title: Effects of gas density and ambient pressure on myocardial contractility in the rat.
abstract: 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.
  
  
  OBJECTIVE
  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).
  
  
  CONCLUSIONS
  At moderately increased ambient pressure, pressure per se increases cardiac contractility independently of the breathing gas density.
authors: Norwegian Underwater Technology Centre, Ytre Laksevåg, Norway.
authors: J
authors: J
authors: Risberg
authors: S
authors: S
authors: Skei
authors: I
authors: I
authors: Tyssebotn
mesh_terms: D000818:Animals; D001274:Atmospheric Pressure; D004242:Diving; D005740:Gases; D006439:Hemodynamics; D018496:Hyperoxia / Q000503:physiopathology; D008297:Male; D009200:Myocardial Contraction / Q000502:physiology*; D010812:Physical Stimulation; D051381:Rats; D017208:Rats, Wistar
chemical_list: D005740:Gases