Cardiorespiratory effects of an inspiratory hold and continuous positive pressure ventilation in goats

Oxygenation remains unaffected by increased inspiration-to-expiration ratio but impairs hemodynamics in surfactant-depleted piglets

OBJECTIVES

Prolongation of inspiratory time is used to reduce lung injury in mechanical ventilation. The aim of this study was to isolate the effects of inspiratory time on airway pressure, gas exchange, and hemodynamics, while ventilatory frequency, tidal volume, and mean airway pressure were kept constant.

DESIGN

Randomized experimental trial.

SETTING

Experimental laboratory of a University Department of Anesthesiology and Intensive Care.

ANIMALS

Twelve anesthetised piglets.

INTERVENTIONS

After lavage the reference setting was pressure-controlled ventilation with a decelerating flow; I:E was 1:1, and PEEP was set to 75% of the inflection point pressure level. The I:E ratios of 1.5:1, 2.3:1, and 4:1 were applied randomly. Under open lung conditions, mean airway pressure was kept constant by reduction of external PEEP.

MEASUREMENTS AND RESULTS

Gas exchange, airway pressures, hemodynamics, functional residual capacity (SF6 tracer), and intrathoracic fluid volumes (double indicator dilution) were measured. Compared to the I:E of 1:1, PaCO2 was 8% lower, with I:E 2.3:1 and 4:1 (p < or = 0.01) while PaO2 remained unchanged. The decrease in inspiratory airway pressure with increased inspiratory time was due to the response of the pressure-regulated volume-controlled mode to an increased I:E ratio. Stroke index and right ventricular ejection fraction were depressed at higher I:E ratios (SI by 18% at 2.3:1, 20% at 4:1; RVEF by 10% at 2.3:1, 13% at 4:1; p < or = 0.05).

CONCLUSION

Under open lung conditions with an increased I:E ratio, oxygenation remained unaffected while hemodynamics were impaired.

Cardiorespiratory effects of pressure-controlled ventilation with and without inverse ratio in the adult respiratory distress syndrome

To assess the cardiorespiratory effects of pressure-controlled ventilation (PCV) and pressure-controlled inverse ratio ventilation (PC-IRV), we compared pressure-controlled ventilation with an inspiratory-to-expiratory time ratio (I/E) of 1/2 (PCV) and of 2/1 (PC-IRV) to volume-controlled ventilation (VCV) with an I/E of 1/2 in 10 patients suffering from the adult respiratory distress syndrome. In all modes, the inspiratory oxygen fraction, tidal volume, respiratory rate, and total positive end-expiratory pressure (PEEPt = applied PEEP + intrinsic PEEP) were kept constant. Each ventilatory mode was applied for 1 h in a randomized order. No significant differences in PaO2 were observed among the three modes. The PaCO2 was lower (p < 0.05) in PC-IRV (39 +/- 4 mm Hg) than in PCV (43 +/- 5 mm Hg) and in VCV (45 +/- 5 mm Hg). The peak airway pressure was significantly lower in PC-IRV than in PCV (p < 0.05) and in PCV than in VCV (p < 0.05), but plateau pressure was not different in the 3 modes. The mean airway pressure (mPaw) was significantly higher (p < 0.05) in PC-IRV (21.4 +/- 0.7 cm H2O) than in PCV (17.1 +/- 0.7 cm H2O) and VCV (16.4 +/- 0.5 cm H2O). As a consequence of this increased mPaw, PC-IRV induced a decrease in cardiac index (CI) (3.3 +/- 0.2 vs 3.7 +/- 0.2 L/min/m2 in VCV; p < 0.05) and hence in oxygen delivery (DO2) (424 +/- 28 vs 469 +/- 38 ml/min/m2 in VCV; p < 0.05). Our results suggest that neither PCV nor PC-IRV bring any benefit over VCV in terms of arterial oxygenation. Moreover, the increase in mPaw induced by PC-IRV may be deleterious to the CI and DO2.

A simple method to estimate functional residual capacity in mechanically ventilated patients

OBJECTIVE

The aim of the present study was to evaluate a simplified method for FRC measurement.

DESIGN

Accuracy and precision of the method were assessed in a physical lung model; reproducibility was tested in 10 mechanically ventilated patients. In each patient FRC was measured at three PEEP levels.

SETTING

Post-operative intensive care unit in a university hospital.

MEASUREMENTS AND RESULTS

Gas flow, CO2 concentration, and O2 concentration were measured during in- and expiration by pneumotachography, a mainstream capnometer and a sidestream O2-analyser. For FRC-measurement inspiratory O2 concentration was changed by 30%. FRC was determined as mean value of a N2 washout and N2 washin procedure. Evaluation of this method in a lung model shows a good correlation between FRC set in the lung model and FRC measured (FRC measured = 1.028*FRG model + 22.92 ml; r2 = 0.957; n = 30). The mean difference was 4.4% of FRC-reference (range -8.4% to +21.7%). Duplicate determinations in 10 mechanically ventilated patients differed by an average of -2.7% (range -30.1% to +27.3%).

CONCLUSION

Our results suggest that the proposed method can be used in daily clinical work.

