Effect of site and intensity of pacing on left ventricular performance

The effect of pacing site and intensity on left ventricular performance is examined. The maximal value of left ventricular developed pressure (PD) and the peak (dp/dt) /P (Vpm) were followed in 11 anesthetized dogs. The following sites were paced: atrial (A), simultaneous atrial and right ventricular (RV), simultaneous atrial, right and left ventricular (RLV) and simultaneous atrial and left ventricular (LV)(, while varying the rate from sinus rate up to 350/min and the intensity from threshold up to 20-120 mA. When all 4 sites were paced in the same experiment, the A pacing resulted in the highest values of Vpm and PD while RV pacing resulted in the lowest values. The higher the pacing intensity the higher the Vpm and PD values tended to be. The strongest correlation coefficient was observed between Vpm and intensity on RV pacing while the same correlation on LV or Rv pacing and between PD and intensity on LV pacing were not significant at the 5% level. On RV pacing a low (les than 0.85) Vpm ratio (Vpm on RV pacing expressed over Vpm on A pacing at the same frequency and intensity) was associated with 65% of 129 pacing runs at a relatively low (less than 35 mA) intensity but with only 35% of 91 runs at a higher intensity (X2 = 19,20, p less than 0.001). The effect of pacing site and intensity on Vpm tended to be more pronounced at faster than at slower pacing frequencies.(ABSTRACT TRUNCATED AT 250 WORDS)

A servomechanical system to control high blood pressure using sodium nitroprusside

A servomechanical device was used to control the intravenous administration of a sodium nitroprusside solution, without using electrical energy or a pump, the aim being the fast and smooth reduction of high arterial pressure (BP). The device senses BP via an intra-arterial catheter which leads to two containers, to one through a narrow tube (slow system) and to the other through a wide tube (fast system). The two systems integrate the BP with time constants that are the product of the tube resistance to flow and the compliance of the containers. The two systems lead to small bellows that interfere with the legs of a clamp regulating an intravenous nitroprusside drip. The distension of the bellows releases the clamp in proportion to the pressure in the slow system and to the difference in pressure between the fast and the slow systems. A screw-spring could determine the desired slow system pressure below which the flow of the nitroprusside solution was stopped. The whole device was applied 11 times in five anaesthetized dogs under a continuous intravenous metaraminol infusion. The BP was always reduced smoothly within 2 to 24 min to a value near that predetermined by the screw-spring. It is concluded that a purely mechanical system involving integral, proportional and derivative components may achieve fast and smooth reduction of elevated BP to a predetermined level, when using fast-acting hypotensive agents.

Intra-aortic balloon assistance without a pump

An intra-aortic compression chamber system (CCS) was utilized in 19 anaesthetized dogs and in seven patients. The CCS consisted of an intra-aortic balloon in the thoracic aorta, an external rigid wall air chamber, and a catheter connecting the other two parts. The CCS, inflated for 2-3 min to a pressure between the systolic and diastolic arterial pressures, caused changes in the following measurements (compared with those immediately before inflation, and after deflation to atmospheric pressure): a significant (P less than 0.001) increase in cardiac output (7.8 +/- 18.8% means S.D.), aortic compliance (12.8 +/- 29.6%), aortic resistance (20.1 +/- 32.7%), cardiac power (or minute work, + 36.7 +/- 44.6%), coronary sinus outflow (+ 23.6 +/- 43.6%) and aortic backflow (+ 28.0 +/- 36.4%); the left ventricular end-diastolic pressure did not change significantly. During a 5-90 min utilization in seven patients with cardiogenic shock, the cardiac output was increased by 4.1 +/- 42.4% (lower than when intra-aortic balloon pumping was applied). It is concluded that the simple device described may be useful in clinical cardiogenic shock, especially when an intra-aortic balloon pump is not available or a portable device is desirable.

Admission process and final qualification of medical students

Several ways of admission are compared as to their effect on the final drop-out rate under an unlimited system of examinations within the Medical School. Out of 8260 students admitted during 10 years via several admission systems, 5927 graduated within 16 years from enrollment. A larger drop-out rate (as well as a longer delay in graduating) was observed with students admitted without entrance examinations. An important number of students entering after examinations (45%) were also late in graduating. Among the admission systems used the findings point to the fact that a closed system provides the school with a majority of competent students but excludes a number of students who proved capable of graduating when enrolled in the same school. The findings also point to the need of improvement of the admission process in the closed system or the need for strict first- or second-year evaluation of students in the open system.

