Oxygenation remains unaffected by increased inspiration-to-expiration ratio but impairs hemodynamics in surfactant-depleted piglets.
OBJECTIVE
Prolongation of inspiratory time is used to reduce lung injury in mechanical ventilation. The aim of this study was to isolate the effects of inspiratory time on airway pressure, gas exchange, and hemodynamics, while ventilatory frequency, tidal volume, and mean airway pressure were kept constant.


METHODS
Randomized experimental trial.


METHODS
Experimental laboratory of a University Department of Anesthesiology and Intensive Care.


METHODS
Twelve anesthetised piglets.


METHODS
After lavage the reference setting was pressure-controlled ventilation with a decelerating flow; I:E was 1:1, and PEEP was set to 75% of the inflection point pressure level. The I:E ratios of 1.5:1, 2.3:1, and 4:1 were applied randomly. Under open lung conditions, mean airway pressure was kept constant by reduction of external PEEP.


RESULTS
Gas exchange, airway pressures, hemodynamics, functional residual capacity (SF6 tracer), and intrathoracic fluid volumes (double indicator dilution) were measured. Compared to the I:E of 1:1, PaCO2 was 8% lower, with I:E 2.3:1 and 4:1 (p < or = 0.01) while PaO2 remained unchanged. The decrease in inspiratory airway pressure with increased inspiratory time was due to the response of the pressure-regulated volume-controlled mode to an increased I:E ratio. Stroke index and right ventricular ejection fraction were depressed at higher I:E ratios (SI by 18% at 2.3:1, 20% at 4:1; RVEF by 10% at 2.3:1, 13% at 4:1; p < or = 0.05).


CONCLUSIONS
Under open lung conditions with an increased I:E ratio, oxygenation remained unaffected while hemodynamics were impaired.
Lichtwarck-Aschoff
M
M
Department of Anesthesiology and Intensive Care, University Hospital, Uppsala, Sweden.
Markström
A M
AM
Hedlund
A J
AJ
Nielsen
J B
JB
Nordgren
K A
KA
Sjöstrand
U H
UH
D000818:Animals; D005652:Functional Residual Capacity / Q000502:physiology; D006439:Hemodynamics / Q000502:physiology*; D018467:Positive-Pressure Respiration, Intrinsic / Q000503:physiopathology*; D011312:Pressure; D011659:Pulmonary Gas Exchange; D011897:Random Allocation; D012121:Respiration, Artificial / Q000379:methods*; D015656:Respiratory Mechanics; D013552:Swine; D013997:Time Factors