Invasive arterial BP monitoring in trauma and critical care: effect of variable transducer level, catheter access, and patient position.

OBJECTIVES: (1) To determine the validity of current recommendations for direct arterial BP measurement that suggest that the transducer (zeroed to atmosphere) be placed level with the catheter access regardless of subject positioning: and (2) to investigate the effect of transducer level, catheter access site, and subject positioning on direct arterial BP measurement. DESIGN: Prospective, controlled laboratory study. SETTING: Large animal laboratory. SUBJECTS: Five Yorkshire pigs. INTERVENTIONS: Anesthetized animals had 16F catheters placed at three access sites: aortic root, femoral artery, and distal hind limb. Animals were placed in supine, reverse Trendelenburg 35 degrees, and Trendelenburg 25 degrees positions with a transducer placed level to each access site while in every position. MEASUREMENTS AND MAIN RESULTS: For each transducer level, five systolic and diastolic pressures were measured and used to calculate five corresponding mean arterial pressures (MAPs) at each access site. When transducers were at the aortic root, MAP corresponding to aortic root pressure was obtained in all positions regardless of catheter access site. When transducers were moved to the level of catheter access, as current recommendations suggest, significant errors in aortic MAP occurred in the reverse Trendelenburg position. The same trend for error was noted in the Trendelenburg position but did not reach statistical significance. CONCLUSIONS: (1) Current recommendations that suggest placing the transducer at the level of catheter access regardless of patient position are invalid. Significant errors occur when subjects are in nonsupine positions. (2) Valid determination of direct arterial BP is dependent only on transducer placement at the level of the aortic root, and independent of catheter access site and patient position.

Metalloproteinase inhibition prevents acute respiratory distress syndrome.

BACKGROUND: The acute respiratory distress syndrome (ARDS) occurs in patients with clearly identifiable risk factors, and its treatment remains merely supportive. We postulated that patients at risk for ARDS can be protected against lung injury by a prophylactic treatment strategy that targets neutrophil-derived proteases. We hypothesized that a chemically modified tetracycline 3 (COL-3), a potent inhibitor of neutrophil matrix metalloproteinases (MMPs) and neutrophil elastase (NE) with minimal toxicity, would prevent ARDS in our porcine endotoxin-induced ARDS model. METHODS: Yorkshire pigs were anesthetized, intubated, surgically instrumented for hemodynamic monitoring, and randomized into three groups: (1) control (n = 4), surgical instrumentation only; (2) lipopolysaccharide (LPS) (n = 4), infusion of Escherichia coli lipopolysaccharide at 100 microg/kg; and (3) COL-3 + LPS (n = 5), ingestion of COL-3 (100 mg/kg) 12 h before LPS infusion. All animals were monitored for 6 h following LPS or sham LPS infusion. Serial bronchoalveolar lavage (BAL) samples were analyzed for MMP concentration by gelatin zymography. Lung tissue was fixed for morphometric assessment at necropsy. RESULTS: LPS infusion was marked by significant (P < 0.05) physiological deterioration as compared with the control group, including increased plateau airway pressure (P(plat)) (control = 15.7 +/- 0.4 mm Hg, LPS = 23.0 +/- 1.5 mm Hg) and a decrement in arterial oxygen partial pressure (P(a)O(2)) (LPS = 66 +/- 15 mm Hg, Control = 263 +/- 25 mm Hg) 6 h following LPS or sham LPS infusion, respectively. Pretreatment with COL-3 reduced the above pathophysiological changes 6 h following LPS infusion (P(plat) = 18.5 +/- 1.7 mm Hg, P(a)O(2) = 199 +/- 35 mm Hg; P = NS vs control). MMP-9 and MMP-2 concentration in BAL fluid was significantly increased between 2 and 4 h post-LPS infusion; COL-3 reduced the increase in MMP-9 and MMP-2 concentration at all time periods. Morphometrically LPS caused a significant sequestration of neutrophils and monocytes into pulmonary tissue. Pretreatment with COL-3 ameliorated this response. The wet/dry lung weight ratio was significantly greater (P < 0.05) in the LPS group (10.1 +/- 1.0 ratio) than in either the control (6.4 +/- 0.5 ratio) or LPS+COL-3 (7.4 +/- 0.6 ratio) group. CONCLUSIONS: A single prophylactic treatment with COL-3 prevented lung injury in our model of endotoxin-induced ARDS. The proposed mechanism of COL-3 is a synergistic inhibition of the terminal neutrophil effectors MMPs and NE. Similar to the universal practice of prophylaxis against gastric stress ulceration and deep venous thromboses in trauma patients, chemically modified tetracyclines may likewise be administered to prevent acute lung injury in critically injured patients at risk of developing ARDS.

