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ABSTRACT |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Myocardial edema and histologic changes consistent with tissue injury are reported in association
with sepsis-induced myocardial depression. The objective of the present study was to determine whether, in the absence of shock, such changes (assessed by studying microvascular albumin flux, tissue
edema, and morphometry) are prerequisites for the development of contractile dysfunction in sepsis.
Sprague-Dawley rats were randomized into groups for either cecal ligation and perforation (CLP) or
sham study. Twenty-four hours after entry of animals into the study, their myocardial function was
assessed with the Langendorff isolated heart technique. Left-ventricular developed pressure (preload: 5 mm Hg) was reduced in CLP animals (34.9 ± 3.3 mm Hg, n = 10) as compared with time-matched controls (46.4 ± 4.0 mm Hg, n = 8, p < 0.05, unpaired t test). This was associated with a
significant reduction in the maximal rate of increase (+dP/dtmax) and decrease (
dP/dtmax) in left ventricular pressure in the CLP group (sham versus CLP, unpaired t test, p < 0.05). Upon reperfusion,
after 30 min of ischemia, left ventricular resting tension was decreased in CLP as compared with sham-treated animals (sham versus CLP, analysis of variance (ANOVA) with repeated measures, p < 0.05).
At 24 h, sepsis was not associated with myocardial edema (wet:dry weight ratio, sham = 4.094 ± 0.098, n = 10; CLP = 4.185 ± 0.066, n = 7), and tissue albumin flux was reduced (sham = 194 ± 27 µl · h
1 · g dry wt1, n = 10; CLP = 100 ± 14 µl · h1 · g dry wt1, n = 7). In tissue processed for electron microscopy, we found no evidence of tissue injury or edema at either 24 or 48 h after CLP. We
conclude that polymicrobial normotensive sepsis causes myocardial contractile depression in the absence of changes in myocardial structure.
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Clinical studies have shown that myocardial contractility is reduced in sepsis in the absence of changes in ventricular preload and afterload (1). The pathogenesis of these changes is of considerable importance, since myocardial dysfunction leading to shock is strongly associated with mortality (2). The cause of
sepsis-induced myocardial dysfunction is incompletely understood. Circulating myocardial depressant substances, obtained
from the serum of septic humans and animals, cause acute reductions in myocardial contractility in isolated myocardial tissue (3). Although these depressant substances have not been
identified, a number of inflammatory mediators (e.g., tumor
necrosis factor-
[TNF-], interleukin-6 [IL-6], IL-2, and endotoxin) have been shown to cause acute reductions in myocardial contractility both in vivo (4) and in vitro (5). In addition
to uncertainties about the mediator responsible for sepsis-induced myocardial dysfunction, it is unclear from the literature whether changes in myocardial contractility are due to
tissue injury, manifested as structural alterations in the myocardium (6), or to modulation of specific intracellular signal-transduction pathways (i.e., nitric oxide/cyclic guanosine monophosphate [cGMP]-mediated pathways) (5, 7) responsible for
contractile function.
We previously reported the presence of myocardial injury, identified with quantitative morphometry and semiquantitative tissue scoring (8), in a normotensive sheep model of sepsis complicating peritonitis. Other studies, summarized in Table 1, have also demonstrated histologic changes associated with sepsis, such as tissue and mitochondrial edema and myocyte necrosis, although few have sought to correlate these changes with myocardial dysfunction (6, 9). Although such data suggest that myocardial tissue injury may lead to myocardial dysfunction, it is unclear from these studies whether structural changes are a necessary prerequisite for the development of myocardial dysfunction in sepsis.
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To further probe the relationship between myocardial structure and function in sepsis, we designed an experiment to characterize the temporal relationship between myocardial dysfunction and structural injury in a model of normotensive sepsis. In rats made septic by cecal ligation and perforation, we hypothesized that changes in myocardial structure would either precede or occur simultaneously with the identification of anomalies in myocardial contractility. We correlated changes in myocardial function, determined with the Langendorff isolated heart preparation, with alterations in: (1) microvascular permeability, assessed by measuring the myocardial tissue wet:dry weight ratio and albumin flux; and (2) myocardial morphometry, assessed with electron microscopy. We assessed tissue morphometry both at a time when contractile dysfunction is known to occur (24 h after insult) and 24 h later, when more severe tissue injury would be expected. We found that myocardial contractility was decreased in the absence of either myocardial edema or morphometric evidence of myocardial parenchymal or microcirculatory injury. These findings support a view that myocardial contractile dysfunction in normotensive sepsis is due either to: (1) subtle injury to subcellular macromolecules vital for normal contractile function; or (2) modulatory effects on regulatory signal-transduction pathways (5, 7).
