The effect of positive end-expiratory pressure on regional pulmonary perfusion during acute lung injury

Imaging the pulmonary vasculature in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is characterized by a redistribution of regional lung perfusion that impairs gas exchange. While speculative, experimental evidence suggests that perfusion redistribution may contribute to regional inflammation and modify disease progression. Unfortunately, tools to visualize and quantify lung perfusion in patients with ARDS are lacking. This review explores recent advances in perfusion imaging techniques that aim to understand the pulmonary circulation in ARDS. Dynamic contrast-enhanced computed tomography captures first-pass kinetics of intravenously injected dye during continuous scan acquisitions. Different contrast characteristics and kinetic modeling have improved its topographic measurement of pulmonary perfusion with high spatial and temporal resolution. Dual-energy computed tomography can map the pulmonary blood volume of the whole lung with limited radiation exposure, enabling its application in clinical research. Electrical impedance tomography can obtain serial topographic assessments of perfusion at the bedside in response to treatments such as inhaled nitric oxide and prone position. Ongoing technological improvements and emerging techniques will enhance lung perfusion imaging and aid its incorporation into the care of patients with ARDS.

Acute right ventricular dysfunction: real-time management with echocardiography

In critically ill patients, the right ventricle is susceptible to dysfunction due to increased afterload, decreased contractility, or alterations in preload. With the increased use of point-of-care ultrasonography and a decline in the use of pulmonary artery catheters, echocardiography can be the ideal tool for evaluation and to guide hemodynamic and respiratory therapy. We review the epidemiology of right ventricular failure in critically ill patients; echocardiographic parameters for evaluating the right ventricle; and the impact of mechanical ventilation, fluid therapy, and vasoactive infusions on the right ventricle. Finally, we summarize the principles of management in the context of right ventricular dysfunction and provide recommendations for echocardiography-guided management.

Monitoring Lung Volumes During Mechanical Ventilation

Respiratory inductive plethysmography (RIP) is a non-invasive method of measuring change in lung volume which is well-established as a monitor of tidal ventilation and thus respiratory patterns in sleep medicine. As RIP is leak independent, can measure end-expiratory lung volume as well as tidal volume and is applicable to both the ventilated and spontaneously breathing patient, there has been a recent interest in its use as a bedside tool in the intensive care unit.

Cardiopulmonary interactions in acute lung injury

PURPOSE OF REVIEW

Through shared anatomy, pressures, and endothelial connections, the respiratory and cardiovascular systems affect each other in complex but clinically important ways.

RECENT FINDINGS

Lung injury has clinically important circulatory effects, especially with regards to right ventricular function. Mechanical ventilation and PEEP produce a host of circulatory consequences, some beneficial, some life-threatening. At the same time, circulatory impairments and treatments can magnify the impact of lung failure.

SUMMARY

Cardiopulmonary interactions underpin current views of fluid management and mechanical ventilation. Understanding cardiopulmonary interactions and their physiological basis has direct clinical relevance.

Effects of prone position and positive end-expiratory pressure on lung perfusion and ventilation

OBJECTIVES

Prone positioning is frequently used during acute respiratory distress syndrome. However, mechanisms by which it improves oxygenation are poorly understood, as well as its interaction with positive end-expiratory pressure. This study was conducted to decipher the respective effects of positive end-expiratory pressure and posture during lung injury on regional lung ventilation, perfusion and recruitment assessed by positron emission tomography.

DESIGN

Experimental study.

SETTING

Research laboratory of a university hospital.

SUBJECTS

Six female piglets.

INTERVENTIONS

After oleic acid-induced lung injury, all animals were studied in supine and prone position at both positive end-expiratory pressure 0 and positive end-expiratory pressure 10 cm H2O.

