoronary bypass surgery.

Effects of successive unilateral ablations of principalis cortex upon performances of delayed alternation and delayed response by monkeys

Monkeys were trained on delayed alternation (DA), and were then subjected to serial unilateral or simultaneous bilateral ablations of the banks of the sulcus principalis of each frontal lobe. When subjects with unilateral lesions were retrained on DA, their performances were intermediate to those of normal and bilateral animals. This interoperative training failed to protect the serially operated monkeys from losses of DA following their second-stage ablations, for they then performed as poorly as one-stage subjects and subjects prepared with serial ablations that were not given practice on the task between the two operations. Additional postoperative tests of delayed responding (DR) showed that both serially and simultaneously ablated subjects also had severe impairments of performance of DR. This result confirmed, in principle, a finding that monkeys with large unilateral ablations, if reoperated after many months for the removal of the contralateral dorsolateral perfrontal cortex, will thereafter exhibit DR deficits that are both severe and endure for a period of years. It contrasted sharply with a recent observation that DR is retained by monkeys subjected to two-stage symmetrical ablations of the principalis cortex, which suggests that recoveries of frontal-lobe functions are powerfully affected by the orders in which serial extirpations are performed.

Functional reductions in left ventricular volume. Minimum chamber size consonant with effective hemodynamic performance

A heart model in dogs was developed to evaluate quantitatively the extent to which left ventricular chamber size could be reduced and yet retain residual mechanical function to perform adequately as a pump. In 9 animals placed on right heart bypass perfusion to control systemic flows; left ventricular performance was estimated from high-fidelity left ventricular pressure and aortic flowmeter recordings and from lateral plane left ventricular angiograms. Studies were made during unrestricted left ventricular filling at varying cardiac outputs and with inflation of a balloon in the left ventricular cavity at a physiological cardiac output. As compared with control data (cardiac output 1.4 L. per minute), balloon inflation to 18.7 ml. caused an increase in total left ventricular end-diastolic volume (from 35.4 to 44.3 ml., p less than 0.001) and left atrial pressure (from 7.8 to 21.2 mm. Hg; p less than 0.001); it also caused a reduction in left ventricular stroke work (from 12.5 to 8.1 Gm.-M., P LESS THAN 0.005) ANd max. dp/dt (from 2,487 to 1,320 mm. Hg per second, p less than 0.05). Importantly, left ventricular stroke volume was unchanged. When compared with preload augmentation (with the balloon deflated), the magnitude of depression of cardiac performance caused by balloon inflation was more fully appreciated (left ventricular stroke work, max. dp/dt, and ejection fraction reduced 69, 61, and 45 per cent, respectively). Even so, with appropriate compensations, principally by the Frank-Starling mechanism, up to 42 per cent of the left ventricular cavity volume could be functionally eliminated with retention of adequate mechanical performance. Such data may have implications regarding the extent of resections possible in patients undergoing surgery for left ventricular aneurysm.

A comparison of hypothalamically induced biting attack with natural predatory behavior in the cat

A comparison of biting attack induced by electrical stimulation of the cat hypothalamus with natural feline predatory attack revealed a high degree of similarity between the two behaviors. All major components of natural attack could be elicited by hypothalamic stimulation in cats not showing spontaneous attack, including stalking approach to prey, holding and pinning of the prey with forepaws, biting directed to the head and neck, prey kicking, prey carrying, and prey eating. Differences between natural and elicited attack appeared to be related to the range of completenes of elicited attack, the greater intensity of elicited attack, and the presence of unrelated responses induced by hypothalamic stimulation. Results support the view that hypothalamic stimulation facilitates sensorimotor mechanisms for separate response components, which are highly sensitive to specific stimulus features of the environment and potential goal objects.

The effect of different methods of protecting the myocardium on lysosomal activation and acid phosphatase activity in the dog heart after one hour of cardiopulmonary bypass

The present study was undertaken to determine the involvement of cardiac lyososomes in injury to the myocardium after cardiopulmonary bypass. Twenty conditioned mongrel dogs, weighing 15 to 18 kilograms, were fasted overnight, anesthetized with sodium pentobarbital (30 mg. per kilogram), intubated, and maintained on positive-pressure ventilation. The femoral artery and femoral vein were cannulated for pressure measurements. After median sternotomy, intravenous heparin was administered (3 mg. per kilogram) before the aorta and the superior and inferior venae cavae were cannulated for bypass. Bypass was instituted with a Travenol modular pump and a Bentley pediatric bubble oxygenator and heat exchanger. The ultrastructural effects on the myocardium and the acid phosphatase activity in the left ventricle were compared in dogs exposed to bypass for 1 hour with varying types of myocardial support: perfusion of the coronary arteries, normothermic ischemic arrest, or selective cardiac hypothermia. The morphology of control hearts and hearts fixed after 1 hour of coronary perfusion were similar. The distribution and structure of subcellular lysosomes were the same and showed identical patterns of acid phosphatase activity. Normothermic ischemic arrest was associated with a loss of glycogen stores, disrupted sarcoplasmic reticulum and T tubules, vacuolization and decrease in matrix density of mitochondria, and separation of the intercalated discs. Lysosomal activity was absent except for occasional residual bodies in the nuclear pole zone of the myocardial cells. Selective cardiac hypothermia produced results superior to those from normothermic ischemic arrest. Although these hearts showed proliferation of the lysosomal compartment, the organelles responsible for excitation-contraction coupling were spared.

An experimental model for studying myocardial ischemia. Correlation of hemodynamic performance and metabolism in the working swine heart

An intact, working swine heart preparation with controlled coronary perfusion is described. In this model, hemodynamic and metabolic functions were correlated in control and ischemic myocardium. A closed-loop, extracorporeal coronary perfusion circuit in series with a perfusion pump and oxygenator was designed to return reoxygenated coronary venous blood at controlled flow rates to the left and right coronary arteries. In 9 swine at normal flows (232 plus or minus 17 ml. per minute), the preparation maintained stable hemodynamic performance and oxygen consumption for a 1 hour period, after which ischemia was induced by reducing coronary flow by 50 per cent. As a result, left ventricular end-diastolic pressure (LVEDP) rose by 227 per cent, whereas heart rate (-17 per cent), aortic pressure (-9 per cent), pressure time/minute (PTM) (-28 per cent), left ventricular work (-47 per cent), and oxygen consumption (-39 per cent) all decreased. The ischemic myocardium shifted from lactate extraction to production. With this model, we can define, over a period of time, several mechanical and metabolic collations as a function of total coronary blood flow in an intact, large animal. We can also test interventions during the acute phases of ischemia in an effort to reduce myocardial damage.