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METHODS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Animal Preparation
One hundred and thirty-one Sprague Dawley rats (weight: 337 ± 2.7 g; Charles River, St.-Constant, Quebec, Canada) were studied. Animals were assigned to one of four protocols, which assessed: (1) baseline myocardial contractility, using the Langendorff isolated heart preparation; (2) recovery of myocardial contractility after in vitro ischemia- reperfusion; (3) tissue injury, using quantitative and semiquantitative scoring of tissue sections processed for electron microscopy; and (4) microvascular integrity, by measuring tissue albumin flux and wet:dry weight ratio.
After the animals were given anesthesia with halothane, internal carotid (PE 50; Intramedic, Sparkes, MD) and external jugular lines (0.25 mm Silastic tubing; Dow Corning, Midland, MI) were inserted under sterile conditions. The lines were tunneled subcutaneously to the back of the neck, where they were attached to a swivel device. Animals were then randomized to either sham-treatment or cecal ligation and perforation (CLP) groups. Sham animals had insertion of lines only, unless otherwise indicated. In the CLP group, a laparotomy was performed and a ligature placed around the cecum immediately distal to the ileocecal valve. The cecum was then punctured twice with an 18-gauge needle. Following recovery of animals from anesthesia, the following infusions were begun in both groups: normal saline at 300 to 400 ml/kg/d, heparin at 400 U/kg/d, and fentanyl at 400 µg/kg/d. Heparin was administered to assure patency of intravascular lines. Water and laboratory chow were available ad libitum. Where indicated, a group of naive animals (animals that were studied without any prior intervention) were included to provide a normal control group. This animal protocol was reviewed and approved by the University of Western Ontario Committee on Animal Care.
Assessment of Baseline Contractility
Twenty-four hours after randomization to either the sham (n = 5) or
CLP (n = 12) group, animals were lightly anesthetized with phenobarbital (30 mg/kg intraperitoneally). The animals were then decapitated
and their hearts rapidly excised, mounted on a nonrecirculating Langendorff apparatus, and perfused at 37° C with Krebs-Henseleit solution (composition in mM: NaCl 120, KCl 4.8, KH2PO4 1.2, MgSO4 1.2, CaCl2 1.25, NaHCO3 25, and glucose 11). The perfusion buffer was
equilibrated with a 95% O2/5% CO2 gas mixture, resulting in a buffer
PaO2 above 600 mm Hg. Hearts were paced at 300 beats/min with a
Grass stimulator (SD5, Quincy, MA) and ventricular pacing wires.
Left-ventricular developed pressure (LVDP) and its first derivative
(LV dP/dt) were monitored with a latex balloon (compliant volume
> 130 µl) positioned in the left-ventricular cavity. Perfusion pressure
relative to the left atrium, which reflects coronary vascular resistance
(CVR) under constant-flow conditions, was monitored through the
Langendorff column by means of a fluid-filled catheter connected to a
pressure transducer (Inflow; Baxter Corp., Toronto, Ontario, Canada). Measurements were then made, including: (1) a Starling curve
over a range of preloads of 5 to 20 mm Hg at a coronary flow rate of
10 ml/min; and (2) a flow-function curve. The latter was generated by
increasing coronary flow from 2.2 to 13 ml/min in a stepwise fashion,
with a 5-min period of equilibration between steps. LVDP, LV dP/dt, and coronary perfusion pressure (CPP) were recorded at the end of
each 5-min interval at a preload of 5 mm Hg. The differential ratio
was calculated by dividing +dP/dtmax by dP/dtmax. Data were recorded on a chart recorder (Model 8188 recorder and modules 13-4615-50, 13-4615-71; Gould, Inc., Cleveland, OH).
Assessment of Contractile Recovery after Ischemia-Reperfusion
In a separate group of animals, the heart was paced at 360 beats/min and intraventricular pressure was measured with a latex balloon (Hugo-Sachs Elektronik, March-Hugstetten, Germany) positioned in the left-ventricular cavity. The animals in this study were naive, and the sham procedure included a sham laparotomy (naive group, n = 11; sham group, n = 8; CLP group, n = 10). After a 35-min equilibration period, baseline contractility was assessed at an LVEDP of 5 mm Hg and coronary flow rate of 9 ml/min. Coronary flow was then stopped and the heart maintained in Krebs-Henseleit solution at 37° C. After 30 min of ischemia, the heart was reperfused at a flow rate of 4 ml/min, which in this preparation produced optimal recovery from ischemia (16). LVDP and resting tension were recorded for 60 min after reperfusion. Pacing was commenced 20 min after reperfusion in order to avoid pacemaker-induced ventricular arrhythmias, which are common during this period (16).