Visual validation of the mechanical stabilizing effects of positive end-expiratory pressure at the alveolar level.

BACKGROUND: Positive end-expiratory pressure (PEEP) reduces ventilator-induced lung injury (VILI), presumably by mechanically stabilizing alveoli and decreasing intrapulmonary shear. Although there is indirect support for this concept in the literature, direct evidence is lacking. In a surfactant depletion model of acute lung injury we observed unstable alveolar mechanics referred to as repeated alveolar collapse and expansion (RACE) as measured by changes in alveolar area from inspiration to expiration (I - E(Delta)). We tested the hypothesis that over a range of tidal volumes PEEP would prevent RACE by mechanically stabilizing alveoli. MATERIALS AND METHODS: Yorkshire pigs were randomized to three groups: control (n = 4), Tween (surfactant-deactivating detergent) (n = 4), and Tween + PEEP (7 cm H(2)O) (n = 4). Using in vivo video microscopy individual alveolar areas were measured with computer image analysis at end inspiration and expiration over consecutive increases in tidal volume (7, 10, 15, 20, and 30 cc/kg.) I - E(Delta) was calculated for each alveolus. RESULTS: Surfactant deactivation significantly increased I - E(Delta) at every tidal volume compared to controls (P < 0.05). PEEP prevented this change, returning I - E(Delta) to control levels over a spectrum of tidal volumes. CONCLUSIONS: RACE occurs in our surfactant deactivation model of acute lung injury. PEEP mechanically stabilizes alveoli and prevents RACE over a range of tidal volumes. This is the first study to visually document the existence of RACE and the mechanical stabilizing effects of PEEP at the alveolar level. The ability of PEEP to stabilize alveoli and reduce shear during mechanical ventilation has important implications for therapeutic strategies directed at VILI and acute respiratory distress syndrome.

Altered alveolar mechanics in the acutely injured lung.

OBJECTIVES: Alterations in alveolar mechanics (i.e., the dynamic change in alveolar size during tidal ventilation) are thought to play a critical role in acute lung injuries such as acute respiratory distress syndrome (ARDS). In this study, we describe and quantify the dynamic changes in alveolar mechanics of individual alveoli in a porcine ARDS model by direct visualization using in vivo microscopy. DESIGN: Prospective, observational, controlled study. SETTING: University research laboratory. SUBJECTS: Ten adult pigs. INTERVENTIONS: Pigs were anesthetized and placed on mechanical ventilation, underwent a left thoracotomy, and were separated into the following two groups post hoc: a control group of instrumented animals with no lung injury (n = 5), and a lung injury group in which lung injury was induced by tracheal Tween instillation, causing surfactant deactivation (n = 5). Pulmonary and systemic hemodynamics, blood gases, lung pressures, subpleural blood flow (laser Doppler), and alveolar mechanics (in vivo microscopy) were measured in both groups. Alveolar size was measured at peak inspiration (I) and end expiration (E) on individual subpleural alveoli by image analysis. Histologic sections of lung tissue were taken at necropsy from the injury group. MEASUREMENTS AND MAIN RESULTS: In the acutely injured lung, three distinct alveolar inflation-deflation patterns were observed and classified: type I alveoli (n = 37) changed size minimally (I - EDelta = 367 +/- 88 microm2) during tidal ventilation; type II alveoli (n = 37) changed size dramatically (I - EDelta = 9326 +/- 1010 microm2) with tidal ventilation but did not totally collapse at end expiration; and type III alveoli (n = 12) demonstrated an even greater size change than did type II alveoli (I - EDelta = 15,418 +/- 1995 microm2), and were distinguished from type II in that they totally collapsed at end expiration (atelectasis) and reinflated during inspiration. We have termed the abnormal alveolar inflation pattern of type II and III alveoli "repetitive alveolar collapse and expansion" (RACE). RACE describes all alveoli that visibly change volume with ventilation, regardless of whether these alveoli collapse totally (type III) at end expiration. Thus, the term "collapse" in RACE refers to a visibly obvious collapse of the alveolus during expiration, whether this collapse is total or partial. In the normal lung, all alveoli measured exhibited type I mechanics. Alveoli were significantly larger at peak inspiration in type II (18,266 +/- 1317 microm2, n = 37) and III (15,418 +/- 1995 microm2, n = 12) alveoli as compared with type I (8214 +/- 655 microm2, n = 37). Tween caused a heterogenous lung injury with areas of normal alveolar mechanics adjacent to areas of abnormal alveolar mechanics. Subsequent histologic sections from normal areas exhibited no pathology, whereas lung tissue from areas with RACE mechanics demonstrated alveolar collapse, atelectasis, and leukocyte infiltration. CONCLUSION: Alveolar mechanics are altered in the acutely injured lung as demonstrated by the development of alveolar instability (RACE) and the increase in alveolar size at peak inspiration. Alveolar instability varied from alveolus to alveolus in the same microscopic field and included alveoli that changed area greatly with tidal ventilation but remained patent at end expiration and those that totally collapsed and reexpanded with each breath. Thus, alterations in alveolar mechanics in the acutely injured lung are complex, and attempts to assess what may be occurring at the alveolar level from analysis of inflection points on the whole-lung pressure/volume curve are likely to be erroneous. We speculate that the mechanism of ventilator-induced lung injury may involve altered alveolar mechanics, specifically RACE and alveolar overdistension.

