Hemodynamic and Oxygen Transport Characteristics of Common Ventilatory Modes: Discussion
The SIMV mode has been promoted as a mean of positive-pressure ventilatory support with minimal effect on venous return by allowing ITP to decrease during the spontaneous inspiratory efforts. Previous studies had suggested that in normal individuals and in those with ventricular dysfunction the steady state cardiac output is not different on SIMV as compared with positive-pressure ventilation with matched ventilatory parameters. Our study shows that SIMV by allowing lower peak and mean airway pressures throughout the respiratory cycle tends to increase Cl compared with the ACV mode, although the differences are only of borderline significance and of questionable clinical importance. Canadian family pharmacy read We show, in addition, that the PSV mode tends to affect MIP and Cl in the same manner as does the SIMV mode, although to a lesser extent. Our study is in agreement with previous reports that the magnitude of the effects of SIMV and PSV modes on Cl are small, such that although there was a trend toward higher oxygen transport and consumption on these modes compared with the ACV mode (Table 4) the differences did not reach statistical significance.
Judged by data presented in Table 3, patients used in this study had elevated pulmonary artery pressures and PVRI. The cause and the clinical importance of a slight but statistically significant rise in pulmonary artery systolic pressure on SIMV and PSV relative to ACV is not clear.
The РаСОг tended to be lower and pH higher in ACV than in other modes (Table 4). A likely explanation for this observation is that patients’ work-of-breathing and the CO2 production were less in ACV.
Although statistically significant, these values were within the normal range and the differences, therefore, were of no clinical importance. Other measures of oxygen transport and tissue oxygenation were similar among the three groups. Croeger and associates compared SIMV and AC modes on ventilatory and hemodynamic parameters in patients without COPD in acute respiratory failure. Their results are similar to ours showing an increase in f, a decrease in Vt, an increase in РаСОг, and a rise in CO on SIMV vs AC mode. No differences in resting energy expenditure and CO2 production between the two ventilatory modes, however, were observed. Hudson and colleagues showed that when critically ill patients without head injury were switched from SIMV to AC mode, respiratory alkalosis worsened and CO2 production decreased. They concluded that the rise in РаСОг during SIMV was presumably related to an increase in the WOB, although the magnitude of differences and the clinical importance of these changes were questionable.