Assessment of Myocardial Structure and Vascular Permeability
Two groups of animals were studied. In the first group, we used electron microscopy to assess myocardial ultrastructure at 24 h and 48 h after entry into the study. In a second group, we assessed microvascular integrity by measuring tissue albumin flux and wet:dry weight ratios (Figure 1).
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Assessment of tissue ultrastructure. At 24 h and 48 h after entry into the study, animals were heparinized (500 U) and anesthetized with phenobarbital (50 mg/kg), and their hearts were rapidly removed and perfused for 30 s with a modified Kreb's solution of the following composition (mM): NaCl 117, KCl 5.9, KH2PO4 1.2, MgSO4 1.2, CaCl2 3.0, NaHCO3 25, glucose 16, calcium ethylene diamine tetraacetate (CaEDTA) 0.5, and albumin 0.025%. The heart was then perfusion-fixed with 6.25% glutaraldehyde in 0.1 M cacodylate buffer (osmolarity 1,000 to 1,100 mOsm) for 3 min (17). A block of tissue was then harvested from the subendocardium of the mid-left-ventricular free wall. Tissue was sampled from the subendocardium, since sepsis-induced histologic changes are most marked in this region (9). One-millimeter blocks of tissue from the endocardium were postfixed in osmium tetroxide and then dehydrated in 100% alcohol. Tissues were then embedded in epoxy resin. One-micron sections from the tissue block were stained with toluidine blue for the purpose of tissue orientation, prior to the cutting of thin sections. One thin section (800 Å to 1,000 Å), large enough to allow at least 10 capillaries to be photographed, was cut from each block, avoiding the possibility that the same capillary may have been sampled twice. All suitable capillaries in each section were photographed in a sequential manner (magnification: ×9,100), using a grid until the necessary number of capillaries were photographed. Capillary sections that included endothelial nuclei were not photographed. Low-magnification sections (×2,100) were prepared in a similar manner. Tissue from a naive group of animals was also studied.
Quantitative measurements of cross-sectioned capillary micrographs were made with computerized image-analysis software (Java, Version 1.20; Jandel Scientific, Corte Madera, CA). Differences in magnification were normalized by introducing a germanium-shadowed carbon replica of a diffraction grating (54,864 lines per inch; Ladd Research Industries, Inc., Burlington, VT). Capillary outer and inner perimeters were measured and used to calculate total capillary and luminal areas. By assuming the capillary to be a circle, the outer (O.D.) and inner equivalent circular diameters (I.D.) were calculated. Mean thickness of the capillary wall was derived from the difference between calculated O.D. and I.D.
Ultrastructural damage to the heart was graded with a modification of Kloner's ultrastructural grading system for myocardial injury (18). A single observer graded the following four parameters in blinded fashion: overall cell morphology, mitochondrial swelling, myofibrillar edema, and glycogen depletion. Overall cell morphometry assessed depletion of glycogen stores, myofibrillar edema, the degree of mitochondrial swelling, and disruption of cristae. Each parameter was evaluated separately on a scoring scale ranging from 0 to 4, with 0 considered normal and 4 representing the most severe cell or organelle disruption. Tissue-injury scores were calculated with both high- and low-power sections. Investigators were blinded to the experimental group during tissue processing for electron microscopy and assessment of tissue-injury scores.
Assessment of microvascular albumin flux and tissue wet:dry weight ratio. In a separate group of rats (n = 38), tissue wet:dry weight ratio and albumin leak were measured (19). Bovine serum albumin (BSA) labeled with 125I was used as a tracer, and BSA labeled with 131I was used as an intravascular volume reference. The tracer was injected intravenously at the beginning of the study (time = 0) and the reference injection was made 60 min later. Three minutes after the reference injection, a sample of arterial blood was collected for counting and the hematocrit was measured. The animal was then killed with a bolus of intravenous sodium pentobarbital and the heart harvested and blotted dry, prior to weighing. After drying for 48 h under an infrared heating lamp, the heart was reweighed and the wet:dry weight ratios was calculated.