Effect of pacing rate and intensity on mechanical alternans amplitude

The aim of this paper is to report the effect of variations in stimulus frequency (SF) and intensity (SI) of right ventricular pacing on the left ventricular mechanical alternans (MA) amplitude, (MAA) which is defined as 100 (S-W)/S, where S is the ventricular pressure of the strong beat and W that of the weak beat. In 30 dogs the right ventricle was paced at SF ranging from the sinus frequency up to 400/min and at SI ranging from threshold up to 270 mA, while the left ventricular pressure was recorded. Increasing SF at near threshold values of SI resulted in an increase of MAA up to a maximal value. Further increase in SF might cause a diminution of MAA. A slight raise in SI than could increase abruptly MAA. Further increase in SI while keeping a constant SF caused a reduction in MAA which was an approximate logarithmic function of SI. Digoxin, adrenaline or calcium reduced or abolished MA, even in experiments which left atrial and mean aortic pressure were kept constant by suitably placed open-air reservoirs. Pindolol increased MAA. Raising the height of the atrial constant pressure reservoir induced the MAA, while lowering its height increased the MAA. Changing the height of the aortic constant pressure reservoir did not produce consistent changes in MAA. The effects of SF and SI on the MAA could possibly be explained on the basis of the hypothesis that the MA is a complex oscillatory phenomenon. A different number of myocardial fibers contracting at each systole depending on SF and SI may play a role in the genesis of MA and may explain the present findings. It is concluded that, in addition to hemodynamic and inotropic factors, stimulating the ventricles at a high SI may ameliorate the MA at a given heart rate.

Direct measurement of myocadial hardness

Myocardial hardness, defined as resistance to a pitting force, was measured on several sites of the myocardium in 20 anesthetized dogs. The durometer used for hardness measurements consisted of an indentor, 4 mm in diameter, pressed perpendicularly on the myocardium by a calibrated spring (elastic modulus 325 N/m). Hardness changes during the cardiac cycle caused movement of the indentor which interrupted a light beam falling on a photoresistance. Changes in illumination of the photoresistance resulted in resistance changes, which were recorded using a Wheatstone bridge and a DC amplifier. Hardness was expressed in mm of spring shortenings. Balanced in (zero hardness), the indentor protruded 7.2 mm beyond the foot. Thus hardness adequate to press the indentor level with the foot defined as 7.2 mm. The left ventricular pressure (LVP) rise followed slightly the hardness rise (from 29.5 +/- 3.6 msec--mean +/- SEM--for the basal area at the right ventricle to 35.8 +/- 5.9 msec for the basal area of the left ventricle. Apical areas were significantly harder in systole than corresponding basal areas and left ventricular areas had a significantly higher systolic hardness (SH) than corresponding right ventricular areas. Changes in LVP by transfusion bleeding caused almost proportional changes in SH. While using a constant pressure reservoir in left atrium and another in aorta, adrenaline caused a significant increase in SH, decrease in diastolic hardness (DH) and increase in hardness width (WH); calcium chloride caused a nonsignificant increase in SH and WH and a significant decrease in DH; practolol caused a nonsignificant decrease in SH and a significant increase in DH and decrease in WH. Ligation of 3--5 branches of the anterior descending coronary artery resulted in a significant increase in DH and reduction in the WH distally to the ligation during the first 11 min. The WH was significantly reduced as a function of time in both areas (healthy and infarcted) taken collectively. The WH was significantly higher over the healthy area than over the infarcted one at corresponding times.

Mechanism of atrioventricular conduction: study on an analogue

A simple analogue of the heart, consisting of neon relaxation oscillators, is presented. The analogue may display several disturbances of the A-V conduction, like normal atrioventricular (A-V) conduction, first-degree heart block, Wenckebach periods, Mobitz II type block, supernormal conduction, complete A-V block, the phenomenon of accrochage in complete A-V block and the absolutely arrhythmic response of the ventricles to a very high atrial rate. The analogue was constructed in the simplest possible way, i.e., using the least possible number of variables. The striking similarities between the properties of relaxation oscillators and cardiac pacemakers on the one hand and between the behavior of the analogue and manifestations of the A-V conduction abnormalities on the other might possibly permit a hypothesis about the mechanism of A-V conduction abnormalities based on the analogue. This mechanism is discussed in detail.

