Comparison of pressure support ventilation and assist-control ventilation in the treatment of respiratory failure.

STUDY OBJECTIVE: To assess whether pressure support ventilation (PSV) could be used as an alternative ventilatory mode to assist-control (A/C) ventilation in the treatment of respiratory failure. DESIGN: A short-term (4-h) prospective study in which the beneficial effect of PSV on respiratory mechanics, gas exchange, arterial oxygenation, cardiovascular hemodynamics, and oxygen consumption was compared with A/C ventilation. SETTING: ICU of a community hospital. PATIENTS: Forty-five patients (mean age, 62.8 [11.8] years) with respiratory failure secondary to COPD, restrictive disorders, or neuromuscular disease requiring mechanical ventilatory support in the ICU were selected for study. INTERVENTIONS: The mean duration of mechanical ventilation before the study was 7.16 (8.64) days. Patients were switched to the PSV mode of the mechanical ventilator for a period of 4 h after which conventional A/C ventilation was resumed. RESULTS: Patients supported with PSV compared with A/C ventilation showed significantly higher tidal volume, minute ventilation, and inspiratory time in association with significantly lower pressure in the airway and I:E ratio. With regard to gas exchange data, an increase in dead space/tidal volume ratio (VD/VT), decrease in PaO2, and statistically but not clinically significant alteration of arterial oxygenation indexes were noted. However, when patients with COPD, restrictive disorders, and neuromuscular disease were compared, significant changes in arterial oxygenation parameters were found only in patients with restrictive disorders. There were significant decreases in heart rate, systolic pulmonary artery pressure, and pulmonary capillary wedge pressure when PSV was applied. Oxygen transport and oxygen consumption were unchanged. CONCLUSIONS: PSV could be a possible alternative to A/C ventilation in patients with respiratory failure. PSV caused an increase in VD/VT in association with a significantly lower pressure in the airway and I:E ratio. Randomized studies are needed to define the long-term benefits of both respiratory modes and the conditions in which PSV may be a valuable alternative to A/C ventilation.

Evaluation of the left ventricular performance through equilibrium radionuclide angiocardiography.

In patients with severe heart failure due to acute myocardial infarction (AMI) breathing with PEEP can be of additional therapeutic value. This study was designed to assess the effects of CPAP through face mask with 15 cm H2O on left ventricular performance in AMI patients, using equilibrium radionuclide angiocardiography (ERA). In response to lung inflation, high levels of PEEP have been shown to decrease heart rate and stroke volume. The sum of the TPF pathological prolongation and the Mean-FR reduction suggests a decrease in the left ventricular compliance determined by the restriction imposed by the positive pressure. The global systolic performance is preserved.

[Noninvasive ventilator support in patients with chronic obstructive pulmonary disease. A comparison of 2 methods]. / Apoyo ventilatorio no invasivo en pacientes con enfermedad pulmonar obstructiva crónica. Comparación de dos métodos.

The external high frequency oscillation around a negative baseline (EHFO-NB) is a new mode of non-invasive ventilatory support which could replace or complement nasal intermittent positive pressure ventilation (NIPPV) for treatment of patients with chronic obstructive pulmonary disease, both in the decompensation phases and as preventive measure in intercrisis periods. This was a prospective study in which tolerance and short term effects on acid-base balance and gas interchange of both NIPPV and EHFO-NB in twenty patients with severe chronic obstructive pulmonary disease were compared. With both methods similar decrease in PaCO2 and increases in pH were observed, with decrease and increase in respiratory frequency when using NIPPV and EHFO-NB, respectively; the latter in association with the high oscillation frequencies used. While an improvement of SaO2 and no changes in PaO2 and intrapulmonary shunt were observed on NIPPV, a decrease was observed in the SaO2 and an increase in the shunt on EHFO-NB. Comfort of patients, improvement of perceived sensation of dyspnea, and the absence of complications were similar with both methods. While on NIPPV, 75 per cent of patients required a short period of training and continuous supervision.

[Airway continuous positive pressure in acute respiratory failure caused by Pneumocystis carinii pneumonia]. / Presión positiva continua de la vía aérea en la insuficiencia respiratoria aguda debida a neumonía por Pneumocystis carinii.

OBJECTIVE: To evaluate the efficiency of continuous positive airway pressure through a face mask in acute respiratory insufficiency (ARI) secondary to Pneumocystis carinii pneumonia. DESIGN: Prospective study. SETTING: Multidisciplinary ICU. PATIENTS: Fifteen patients with ARI secondary to Pneumocystis carinii pneumonia were studied. INTERVENTIONS: Initially al patients received high flow oxygen therapy through a face mask for 60 minutes (Pre-CPAP phase); then CPAP through a face mask, with identical FiO2 and for a similar period of time (Pst-CPAP phase). At the end of each phase the following parameters were evaluated: respiratory rate, heart rate, arterial gases, acid-base balance, and respiratory muscle motility. MEASUREMENTS: After oxygen therapy all fifteen patients had similar variables. After 60 minutes with CPAP through a face mask, significant improvements were noted for respiratory rate, heart rate, muscular effort, PaO2, SaO2, and PaO2/FiO2 (p < 0.001) in eleven patients, who survived after a mean stay of 8.5 days in the ICU with no evidence of major complications. In contrast, CPAP failed in four patients as respiratory rate, heart rate and vigorous muscle effort remained unchanged and, although PaO2 and SaO2 increased, the obtained values were significantly lower than in the remaining patients. Consequently, they underwent intubation and mechanical ventilation and after a mean stay of fourteen days with this ventilatory option died. CONCLUSIONS: These results confirm that CPAP through a face mask is an effective means to improve oxygenation in patients with Pneumocystis carinii pneumonia who develop hypoxemic respiratory insufficiency. Its early introduction in hospital protocols can help improving the prognosis in certain patients with severe PCP and avoid their admission to ICU, which would be thus preserved for those patients requiring mechanical ventilation.