MEASUREMENTS AND MAIN RESULTS

In each experimental condition, regional lung perfusion and ventilation were assessed with positron emission tomograph using intravenous 15O-labeled water and inhaled nitrogen-13. Nonaerated lung weight was assessed with positron emission tomograph, and alveolar recruitment was defined as the difference of nonaerated lung weight between conditions. Positive end-expiratory pressure was associated with significant alveolar recruitment (130 +/- 85 and 65 +/- 29 g of lung in supine and prone position, respectively [p < 0.05 vs. 0]), whereas recruitment induced by posture was not statistically significant (77 +/- 97 g with positive end-expiratory pressure 0 and 13 +/- 19 g with positive end-expiratory pressure 10 [p > 0.05 vs. 0]). Regardless the posture, positive end-expiratory pressure redistributed both perfusion and ventilation toward dependent regions. Recruitment by positive end-expiratory pressure was restricted to dorsal regions in supine position, but extended diffusely along the ventral-to-dorsal dimension in prone position. Prone position was associated with recruitment in dorsal regions with concomitant derecruitment in ventral regions, magnitude of this being reduced by positive end-expiratory pressure. Prone position redistributed ventilation toward dorsal and ventral regions at positive end-expiratory pressure 0 and positive end-expiratory pressure, respectively. Finally, prone position redistributed perfusion toward ventral regions, to an extent amplified by positive end-expiratory pressure.

CONCLUSIONS

Positive end-expiratory pressure and posture act synergistically by redistributing lung regional perfusion toward ventral regions, but have antagonistic effects on regional ventilation.

Anatomical and functional intrapulmonary shunt in acute respiratory distress syndrome

OBJECTIVES

The lung-protective strategy employs positive end-expiratory pressure to keep open otherwise collapsed lung regions (anatomical recruitment). Improvement in venous admixture with positive end-expiratory pressure indicates functional recruitment to better gas exchange, which is not necessarily related to anatomical recruitment, because of possible global/regional perfusion modifications. Therefore, we aimed to assess the value of venous admixture (functional shunt) in estimating the fraction of nonaerated lung tissue (anatomical shunt compartment) and to describe their relationship.

DESIGN

Retrospective analysis of a previously published study.

SETTING

Intensive care units of four university hospitals.

PATIENTS

Fifty-nine patients with acute lung injury/acute respiratory distress syndrome.

INTERVENTIONS

Positive end-expiratory pressure trial at 5 and 15 cm H2O positive end-expiratory pressures.

MEASUREMENTS AND MAIN RESULTS

Anatomical shunt compartment (whole-lung computed tomography scan) and functional shunt (blood gas analysis) were assessed at 5 and 15 cm H2O positive end-expiratory pressures. Apparent perfusion ratio (perfusion per gram of nonaerated tissue/perfusion per gram of total lung tissue) was defined as the ratio of functional shunt to anatomical shunt compartment. Functional shunt was poorly correlated to the anatomical shunt compartment (r2 = .174). The apparent perfusion ratio at 5 cm H2O positive end-expiratory pressure was widely distributed and averaged 1.25 +/- 0.80. The apparent perfusion ratios at 5 and 15 cm H2O positive end-expiratory pressures were highly correlated, with a slope close to identity (y = 1.10.x -0.03, r2 = .759), suggesting unchanged blood flow distribution toward the nonaerated lung tissue, when increasing positive end-expiratory pressure.

CONCLUSIONS

Functional shunt poorly estimates the anatomical shunt compartment, due to the large variability in apparent perfusion ratio. Changes in anatomical shunt compartment with increasing positive end-expiratory pressure, in each individual patient, may be estimated from changes in functional shunt, only if the anatomical-functional shunt relationship at 5 cm H2O positive end-expiratory pressure is known.

Relation between shunt, aeration, and perfusion in experimental acute lung injury

RATIONALE

In a pulmonary process characterized by spatially heterogeneous loss of aeration, the impairment of gas exchange is expected to depend on the regional distribution of perfusion relative to that of aeration.

OBJECTIVES

To investigate how regional aeration, shunt, and perfusion are interrelated at different levels of end-expiratory pressure and how their interplay relates to global shunt fraction in acute lung injury.

METHODS

Regional shunt and perfusion were assessed by imaging with positron emission tomography the pulmonary kinetics of [(13)N]nitrogen infused in saline solution in five sheep after lung lavage. The lung field was divided in six horizontal regions.

MEASUREMENTS AND MAIN RESULTS

Each animal showed an inverse relation between regional shunt (Fs) and gas (Fg) fractions: Fs = -m . Fg + Fs(0). This relation was similar among animals (m = 1.25 +/- 0.14, Fs(0) = 0.75 +/- 0.15) and invariant with end-expiratory pressure, despite lack of correlation between global shunt and gas fractions and large interanimal variability in global shunt fraction. When this relation was used to estimate global shunt fraction as a perfusion-weighted average of the estimates of regional shunt fraction derived from regional gas fraction, 72% of the interanimal variability in global shunt fraction could be explained.

