Cardiac Synchronized Jet Ventilation Fails to Increase Cardiac Output
Cardiac Synchronized Jet Ventilation Fails to Increase Cardiac Output
Abstract & Commentary
Synopsis: Comparing synchronized jet ventilation (SJV) with conventional mechanical ventilation (CMV), no significant effects on cardiac hemodynamics were seen in 20 patients following cardiac revascularization surgery. This was also true in those patients with reduced cardiac ejection fraction.
Source: Romand JA, et al. Anesthesiology 2000;92: 24-30.
Some patients with severe ventricular failure have been shown to have increased cardiac output when ventilated with high-frequency jet ventilation timed and synchronized with the cardiac ejection cycle compared to conventional ventilation. These preliminary observations were seen with high jet pressures producing large ventilation volumes and marked hyperventilation. This study was designed to evaluate the effect of synchronization of jet ventilation compared to conventional ventilation on cardiac output when gas exchange was normal.
Twenty patients with normal or only moderately depressed left ventricular function and no evidence of chronic obstructive pulmonary disease or right ventricular dysfunction were studied following coronary bypass surgery. Patients were sedated with midazolam and morphine, ventilated at 12 breaths per minute with 8-10 mL/kg tidal volumes to achieve a PaCO2 of 40 mmHg. Positive end-expiratory pressure (PEEP) was set at 5 cm H2O, and inspired oxygen fraction was adjusted to maintain SaO2 greater than 93%. Pulmonary capillary wedge pressure was maintained at 10 mmHg with continuous infusion of normal saline and boluses as needed. A continuously reading oximetric pulmonary artery catheter was used to monitor mixed venous oxygen saturation (SVO2). Immediate changes in SVO2 were used to identify the effect on cardiac output and to optimize jet timing while changing the jet firing throughout the cardiac cycle. Jet ventilation was delivered with no entrainment and passive exhalation, via an Accutronic AMS 100 ventilator (Jona, Switzerland).
After two hours to confirm stability, patients were studied during five sequential 30-minute periods of ventilation: CMV, SJV, CMV, SJV, and return to CMV. The first SJV period was used to optimize the timing of the jet ventilation by dividing the electrocardiographic R-R interval into 10 parts, moving the delivered jet to each point and observing the effect on SVO2. The second period was the test period using the triggering determined during the first period. Thermodilution cardiac output measurements, determined in triplicate, were compared during SJV and CMV, with each patient serving as his or her own control. The cardiac hemodynamic effects in patients with ejection fractions greater and less than 45% were grouped and compared.
No systematic differences were identified in cardiac output during any form of ventilation. This was also true in the 10 patients with low ejection fractions (EF = 35 ± 6%). Heart rate averaged 86 ± 15 beats/minute, venous admixture was 27%, oxygen extraction was 40%, and cardiac index averaged 2.5 L/min/m2 under all conditions.
COMMENT BY CHARLES G. DURBIN, Jr., MD, FCCM
From this small study, it appears that synchronized jet ventilation offers no hemodynamic advantage in patients with modestly reduced cardiac function. There are several limitations to generalizing the conclusion that SJV offers no advantages. While Romand and colleagues attempted to maintain the same blood gases during the test periods, the PaCO2 was lower during SJV than during CMV (32 compared to 35 mmHg). This may have counteracted any advantage of SJV on cardiac ejection by altering venous return and cardiac muscle function.
Another limitation was the relatively low filling pressure goal, 10 cm H2O. This could have made the effects of airway pressure on cardiac ejection less apparent. The fact that others have seen increased cardiac output with SJV in severe heart failure suggests that perhaps an effect on ventricular filling may be more important in improving cardiac function as the presumed effect on cardiac ejection.
An interesting sidelight of this study is the way jet timing was optimized using the immediate effects on the SVO2. By observing the immediate change in the continuously monitored SVO2, the effects of changes in cardiac output were rapidly identified. This method can be used to optimize vasoactive drug infusions and arteriovenous (A-V) pacing intervals, as well as ventilation changes. While several factors determine SVO2 (for example, hemoglobin level, oxygen consumption, hemoglobin oxygen affinity, and cardiac output), only changes in cardiac output can cause the rapid response observed in the seconds following an intervention. Thus, continuously monitoring SVO2 allows hemodynamic optimization of many interventions in real time.
In conclusion, while this well-designed and executed study increases our understanding of the effects of synchronization of ventilation on cardiac function, it leaves many unanswered questions.
Compared to conventional ventilation, synchronized jet ventilation:
a. increases the risk of pneumothorax.
b. depresses myocardial contractility.
c. improves cardiac output.
d. decreases time to extubation.
e. achieves comparable hemodynamics.
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