[Acute respiratory failure in chronic obstructive lung disease. Effective weaning and excess base]. / Fallo respiratorio agudo en la enfermedad pulmonar obstructiva crónica. Desconexión eficaz y exceso de base.

To establish whether the weaning from assisted mechanical ventilation (AMV) should be accompanied by continuous respiratory support (synchronized intermittent mandatory ventilation [SIMV]) or discontinuous support (O2 in T) in patients affected by chronic obstructive lung disease (COLD) who are recovering from acute respiratory failure (ARF), and also to identify any possible predictive value of gasometric measurements. Sixteen patients with COLD and ARF were studied prospectively during their stay in the intensive care unit. Ten had acute bronchitis, 3 had left ventricular failure and 2 had pneumonia. In 1 case the etiology was unknown. The following protocol was used for the first attempt at weaning: 1) SIMV for 30 min, 2) return to rest period with AMV for 2 hours, 3) O2 in T-tube for 30 min. After 30 min both SIMV and O2 in T had produced a rise in PaCO2 to 55.1 and 54.6 mmHg, respectively (p < 0.001), with a subsequent lowering of pH to 7.32 and 7.36 (p < 0.001). When weaning was well tolerated, pH decreased significantly due to an increase in PaCO2 with both techniques, while base excess (BE) remained stable. Values of pH also decreased significantly when weaning was poorly tolerated, and the fall was greater with SIMV; increases in PaCO2 were similar, but decreases in BE were significant. When pH is kept within normal range by a high BE, the withdrawal of AMV, accompanied by either support system, is usually well-tolerated.

The effect of hypo-, normo-, and hypercapnia induced by mechanical ventilation on intrapulmonary shunt.

The effects of hypo-, normo- and hypercapnia on the variations in arterial oxygenation and their indices in critical patients with acute respiratory failure (ARF) receiving mechanical ventilation are studied. It is a prospective and randomized study carried out in multidisciplinary ICU. Fifteen ARF patients, intubated and mechanically ventilated, were studied within the first 48 h of evolution. Three stages were delimited: I) 30 min after the beginning of anaesthesia; II) 30 min after adding 30 cm of dead space (VD); III) 30 min after replacing the previous VD with VD of 60 cm. Ventilation parameters and FiO2 were kept stable. Stage I was characterized by respiratory alkalosis and stage II by normal acid-base balance with an increase in PaO2 (p < 0.01) and a decrease in intrapulmonary shunt (Qsp/Qt) (p < 0.001); the indices alveolar to arterial oxygen tension gradient [P(A-a)O2], respiratory index (R.I.) and estimated shunt (Est Shunt) also decreased significantly, whereas arterial to alveolar oxygen tension ratio (PaO2/PAO2) and arterial oxygen tension to inspired oxygen fraction ration (PaO2/FiO2) increased significantly. In stage III there was pure hypercapnic acidosis, with decreases in PAO2 (p < 0.001), P(A-a)O2 (p < 0.01) and R.I. (p < 0.05), while PaO2, Qsp/Qt, Est Shunt, PaO2/PAO2 and PaO2/FiO2 remained stable with respect to the previous situation. The observed PaO2 differs (p < 0.05) from the expected PaO2 in stage III. It is suggested that local or regional modifications of pulmonary perfusion are responsible for the observed variations. The P(A-a)O2 and R.I. indices do not make it possible to differentiate the causes of arterial hypoxemia in the presence of hypercapnia.

Modifications of tissular oxygenation and systemic hemodynamics after the correction of hypocapnia induced by mechanical ventilation.

The modifications of systemic hemodynamics, oxygen transport and tissular oxygenation in mechanically-ventilated critical ARF (acute respiratory failure) patients, after the correction of its hypocapnia by addition of dead space (VD) are determined. The prospective and randomized study was carried out in a multidisciplinary ICU. Fifteen ARF patients were studied within the first 48 hours of evolution. All the patients were intubated and mechanically ventilated. Three stages were delimited: I) 30 min after the beginning of anesthesia; II) 30 min after adding 30 cm of VD; III) 30 min after replacing the previous VD with a VD of 60 cm. Similar steady states had been reached when the measurements were taken. Ventilation parameters and FiO2 were kept stable. In stage I the patients presented a pure respiratory alkalosis and, with respect to hemodynamics, a hyperdynamic situation. In stage II the acid-base balance was normalized with a continuation of the hyperdynamic situation and an increase in mixed venous oxygen tension and saturation (PvO2 and SvO2) (p < 0.001). Stage III was characterized by a pure hypercapnic acidosis and an increase in capillary wedge pressure (CWP) (p < 0.05), right atrial pressure (RAP) (p < 0.001) and cardiac output (Qt) (p < 0.001); simultaneously, the systemic vascular resistances (SVR) decreased (p < 0.01), the PvO2, SvO2 and oxygen delivery (DO2) increased (p < 0.001); oxygen utilization coefficient (OUC) decreased (p < 0.01). The results suggest that the variations in PvO2 and SvO2 are a direct consequence of the modifications in blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical use of the arterial/alveolar oxygen tension ratio.

A prospective study to verify the behavior of the arterial/alveolar oxygen tension ratio (PaO2/PAO2) at different fractions of inspired oxygen (FIO2), was performed in 15 adult hypoxemic patients. This index was useful not only for predicting the arterial oxygen tension but also for choosing the necessary oxygen supplementary levels. PaO2/PAO2 was a less reliable index in patients with mild intrapulmonary shunts, probably because of changes in the distribution of alveolar ventilation/perfusion ratios.