The effects of inversed ratio ventilation (IRV) on arterial oxygenation during mechanical ventilation in patients with acute respiratory failure

We investigated the effects of inversed ratio ventilation by altering the inspiratory:expiratory (I:E) ratio and assessing the time course changes in the intrapulmonary shunting (Qs/Qt) in 14 patients with acute respiratory failure. Stepwise prolongation of the I:E ratio from 1:1.9 to 2:1 and then to 2.6 or 4:1 was applied when PEEP failed to raise the PaO2 above 80 mmHg while breathing oxygen. A significant decrease in Qs/Qt was observed following prolongation of the I:E ratio from 1:1.9 (Qs/Qt = 45 +/- 9%) to 2:1 (Qs/Qt = 29 +/- 9%) but not with further prolongation of the I:E ratio (Qs/Qt = 27 +/- 7%). Improvement of the pulmonary ventilation/perfusion imbalance became more marked with continued IRV and a significant increase in PaO2 was observed at 6 h after initiating prolongation of the inspiratory time (P less than 0.05). There were no significant changes in hemodynamics, PaCO2, or peak inspiratory pressure during IRV. This ventilatory pattern may be indicated when PEEP fails to improve PaO2, but prolongation of the inspiratory time above an I:E ratio of 2:1 did not produce a greater improvement in Qs/Qt and further increases in PaO2 did not occur after more than 10 h of IRV in our 14 patients.

Combined effects of inversed ratio ventilation (IRV) with positive end-expiratory pressure ventilation (PEEP) on cardiorespiratory function in acute respiratory failure

Combined effects of inversed ratio ventilation (IRV) with positive end-expiratory pressure (PEEP) on cardiorespiratory function were examined in 24 patients with acute respiratory failure. Patients were divided into two groups: the IRV group (n = 12) who showed no significant increase in Pa(O)(2) with a 6 cmH(2)O of PEEP and PEEP group (n = 12) who were ventilated mechanically with PEEP only at maximum level of 10 cmH(2)O. In IRV group step-wise prolongation of the I : E ratio from 1 : 1.9 to 2.6 : 1 or 4 : 1 was applied as a Pa(O)(2) was improved and in PEEP group also level of PEEP was increased from 0, 5 to 10 cmH(2)O after one hour period irrespective of Pa(O)(2). Inversed ratio ventilation and PEEP increased significantly Pa(O)(2)/F i(O)(2), the increase being observed 6 hrs (I : E = 2 : 1) and 2 hrs (10 cmH(2)O) after starting IRV or PEEP. Further improvement of oxygenation was not observed in IRV even if I : E ratio was prolonged up to 2.6 : 1 or 4 : 1. These results suggested that combinations of IRV with PEEP were effective and an I : E ratio of 2 : 1 may be optimal, and IRV is advantageous compared to PEEP, but will take more long time to improve oxygenation than PEEP.

Ventilation and ventilators--an update

In the five years which have passed since the previous review, the literature has been concerned more with the ways in which ventilators may be applied to patients and the effects of differing patterns of ventilation than with the design philosophy of the ventilators themselves. This account should be read in conjunction with that of 1982 [1].

Continuous monitoring of alveolar and inspiratory concentrations of anesthetic and respiratory gases is difficult and potentially unsafe

One essayist suggests that continuous monitoring of alveolar and inspiratory concentrations of anesthetic and respiratory gases has little or no positive effect on patient outcome and may even be detrimental to patients. Such monitoring, he says, tends to remove anesthesiologists from personal contact with their patients. He recommends careful monitoring of fresh gas concentrations leaving the anesthetic machine, careful monitoring of inspired gas in a circle absorption breathing system, and improved training of anesthesiologists to prevent human error. Another essayist suggests that continuous monitoring of alveolar and inspiratory concentrations of anesthetic and respiratory gases is cost-effective and relatively simple. He says that such monitoring, without being a source of legal problems for its users, improves the quality of patient care.

Inverse ratio ventilation compared with PEEP in adult respiratory failure

We have compared the cardiorespiratory effects of an inspiratory: expiratory (I:E) ratio of 4:1 with a ratio of 1:2 in 10 adult patients requiring intermittent positive pressure ventilation (IPPV) for acute respiratory insufficiency. Further comparisons were made with IPPV with positive end-expiratory pressure (PEEP) which was adjusted to achieve an equal external end-expiratory volume (EEEV) to that produced by the 4:1 ratio, as determined by respiratory inductive plethysmography, and with an I:E ratio that only changed the EEEV minimally (IRV-min). Percentage pulmonary shunt (Qs/Qt) was reduced equally with PEEP and with the 4:1 I:E ratio but both patterns reduced cardiac output and oxygen delivery. IRV-min also reduced Qs/Qt significantly but had no effect on cardiac output so that oxygen delivery was increased. The dead space to tidal volume ratio (VD/VT) during IPPV-4:1 and IRV-min was reduced significantly when compared with that during IPPV-1:2. The clinical implications of the findings suggest that for some ITU patients, a modest increase in I:E ratio to between 1.1:1 and 1.7:1 may produce better gas exchange without significantly effecting the cardiac output.