Autotriggering caused by cardiogenic oscillation during flow-triggered mechanical ventilation.

OBJECTIVES: We noticed that in some patients after cardiac surgery, when flow triggering was used, cardiogenic oscillation might be autotriggering the ventilatory support. In a prospective study, we evaluated the degree of cardiogenic oscillation and the frequency rate of autotriggering. We suspected that autotriggering caused by cardiogenic oscillation was more common than clinically appreciated. DESIGN: Prospective, nonrandomized, clinical study. SETTING: Surgical intensive care unit in a national heart institute. PATIENTS: A total of 104 adult patients were enrolled after cardiac surgery. INTERVENTIONS: During the study period, patients were paralyzed and ventilated with intermittent mandatory ventilation at a rate of 10 breaths/min, pressure support of 10 cm H2O, and flow triggering with a sensitivity of 1 L/min. MEASUREMENTS AND MAIN RESULTS: Because the patients would not be able to breathe spontaneously, we counted pressure-support (PS) breaths as instances of autotriggering. Then, we classified the patients into two groups according to the number of PS breaths: an "AT group" (PS breaths of >5/min) and a "non-AT group" (PS breaths of < or =5/min). If autotriggering occurred, we decreased the sensitivity so autotriggering disappeared (threshold triggering sensitivity). The intensity of cardiogenic oscillation was assessed as the flow and airway pressure at the airway opening. A total of 23 patients (22%) demonstrated more than five autotriggered breaths/min. During mechanical ventilation, the inspiratory flow fluctuation caused by cardiogenic oscillation was significantly greater in the AT group than in the non-AT group (4.67+/-1.26 L/min vs. 2.03+/-0.86 L/min; p<.01). The AT group also showed larger cardiac output, higher ventricular filling pressures, larger heart size, and lower respiratory system resistance than the non-AT group. As the inspiratory flow fluctuation caused by cardiogenic oscillation increased, the level of triggering sensitivity also was increased to avoid autotriggering. In the AT group with 1 L/min of sensitivity, the respiratory rate increased (19.9+/-2.7 vs. 10+/-0 breaths/min, p<.01), Paco2 decreased (30.8+/-4.0 torr [4.11+/-0.36 kPa] vs. 37.6+/-4.3 torr [5.01+/-0.57 kPa]; p < .01), and mean esophageal pressure increased (7.7+/-3.0 vs. 6.9+/-3.0 cm H2O; p<.01) compared with the threshold triggering sensitivity. CONCLUSIONS: Autotriggering caused by cardiogenic oscillation is common in postcardiac surgery patients when flow triggering is used. Autotriggering occurred more often in patients with more dynamic circulation. Autotriggering caused respiratory alkalosis and hyperinflation of the lungs.

Comparison of four methods for measuring elevation of FRC in mechanically ventilated infants.

The aim of the study was to compare measurements of the elevation of functional residual capacity (FRC) above the relaxation volume obtained in 34 mechanically ventilated infants (median weight 2.6 kg, range 1.2-9) from four different methods: (1) direct measurement of the complete exhalation volume after brief disconnection from the ventilator, (2) calculated measurement from total positive end-expiratory pressure (PEEP) measured by end-expiratory occlusion of the breathing circuit, (3) extrapolated evaluation from the mathematical model of Brody, (4) extrapolated evaluation from the passive expiration method. We considered the direct measurement (1) as the "gold standard". Measurements obtained by total PEEP (2) and by the Brody's mathematical model (3) provided similar results than the direct measurement. Conversely, graphical extrapolation from the passive expiration method (4) underestimated the elevation of FRC. In conclusion, we suggest using the mathematical extrapolation from the Brody's model to evaluate the elevation of FRC in mechanically ventilated infants: this method is non-invasive, does not require disruption of gas flow, can be easily performed with all the neonatal ventilators, and allows continuous breath-by-breath measurements.

Dynamic measurement of intrinsic PEEP does not represent the lowest intrinsic PEEP.

OBJECTIVE: Dynamic intrinsic PEEP (PEEPi-dyn) is the airway pressure required to overcome expiratory flow and is considered to represent the lowest regional PEEPi. However, there are few data to validate this assumption. We investigated if PEEPi-dyn represents the lowest PEEPi. SETTING: The animal laboratory at the Osaka University Medical School. MEASUREMENTS AND RESULTS: We compared static PEEPi (PEEPi-stat) and PEEPi-dyn in healthy animals. Five adult white rabbits (2.77+/-0.05 kg) were anesthetized, tracheostomized, and intubated with several different sizes of endotracheal tubes (ETT) (2.0, 2.5, 3.0, 3.5, or 4.0 mm i.d.). The animals were paralyzed and ventilated (Siemens Servo 900C). Baseline ventilator settings were at a rate of 50/min, inspiratory:expiratory (I:E) ratio of 2:1 or 4:1, and minute ventilation was manipulated to create 3 or 5 cm H2O PEEPi-stat. PEEPi-stat was measured using the expiratory hold button of the ventilator. PEEPi-dyn showed large variations. In all ventilator settings, PEEPi-dyn was higher than PEEPi-stat (p<0.001). The larger the ETT, the higher the PEEPi-dyn at an I:E ratio of 2:1 (p<0.05). The higher the minute ventilation, the greater the difference between PEEPi-stat and PEEPi-dyn. The tidal volume and the difference showed a significant correlation (r2 = 0.514, p<0.001). CONCLUSIONS: The value of PEEPi-dyn was dependent on ventilatory settings, and PEEPi-dyn does not necessarily represent the lowest regional PEEPi within the lungs.