Change in Intrathoracic Pressure in Rats with Spontaneous and Controlled Ventilation during Microgravity by Parabolic Flight

We previously reported that the intrathoracic pressure (ITP) decreases and the transmural pressure of the aortic wall (TMP) increases during 4.5 s of microgravity (μG) induced by free drop. To examine the ITP response to a longer period of μG in the absence of the respiratory rate (RR) decrease, i.e., bradypnea, which occurs at the onset of μG, we measured the aortic blood pressure at the diaphragma level (AP) and ITP. We then calculated the TMP at the aortic arch level during 20 s of μG induced by parabolic flight in anesthetized rats (n = 7) with either spontaneous ventilation (SPN-V) or controlled ventilation (CONT-V). In the SPN-V group, the bradypnea was observed in all rats after the onset of the μG (RR change −13.9 ± 2.9/min). The ITP during μG (−9.3 ± 0.9 mmHg) was significantly lower than that during 1 G (−7.7 ± 0.9 mmHg), and the TMP was significantly increased during μG (112 ± 6 mmHg) compared to 1 G (103 ± 5 mmHg). Similar changes in ITP and TMP were observed in the CONT-V group: During μG and 1G, respectively, the ITP was −8.4 ± 0.6 mmHg and −5.9 ± 0.7 mmHg, and the TMP was 112 ± 6 mmHg and 101 ± 6 mmHg, whereas no change in RR was observed because of the controlled ventilation. These results show that the ITP decreases and the TMP increases during μG, and they are not affected by a disturbance of respiratory rhythm.