CONCLUSIONS

Despite large interanimal variability in global shunt fraction, there was a consistent inverse relation between regional shunt and gas fractions, independent of end-expiratory pressure. Most of the interanimal variability in global shunt fraction could be explained by the combined effect of this relation and the distribution of perfusion on regional shunt, rather than by differences in global aeration.

Computed Tomography Assessment of Positive End-expiratory Pressure-induced Alveolar Recruitment in Patients with Acute Respiratory Distress Syndrome

Computed tomography (CT) assessment of positive end-expiratory pressure (PEEP)-induced alveolar recruitment is classically achieved by quantifying the decrease in nonaerated lung parenchyma on a single juxtadiaphragmatic section (Gattinoni's method). This approach ignores the alveolar recruitment occurring in poorly aerated lung areas and may not reflect the alveolar recruitment of the entire lung. This study describes a new CT method in which PEEP-induced alveolar recruitment is computed as the volume of gas penetrating in poorly and nonaerated lung regions following PEEP. In 16 patients with acute respiratory distress syndrome a thoracic spiral CT scan was performed in ZEEP and PEEP 15 cm H(2)O. According to the new method, PEEP induced a 119% increase in functional residual capacity (FRC). PEEP-induced alveolar recruitment was 499 +/- 279 ml whereas distension and overdistension of previously aerated lung areas were 395 +/- 382 ml and 28 +/- 6 ml, respectively. The alveolar recruitment according to Gattinoni's method was 26 +/- 24 g and no correlation was found between both methods. A significant correlation was found between PEEP-induced alveolar recruitment and increase in Pa(O(2)) only when recruitment was assessed by the new method (Rho = 0.76, p = 0.003), suggesting that it may be more accurate than Gattinoni's method.

Effect of partial liquid ventilation on pulmonary vascular permeability and edema after experimental acute lung injury

We evaluated the effects of partial liquid ventilation (PLV) with two different dosages of the perfluorocarbon LiquiVent (perflubron) on pulmonary vascular permeability and edema formation after oleic acid (OA)-induced acute lung injury in dogs. We used imaging with positron emission tomography to measure fractional pulmonary blood flow, lung water concentration (LWC), and the pulmonary transcapillary escape rate (PTCER) of (68)Ga-labeled transferrin at 5 and 21 h after lung injury in five dogs undergoing conventional mechanical ventilation (CMV), five dogs undergoing low-dose PLV (perflubron at 10 ml/kg), and four dogs undergoing high dose PLV (perflubron at 30 ml/kg). A positive end-expiratory pressure of 7.5 cm H(2)O was used in all dogs. After OA (0.08 ml/kg)- induced lung injury, there were no significant differences or trends for PTCER or LWC at any time when the PLV groups were compared with the CMV group. However, lung tissue myeloperoxidase activity was significantly lower in the combined PLV group than in the CMV group (p = 0.016). We conclude that after OA-induced lung injury, the addition of PLV to CMV does not directly attenuate pulmonary vascular leak or lung water accumulation. Rather, the benefits of such treatment may be due to modifications of the inflammatory response.

Regional pulmonary blood flow during partial liquid ventilation in normal and acute oleic acid-induced lung-injured piglets

OBJECTIVE

To determine the spatial distribution of pulmonary blood flow in three groups of piglets: partial liquid ventilation in normal piglets, partial liquid ventilation during acute lung injury, and conventional gas ventilation during acute lung injury.

DESIGN

Prospective randomized study.

SETTING

A university medical school laboratory approved for animal research.

SUBJECTS

Neonatal piglets.

INTERVENTIONS

Regional pulmonary blood flow was studied in 21 piglets in the supine position randomized to three different groups: a normal group that received partial liquid ventilation (Normal-PLV) and two acute lung injury groups that received an oleic acid-induced lung injury: partial liquid ventilation during acute lung injury (OA-PLV) and conventional gas ventilation during acute lung injury (OA-Control). Acute lung injury was induced by infusing oleic acid (0.15 mL/kg iv) over 30 mins. Partial liquid ventilation was instituted with perflubron (LiquiVent, 30 mL/kg) after 30 mins in the Normal-PLV and OA-PLV groups.

