Effects of gas density and ambient pressure on myocardial contractility in the rat.

BACKGROUND: Cardiac contractility and myocardial blood flow have been shown to increase when anesthetized and awake rats were exposed to normoxic 0.5 MPa ambient pressure, independent of inert gas composition. Similar changes have been demonstrated in anesthetized rats breathing a dense (relative density (RD) 5) SF6-O2 gas mixture at normobaric pressure. HYPOTHESIS: The purpose of the present study was to further explore whether cardiac contractility increases during hyperbaric exposure as a response to the elevated atmospheric pressure per se or rather as a response to increased breathing gas density. METHODS: Arterial pressure, left ventricular pressure (LVP), central venous pressure and intra-esophageal pressure were monitored in anesthetized rats during simulated dives. The rats were exposed to various gas mixtures (air, SF6-N2-O2, He-N2-O2), partial pressures of O2 (PIO2 0.02 and 0.03 MPa) ambient pressure (PTot 0.1-0.3 MPa) and gas density (RD 1-10.1). RESULTS: Cardiac contractility increased briefly by 5-10% (p < 0.05) during mild hyperoxia (PIO2 0.03 MPa). A concomitant stepwise increase in RD (1-10.1) and PTot (0.1-0.3 MPa) by adding SF6 to air, increased maximal rate of LVP rise (+dP/dt) and fall (-dP/dt) by 30% (p < 0.01). Two groups of rats exposed to either a high density SF6-N2-O2 (RD 5.5) or a normal density He-N2-O2 (RD 1.2) breathing gas of identical PTot 0.2 MPa demonstrated similar rise in dP/dt (peak 72%, p < 0.05). CONCLUSION: At moderately increased ambient pressure, pressure per se increases cardiac contractility independently of the breathing gas density.

[Establishing smoking habits among adolescents. A study of 9th-grade students]. / Etablering av røykevaner blant ungdom. En undersøkelse blant elever i niendeklasse.

A cross-sectional study of 661 9th grade pupils in Bergen municipality was conducted in November 1992. The pupils completed an anonymous questionnaire on their smoking habits. 25% of the girls and 16% of the boys reported smoking daily. Among the daily smokers, the average daily consumption of cigarettes was seven for both sexes. Most of the experimenting with cigarettes took place from the 6th to the 8th grade. Most youngsters tried their first cigarette when outdoors with friends. 27% of the regular smokers reported smoking at home, and 40% at school. In spite of legislation banning the sale of cigarettes to persons below the age of 16 years, 75% of the smokers bought the tobacco themselves.

Repeated normoxic hyperbaric exposures induce haemodynamic and myocardial changes in rats.

The effect of repeated exposure to ambient pressures of 5 bar (500 kPa), in atmospheres comprising normal partial pressures of oxygen [0.2 bar (20 kPa)] and nitrogen [0.8 bar (80 kPa)] and 4 bar (400 kPa) helium, on cardiac function and morphology was assessed in conscious rats. Ten test rats underwent chamber dives daily for 40 consecutive days, and ten control rats were exposed in the same chamber for an equal period of time, but in air at 1 bar (100 kPa). Cardiac output (Qc) and myocardial blood flow (Qmyocardial) were determined by the microsphere method. After 40 days, the body mass was 7% greater in the control than in the test rats (P < 0.05), although they were given exactly the same amount of standard food. The test rats had a significantly higher (7% absolute, 12% ventricular mass to body mass, P < 0.05) heart mass (left ventricular myocardium, including the ventricular septum) than the control rats. The percentage tissue dry mass of the right and left ventricles was equal in the two groups. Microscopic examination revealed a number of small focal necroses in the left ventricle of the test rats but none in the control rats. The left ventricular pressure (LVP) and the maximum velocity of LVP increase (contractility) and decrease were significantly increased (25%-96%, P < 0.001) in the pre-exposed compared to the control rats at 1 bar (100 kPa). The systolic arterial pressure, heart rate and respiratory frequency were similar in the two groups at 1 bar (100 kPa).(ABSTRACT TRUNCATED AT 250 WORDS)