The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

A COMPARISON OF THE VENTILATORY EFFICIENCY OF ASSISTED VENTILATION WITH FIXED INSPIRATORY FLOW RATES TO VOLUME SUPPORT VENTILATION WITH VARIABLE INSPIRATORY FLOW.

David C. Shelledy, PhD. RRT, Robert W. Lawson, MS, RRT, Oliver J. Drumheller, EdD, RRT, The University of Texas Health Science Center at San Antonio, San Antonio, Texas

BACKGROUND: Volume support (VS) is a newer mode of mechanical ventilation that combines aspects of pressure support and volume ventilation. We compared the ventilatory efficiency of VS with variable inspiratory flow to traditional assist ventilation (AV) with three different fixed inspiratory flow (FIF) rates. METHOD: A prospective, randomized blocks repeated measures design with subjects serving as their own controls was used to study 10 healthy volunteers, aged 22 to 49. Baseline spontaneous ventilation data collection was followed by mechanical ventilation by mouthpiece using variable flow VS and AV with FIFs of 40, 60 and 80 L/min applied in random sequence. All modes used a VT of 10cc/kg of ideal body weight and a sensitivity of -1 cm H_{2}O [-0.0981 kPa] delivered via a Siemens Servo 300 ventilator (Siemens-Elema, Solna, Sweden). Data was collected continuously for 5 minutes using the MetaScope metabolic cart (Colorado MEDtech Inc., Boulder, Co.) and the mean values were reported. Ventilatory equivalent for oxygen (VEQ) is a measure of the efficiency of the ventilatory pump at various workloads and was calculated by dividing V_{E} (BTPS) by VO_{2} (STPD). Results:

Means (SD)

Mode Spontaneous Assist Ventilation VS

flow rate varies 40L/min 60 L/min 80L/min varies

rate(bpm) 15(4) 18*(7) 20(8) 21(8) 20(9)

V_{T}(ml) 608(160) 1033(320) 998(234) 995(230) 918(138)

V_{E}(L/min) 8.9(2.7) 17.2(5.8) 18.9(6.2) 20.3(6.4) 18.5(9.6)

VO_{2}(ml/min)263(37) 277(52) 292(47) 276(37) 279(62)

VEQ(L/L) 34.0(9.6) 61.5(13.2) 64.6(16.7) 72.8**(16.5) 64.7(20.7)

* significantly different (p = .03) than AV with FIF 80 L/min.

** significantly different (p < .05) than AV with FIF 40, AV with FIF 60 and VS.

There were significant differences in VEQ (p =.007) and respiratory rate (p=.039) using ANOVA. There were no significant differences in V_{E} (p= .14). V_{T} (p=.37) or VO_{2} (p= .35). Neuman-Keuls pair wise follow-up comparisons found that a FIF of 80 L/min produced a significantly greater VEQ than FIF 40 L/min (p= .005), FIF 60 L/min (p=.032) and VS (p =.014). AV with an FIF of 40 L/min produced a significantly lower rate than FIF 80 L/min (p=.03). There were no significant differences in VEQ between FIF 40 L/min and FIF 60 L/min (p=.32); FIF 40 L/min and VS (p=.55); FIF 60 L/min and VS (p=.97). Conclusions: AV with a FIF of 80 L/min was superior in ventilatory efficiency to VS. AV with FIF 40 L/min and AV with FIF 60 L/min. VS did not significantly improve ventilatory efficiency when compared to AV, regardless of inspiratory flow rate used.

Reference: OF-96-143

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