Cardiorespiratory Effects of Pressure Controlled Inverse Ratio Ventilation in Severe Respiratory Failure: Methods

In: Respiratory Failure

24 Jan 2015

Nine patients (Table 1) with severe ARDS, as manifested by diffuse pulmonary infiltrates on chest roentgenograms, arterial hypoxemia with widened A-a gradients despite supplemental oxygen, pulmonary capillary wedge pressures (WP) less than 20 mm Hg, and decreased static and dynamic thoracic compliance were entered in the study. In each case, the patient was placed on PC-IRV at the request of the attending physician, who judged the patient to be failing conventional volume contrblled ventilation with conventional ratios (VC-CRV). In all cases, a flow-directed pulmonary artery (Swan-Ganz) catheter with a fiberoptic channel for continuous measurement of mixed venous oxygen saturation had been placed previously for hemodynamic and cardiorespiratory monitoring. The mixed venous oxygen saturation determination from the pulmonary artery catheter was calibrated and verified by using a mixed venous blood sample in which the oxygen saturation was measured by COoximetry. All patients had indwelling arterial catheters and pulse oximeters. All patients previously had been placed on a Servo-controlled ventilator, operating in the volume controlled ventilation mode. Prior to initiation of the PC-IRV trial, all patients were sedated with appropriate doses of benzodiazepines and paralyzed with vecuronium or atracurium by continuous intravenous infusion, after an initial bolus dose. More info

In each patient, immediately prior to initiation of the PC-IRV trial, a full set of hemodynamic and cardiorespiratory variables was measured. This included measurement of arterial systolic and diastolic pressure, heart rate, right atrial pressure, pulmonary artery systolic and diastolic pressure, and pulmonary capillary wedge pressure. Cardiac output was measured in triplicate by the thermodilution technique at end expiration. The timing of injection for cardiac output measurement was supervised by one of the authors (E.A.) to verify that the injection was initiated at the same point in the respiratory cycle. Determination of arterial blood gases (Pa02, PaC02, pH), arterial oxygen saturation (SaOJ, calculated from the arterial blood gas and measured directly with the pulse oximeter, and mixed venous oxygen saturation (SvO*) were made. Inspired oxygen concentration (Flo*), PEEP, and peak airway pressure during inspiration were measured.
The patient then was placed on PC-IRV with an I:E ratio of 2:1, using 67 percent inspiratory time and zero percent pause. The respiratory rate was adjusted to keep the end-expiratory pressure at the same level as the PEEP utilized during volume controlled ventilation. A strip recorder was interfaced with the ventilator to verify that the above goals had been achieved and that zero expiratory flow was not reached prior to the triggering of the next breath.
After a 30-minute stabilization period on PC-IRV with an I:E ratio of 2:1, another full set of cardiorespiratory parameters, as described above, was obtained. The Flo* and PIP again were measured. All of the PC-IRV trials were able to be continued at least for the 30-minute measurement period. In patients with improved oxygenation and without hemodynamic compromise, the PC-IRV was continued for periods as long as 96 hours.
Table 1—Patient Characteristics

Patient Age/Sex Major Clinical Problems PEEP (cm H*0) Flo*
1 44/F Liver failure, GI bleed, hepatorenal syndrome 15 1.0
2 42/M Chronic myelogenous leukemia, post bone marrowtransplantation, CMV pneumonia 15 1.0
3 61/M GI bleed, renal failure, aspiration pneumonia 12 1.0
4 67/F Myelofibrosis, sepsis, pulmonary hemorrhage 10 1.0
5 33/F SLE, Pneumocystis carinii pneumonia, hypertension 12 0.8
6 40/M Acute lymphocytic leukemia, DIC, renal failure, sepsis 6 0.7
7 34/M Chronic myelogenous leukemia, post bone marrowtransplantation, CMV pneumonia 14 1.0
8 46/F Acute myelogenous leukemia, post bone marrowtransplantation, Enterobacter cloaceae sepsis 7.5 0.6
9 52/F Pneumoccocal sepsis, SLE 17 0.9

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