Low Levels Of Positive End-expiratory Pressure Research Articles

Impact of PEEP on lung mechanics and work of breathing in severe airflow obstruction

Positive end-expiratory pressure (PEEP) has generally been withheld from the treatment of patients with chronic airflow obstruction (CAO), in view of the risk of hyperinflation and lack of documented benefit. We studied 10 mechanically ventilated patients with exacerbated CAO and air trapping to determine the impact of PEEP on lung mechanics, alveolar pressure, and the work of breathing. PEEP levels of 5 and 10 cmH2O were applied to patients whose end-expiratory alveolar pressures were documented to be positive when breathing against ambient pressure (the auto-PEEP effect). All patients were studied under two conditions: every breath machine assisted (AMV) and every breath machine controlled (paralyzed, CMV). PEEP improved expiratory resistance without substantially increasing peak static pressure. Inspiratory resistance remained unchanged. The difference between the end-expiratory values of alveolar and central airway pressure narrowed as PEEP increased. Adding PEEP improved the effective triggering sensitivity of the ventilator, diminished ventilatory drive, and reduced the mechanical work of breathing during the machine-assisted ventilatory cycle. Our results indicate that low levels of PEEP may improve lung mechanics and reduce the effort required of mechanically ventilated patients with severe airflow obstruction, without substantially increasing the hazards of hyperinflation.

The Effect of Incremental Positive End-Expiratory Pressure on Right Ventricular Hemodynamics and Ejection Fraction

The effects of incremental positive end-expiratory pressure (PEEP) on right ventricular (RV) function were evaluated in 36 (n = 36) ventilated patients. Positive end-expiratory pressure was increased from 0 (baseline) to 20 cm H2O in 5-cm H2O increments and RV hemodynamics and thermally derived right ventricular ejection fraction (RVEF), right ventricular end-diastolic volume index (RVEDVI), and right ventricular end-systolic volume index (RVESVI) were computed. Right ventricular contractility was determined from the analysis of RV systolic pressure-volume relations. Right ventricular ejection fraction declined from 42 +/- 8% at baseline to 30 +/- 9% at 20 cm H2O PEEP. Right ventricular end-diastolic volume index declined between 0 and 5 cm H2O PEEP (103 +/- 42 to 92 +/- 34 ml.m-2) and then increased to 113 +/- 40 at 20 cm H2O PEEP. Right ventricular end-systolic volume index increased from 60 +/- 31 ml.m-2 at baseline to 79 +/- 34 ml.m-2 at 20 cm H2O PEEP. The slope (E) of the relation of RV peak systolic pressure to RV end-systolic volume index decreased from 0.26 mm Hg.m2.ml-1 between PEEP of 0-15 cm H2O to 0.05 mm Hg.m2.m-1 at PEEP greater than 15 cm H2O. It is concluded that low levels of PEEP have a predominant preload reducing effect on the RV. Above 15 cm H2O PEEP, RV volumes increase and E decreases, consistent with increased RV afterload and a decline in RV contractility.

Effect of positive-end expiratory pressure on accuracy of thermal-dye measurements of lung water

The thermal-dye indicator dilution technique of measuring extravascular lung water (EVLW- TD) correlates well with gravimetric measurement of lung water (EVLW- GR) when no positive-end expiratory pressure (PEEP) is used in both normal and edematous lungs. PEEP is often used to raise arterial oxygen tension in acute respiratory failure. This study was designed to answer the question: Does PEEP have an effect on the accuracy of the EVLW measurement by thermal dye? Sixteen mongrel dogs were anesthetized and intubated. Arterial and PA catheters were placed. They were divided into three groups and ventilated at PEEP levels of 5, 10, or 15 cm H 2O. Fluids were given to minimize decrease in cardiac output with institution of PEEP. They were maintained for 5 hr with measurement of vital signs made hourly and measurement of blood gases and EVLW-TD made at baseline, 1, 3, and 5 hr. After final measurements, gravimetric determination of EVLW was done. Correlation between EVLW TD and EVLW GR remained good when low levels of PEEP were used. At 5 cm H 2O PEEP, EVLW TD was 7.5 ± 0.9 and EVLW GR was 5.4 ± .3. At 10 cm H 2O PEEP, they were 10.0 ± 0.9 and 6.5 ± 0.3. The correlations were 0.87 and 0.97, respectively. However, at 15 cm H 2O PEEP, EVLW TD was 11.4 ± 1.3 and EVLW GR was 7.5 ± 0.6, with a correlation of only 0.59. The correlation between the two techniques seems to break down with higher levels of PEEP in dogs with normal lungs.