MEASUREMENTS AND MAIN RESULTS

Arterial and venous blood gases, hemodynamics, and pulmonary mechanics were measured every 15 mins throughout the hour-long study. Pulmonary blood flow was assessed by fluorescent microsphere technique at baseline and after 30, 45, and 60 mins. In the Normal-PLV piglets, pulmonary blood flow decreased from baseline (before injury or partial liquid ventilation) in the most dependent areas of the lung (F ratio = 3.227; p < .001). In the OA-PLV piglets, pulmonary blood flow was preserved over time throughout the lung (F ratio = 1.079; p = .38). In the OA-Control piglets, pulmonary blood flow decreased in the most dependent areas of the lung and increased from baseline in less dependent slices over time (F ratio = 2.48; p = .003).

CONCLUSIONS

The spatial distribution of regional pulmonary blood flow is preserved during partial liquid ventilation compared with gas ventilation in oleic acid-induced lung injury.

PEEP only partly restores disturbed distribution of regional pulmonary blood flow in lung injury

The effects of lung injury, positive end-expiratory pressure (PEEP), and norepinephrine on heterogeneity of regional pulmonary blood flow (rPBF, radioactive microspheres) were investigated. We hypothesized that lung injury increases heterogeneity of rPBF and that PEEP ventilation reduces these effects. Heterogeneity of rPBF is scale dependent and was therefore assessed in detail. Local correlation (p), relative dispersion (RD), fractal dimension (D), perfusion gradients, and histograms of rPBF each measures a different aspect of heterogeneity. In eight anesthetized dogs, lung injury was induced with oleic acid and glass bead injection. Afterward, PEEP of 10-20 cmH2O was instituted. Norepinephrine was infused at 20 cmH2O PEEP. Heterogeneity increased upon lung injury (p, 0.44 +/- 0.09 vs. 0.24 +/- 0.09; RD, 0.36 +/- 0.06 vs. 0.64 +/- 0.12; both P < or = 0.05), but fractal dimension remained constant. PEEP did not change p, RD, or D. Perfusion gradients were reversed after lung injury (right, -27 +/- 18 vs. 196 +/- 115%; -24 +/- 18 vs. 282 +/- 184%; P < or = 0.05). PEEP (10 cmH2O) reduced gradients (116 +/- 73 and 143 +/- 62%, respectively; P < or = 0.05). Norepinephrine, in part, further reduced gradients (right, 50 +/- 58%; P < or = 0.05; left, 102 +/- 94%; P = NS). We conclude that oleic acid- and glass bead-induced lung injury produces abnormal distribution of rPBF. Of these changes, application of PEEP only reverses perfusion gradients.

Distribution of pulmonary blood flow and total lung water during partial liquid ventilation in acute lung injury

BACKGROUND

Gas exchange is improved during partial liquid ventilation (PLV) with perfluorocarbon in animal models of acute lung injury. The mechanisms are not fully defined. We hypothesize that redistribution of pulmonary blood flow (PBF) along with redistribution of, and decrease in, total lung water (TLW) during PLV may improve oxygenation.

METHODS

We characterized PBF and TLW in anesthetized adult dogs by using positron emission tomography with H2(15)O. Measurements of gas exchange, PBF, and TLW were made before and after acute lung injury was induced with intravenous oleic acid. The same measurements were made during PLV (with 30 ml/kg perfluorocarbon) and compared with gas ventilated (GV) controls.

RESULTS

Oxygenation was significantly improved during PLV. PBF redistributed from the dependent zone of the lung to the nondependent zones, thus potentially improving ventilation/perfusion relationships. However, a similar pattern of PBF redistribution was observed during GV such that there was no significant difference between groups. TLW redistributed in a similar pattern during PLV. By quantitative measurements, PLV ameliorated the continued accumulation of TLW compared with GV animals.

CONCLUSIONS

We conclude that PBF and TLW redistribution and attenuation of increases in TLW may contribute to the improvement in gas exchange during PLV in the setting of acute lung injury.