The mechanism of lung volume change during mechanical ventilation.

To understand ventilator-induced lung injury (VILI) during positive pressure ventilation, mechanisms of normal alveolar mechanics must first be established. Isotropic "balloonlike" alveolar volume (VA) change has been viewed as the prevailing mechanism of normal lung volume (VL) changes. We hypothesized that change in VL is predominantly caused by alveolar recruitment-derecruitment (R/D). Fifteen mongrel dogs were anesthetized and intubated with a tracheal divider. Through a thoracotomy incision, in vivo microscopy of subpleural alveoli was performed as the degassed lung was inflated to 80% TLC, and then deflated to residual volume (RV). Still photomicrographs were evaluated to determine if change in VL is due to change in VA or R/D of alveoli. We noted a steady, significant increase in alveolar recruitment as VL increased to 80% TLC (p < 0.05). However, VA increased significantly, but only to 20% TLC (p < 0.05). Once recruited, alveoli did not demonstrate any further volume change, whereas the lung as a whole maintained a normal pressure/volume relationship. In our model, changes in VL predominantly are caused by R/D.

Matrix metalloproteinase inhibitor: differential effects on pulmonary neutrophil and monocyte sequestration following cardiopulmonary bypass.

UNLABELLED: Acute respiratory distress syndrome (ARDS) following cardiopulmonary bypass (CPB), also known as "post-pump" or "post-perfusion syndrome" (PPS), results from sequential priming and activation of neutrophils. We hypothesized that chemically modified tetracycline (CMT-3) an inhibitor of neutrophil matrix metalloproteinase (MMP) and elastase, would prevent PPS. We performed histometric analysis of lung tissue from our porcine PPS model to correlate cellular sequestration and histologic injury with CMT-3 treatment. METHODS: Yorkshire pigs were randomized into five groups: Control (n = 3); CPB (n = 5); femoral-femoral bypass 1 hour; LPS (n = 7), Escherichia coli lipopolysaccharide (1 microgram/kg); CPB + LPS (n = 6); and CPB + LPS + CMT (n = 5), sequential insults and CMT-3. Protocol histometric analysis defined cellular and tissue components of lung injury. RESULTS: CMT-3 decreased neutrophil sequestration in the CPB + LPS + CMT-3 group (p < 0.0001 vs. CPB + LPS). There were no differences in monocytes between CPB + LPS and CPB + LPS + CMT treatment groups. CONCLUSIONS: CMT-3 attenuates neutrophil sequestration but has no effect on mononuclear sequestration in our PPS model. This finding supports current research on leukocyte chemokines and has important implications regarding mechanisms of CMT-3. Despite lack of monocyte response to CMT-3, PPS was prevented by inhibiting neutrophils alone; confirming the primary role of neutrophils in PPS.