Effects of Positive End-Expiratory Pressure on Carotid Blood Flow during Closed-Chest Cardiopulmonary Resuscitation in Dogs

The effect of continuous increases in intrathoracic pressure by positive end-expiratory pressure (PEEP) on common carotid artery blood flow (CCABF) and arterial blood gas tensions and pH during standard closed-chest cardiopulmonary resuscitation was studied in dogs. CCABF was measured at various levels of PEEP by means of the bubble flowmeter in hypervolemic, normovolemic, and hypovolemic dogs. PEEP was increased and then decreased in 5-cm H2O increments with the highest level being 20 cm H2O. Arterial blood samples were obtained approximately 15 minutes before ventricular fibrillation was induced with a transthoracic electric shock during zero end-expiratory pressure during increases in PEEP to 20 cm H2O, and again during decreases to zero end-expiratory pressure. CCABF increased stepwise with increasing levels of PEEP in hypervolemic dogs, whereas it decreased stepwise with increasing levels of PEEP in hypovolemic dogs. In normovolemic dogs, CCABF increased only when the PEEP was 5 cm H2O. Severe metabolic acidosis developed in both hypovolemic and normovolemic dogs at the end of 18 minutes of cardiopulmonary resuscitation. It is speculated that PEEP can either increase CCABF by increasing intrathoracic pressure during chest compression or decrease the CCABF by decreasing venous return during chest compressions, depending on the circulating blood volume. Data suggest that even low levels of PEEP are contraindicated in hypovolemia. Although changes in cerebral perfusion caused by PEEP were not investigated, it is suggested that some degree of PEEP may be beneficial in the hypervolemic state during cardiopulmonary resuscitation.

Effect of positive end-expiratory pressure and body position in unilateral lung injury

Positive end-expiratory pressure (PEEP), by increasing lung volume in acute lung injury, may recruit terminal air spaces in the involved regions, but may also distend noninvolved regions increasing extravascular lung water and worsening gas exchange. We investigated the effect of increasing levels of PEEP on arterial oxygenation in 26 anesthesized dogs with unilateral acid pneumonitis and studied the influences of gravity and distribution of the injury on this effect. Arterial PO2 was consistently higher when the noninjured lung was dependent than in the supine or injured lung-dependent positions. Low levels of PEEP (5, 10 cmH2O) improved arterial oxygenation and reduced intrapulmonary physiological shunt. However, 15 cmH2O PEEP resulted in worsening of gas exchange, increased dead space ventilation, and diminished static compliance. The adverse effects of high levels of PEEP on arterial oxygenation were similar whether the injured lung was dependent or not and were evident a lower levels of PEEP in one group of dogs in which the unilateral injury was more diffuse and in which the upper and middle lobes were also involved. Thus, the compressive effects of high levels of PEEP on alveolar capillaries in the noninjured lung are influenced by the extent and distribution of injury in the injured lung, but not by local forces governing regional blood flow distribution.

Respiratory and cardiovascular responses to PEEP in artificially ventilated patients after cardiopulmonary bypass surgery.

The respiratory and haemodynamic effects of incremental levels of positive and expiratory pressure (PEEP) to 9 cm H2O were studied in ten adult patients 3--6 hours after uneventful cardiopulmonary bypass surgery. Functional residual capacity was increased and deadspace-tidal volume ratio tended to fall, the latter approaching significance at +6 and +9 cm PEEP. Thus lung volume was increased and there was a tendency to improved gas distribution to the alveoli. However there was no significant change in PaO2, alveolar-arterial oxygen tension difference or venous admixture. Cardiac index, and left ventricular strokework index were marginally depressed at 6 cm PEEP and further at 9 cm, while right atrial pressure and pulmonary artery occlusion pressure were raised at 9 cm PEEP. It would appear that low levels (3--6 cm) of PEEP do not improve gas exchange in the lungs to any worthwhile degree, and levels (6--9 cm) may impair cardiac performance.

Hemodynamic alterations with positive end-expiratory pressure: the contribution of the pulmonary vasculature.

The hemodynamic responses to positive end-expiratory pressure (PEEP) ventilation have been evaluated. A rise in pulmonary capillary wedge pressure which follows the use of PEEP may not reflect changes in left atrial pressure and may be associated with a fall in cardiac output. This increase in pulmonary capillary wedge pressure suggests that there is a high resistance to flow in the pulmonary circulation and that a lower level of PEEP should be used.