Hemodynamic and Oxygen Transport Characteristics of Common Ventilatory Modes: Patients
This study was approved by our Institutional Review Board. Each patient or their next of kin signed a written informed consent form.
We studied 12 patients admitted to the medical intensive care units (Veterans Affairs and University of Cincinnati Medical Centers) who were already on mechanical ventilatory support and were being hemodynamically monitored as determined by the primary medical team. Patients with a variety of underlying acute or chronic pathophysiologic disorders or both were accepted (Table 1) as long as they met the following inclusion criteria: (1) being awake and arousable, (2) being able to breath spontaneously, and (3) being in “stable” respiratory condition, which meant the absence of any significant or acute change in lung compliance and resistance at the time of the study by processes such as severe unresponsive bronchospasm, copious tracheobronchial secretions, impending or rapidly developing pulmonary edema, significant intrinsic positive end-expiratory pressure (PEEP), etc. Patients, therefore, must have been considered to be optimally ventilated on the ACV mode to be included in the study.
All patients were on the ACV mode at the beginning of the study period. Buy allegra online Click Here The ACV setting was determined by the medical team to provide optimum ventilatory support for the given patient using the Bear 5 (Bear Medical Systems, Inc, Riverside, Calif) or Puritan-Bennett 7200 (Puritan-Bennett Corp., Overland Park, Kan) ventilators. The sensitivity was set at —2 cm H2O.
While on the SIMV mode, patients received synchronized machine-generated breaths with the setting exactly as on ACV mode, but with the breathing frequency (f) arbitrarily set at 75 percent of the f while on the ACV. This allowed patients to initiate additional spontaneous breaths with different frequencies and in an unpredictable manner. Since the study protocol represented the first attempt at weaning in all patients, the SIMV rate at 75 percent of the f on ACV was chosen to avoid undue stress to the patients and to prevent early fatigue.
The PSV mode was set using pressures necessary to achieve a similar expiratory tidal volume (Vt) as when on the ACV mode. This, we believed, was a critical issue because support can be provided over a wide range of pressures with different hemodynamic and oxygen transport effects accordingly; providing equal Vt was one way of standardizing the level of ventilatory support so that data on ACV vs PSV modes could be compared. A back-up minute ventilation (augmented mandating ventilation) equal to one on the ACV mode, was set on the ventilator in case patients’ spontaneous respiratory efforts failed.
Table 1—Clinical Characteristics of Patients
Patient No. | Age, Yr/Sex | Diagnosis | FIo2, % | PS, cm H20 |
1 | 69/M | Pneumonia, noncardiogenic pulmonary edema | 50 | 15 |
2 | 73/M | Congestive heart failure, coronary artery disease | 40 | 22 |
3 | 78/F | Biventricular heart failure, systemic hypertension | 35 | 10 |
4 | 70/F | COPD, congestive heart failure | 40 | 15 |
5 | 79/F | COPD, pneumonia, congestive heart failure | 30 | 15 |
6 | 58/M | Congestive heart failure, coronary artery disease | 40 | 15 |
7 | 84/F | Congestive heart failure | 40 | 20 |
8 | 54/F | COPD, respiratory failure | 35 | 20 |
9 | 43/F | Pneumonia, sepsis, systemic hypertension | 30 | 25 |
10 | 60/M | COPD, respiratory failure | 50 | 15 |
11 | 56/F | Respiratory failure, pulmonary fibrosis | 40 | 35 |
12 | 54/F | Pneumonia, sepsis | 30 | 15 |