Mechanism of escape, extrasystolic, and parasystolic arrhythmias. Study on an electrical analogue

A simple analogue of the heart consisting of a system of neon relaxation oscillators is presented. The analogue may display rhythm patterns similar to sinus rhythm, escape rhythm, isorrhythmic dissociation with synchronization, atrial extrasystoles, ventricular extrasystoles, and parasystole. The strict rules followed by these arrhythmias, as well as the deviations from the rules commonly followed by the equivalent heart arrhythmias, may be easily reproduced on the analogue. Such features are the Treppe phenomenon and captured beats in escape rhythm, fixed coupling intervals in extrasystoles, partial or complete atrioventricular block in very premature atrial extrasystoles, prolongation of the period following an atrial extrasystole, interpolated premature beats, complete compensatory pause and the rule of bigeminy in ventricular extrasystoles, slight instability of the parasystolic period, multiple length parasystolic periods slightly different from the exact multiples of the parasystolic idioperiod, preference of the parasystoles for certain phase in the sinus cycle, synchronization at a phase difference and fluctuation repeatedly and without interruption from a parasystolic to an extrasystolic rhythm and synchronization in escape rhythm with isorrhythmic dissociation. The mechanisms involved in these phenomena are discussed in detail. The striking similarity between the properties of the cardiac pacemakers and those of the relaxation oscillators on the one hand and betwen the rhythm patterns of the heart and those of the analogue on the other may permit the hypothesis that the mechanisms operating in the analogue may be used in analyzing and understanding heart arrhythmias.

Atrioventricular block response to exercise and intraventricular conduction at rest

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Heart rate and intermittent Wolff-Parkinson-White syndrome

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Maintenance of sinus rhythm after atrial defibrillation

The long-term results of 356 episodes of atrial defibrillation by DC shock carried out in 250 patients are reviewed. In 78 per cent of the cases, atrial fibrillation recurred by the end of 12 months in spite of the administration of quinidine or procainamide or propranolol, singly or combined. However, when no antidysrhythmic drugs were used the recurrence rate was higher at all periods of observation and 100 per cent at the end of 12 months. The difference in the overall results between the untreated group and the treated groups with the exception of the propranolol group is significant (p less than 0.05). In addition, the propranolol group showed an obviously reduced recurrence rate at 1 month, but afterwards the results tended to approximate those in the untreated group. The period during which sinus rhythm could be maintained in patients who underwent valve operations was on the whole the same as in patients who did not have heart surgery. However, in a small number of patients sinus rhythm was maintained for longer periods after mitral valve replacement than before it. Sinus rhythm was maintained for significantly longer periods when atrial fibrillation lasted less than 1 year than when it had been present for more than 1 year (p less than 0.001). It is concluded that there is justification for a judicious antidysrhythmic prophylaxis after atrial defibrillation.

Direct current shock and antidysrhythmic drugs

A review is given of 457 episodes of atrial fibrillation that occurred in 318 patients and were treated by DC shock. Antidysrhythmic drugs, such as quinidine, procainamide, and propranolol, given singly or in combination, were used concomitantly in 389 instances, and DC shock alone was given in 68 instances. The combined effects of quinidine and DC shock, and of procainamide and DC shock were studied in the experimental animal.Combined DC shock and drug therapy gave a higher conversion rate than DC shock alone, and a statistically significant difference was found in respect of the group of patients receiving procainamide and propranolol together (p<0.01). Antidysrhythmic drugs failed on the whole to reduce the incidence of DC shock-induced dysrhythmias. However, the incidence of certain digitalis and DC shock-induced dysrhythmias was significantly less when propranolol and procainamide were given as pretreatment than when procainamide or quinidine was given alone (p<0.01). In animal experiments, quinidine had no protective action against digitalis and DC shock-induced ectopic tachycardias. Clinical and experimental observations suggest that the cardiotoxicity of these drugs may be enhanced by DC shock. Immediate or delayed post-shock rhythm disorders can be drug related and, therefore, great caution should be exercised in the use of antidysrhythmic drugs in conjunction with DC shock therapy.