Blood and tissue samples were counted in an automatic gamma counter (Model 1185; Searle Analytic, IL). Counts were corrected for background and crossover from 131I to 125I. The apparent distribution volumes of the tracer and reference injections were calculated by dividing the counts per gram of tissue by the counts per milliliter of plasma (drawn at 63 min). Because the reference injection remains in the circulation for only 3 min, its value closely approximates the intravascular volume in a tissue sample. Albumin leak, expressed as µl/g dry weight of muscle, was calculated from the difference in the distribution volumes of the tracer and reference injections for each tissue sample.
Statistical Analysis
Values are expressed as mean ± SEM. Statistical significance was assessed at the p < 0.05 level. Analysis of variance (ANOVA) with and without repeated measures was done with the SAS/PROC GLM (general linear models) procedure (20) (SAS Version 6.08; SAS Institute Inc., Cary, NC). Power calculations were done with Statmate V1.0 (Graphpad Software, San Diego, CA, 1991).
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RESULTS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Model
At 24 h to 48 h after laparotomy, all CLP animals demonstrated reduced activity, piloerection, and exudation around the eyes and nose. In contrast, sham-treated animals demonstrated full recovery from anesthesia and surgery, and had no detectable sign of disease. Generalized peritonitis was confirmed at postmortem in all CLP animals. Physiologic data for animals in which morphometry and in vitro myocardial function was studied are presented in Tables 2 and 3. A mild reduction in blood pressure was observed in CLP animals at 24 h and 48 h, in association with hematologic changes consistent with sepsis, such as a reduction in the leukocyte and platelet counts.
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In vitro Myocardial Function
Figure 2 summarizes the in vitro cardiac perfusion data for animals studied 24 h after CLP. Significant myocardial dysfunction, manifested as a reduction in LVDP, +dP/dtmax, and dP/
dtmax, and an increase in the +dP/dtmax/dP/dtmax ratio, was
demonstrated in CLP animals. An increase in CPP and CVR
was seen in CLP animals, although this difference was not statistically significant.
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sepsis effect, p < 0.0005; sham
group, n = 5; CLP group, n = 12.
Effect of Ischemia-Reperfusion on Myocardial Function
Twenty-four hours after the CLP or sham procedure, we examined the effect of sepsis on baseline myocardial function
and recovery after 30 min of warm (37° C) ischemia. Prior to
ischemia-reperfusion, LVDP at a preload of 5 mm Hg was reduced in CLP animals (34.9 ± 3.3 mm Hg, n = 10) as compared with time-matched controls (46.4 ± 4.0 mm Hg, n = 8;
p < 0.05, unpaired t test). This was associated with a significant reduction in the maximal rate of increase (+dP/dtmax) and decrease (dP/dtmax) in left-ventricular pressure in the CLP group (sham versus CLP, unpaired t test, p < 0.05; Figures 2, 3). Resting pressure, which was adjusted to 5 mm Hg prior to
ischemia, increased rapidly in all groups with the onset of reperfusion (Figure 4). In the CLP group this increase in resting
pressure was significantly attenuated (ANOVA with repeated
measures: p < 0.05). There was no difference among the groups
with respect to recovery of systolic function after reperfusion,
either in terms of the absolute function or function normalized
to baseline (i.e., preischemia; Figure 5).
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Structural Myocardial Changes
There was no difference between experimental groups in the tissue wet:dry weight ratio at 24 h or 48 h (Figure 6A). At 24 h, albumin flux was decreased in CLP animals as compared with time-matched controls (unpaired t test, p < 0.05, Figure 6B). Table 4 shows the data for animals from which histologic sections of subendocardial tissue were processed for electron microscopy (n = 29). No evidence of capillary edema or morphologic tissue injury was seen at either 24 h or 48 h after CLP.
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DISCUSSION |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Previous animal studies have shown an association between sepsis and histologic evidence of parenchymal and microvascular injury (9). These data support an opinion that sepsis-induced myocardial dysfunction may be explained by structural injury to the contractile apparatus or microcirculation of the heart (6). However, an alternative view is that circulating depressant substances are responsible for sepsis-induced myocardial dysfunction (21). To help resolve this question, we undertook the current study to determine the relationship between tissue structure and function in an animal model of normotensive intraabdominal sepsis (CLP). We made two novel findings in this study. First, we found that baseline myocardial contractile function was reduced in the absence of significant alterations in tissue extravascular water content, albumin flux, or other histologic markers of tissue injury. Therefore, demonstrable tissue injury is not a prerequisite for sepsis-induced myocardial dysfunction. Second, we noted that sepsis protected animals against ischemia-reperfusion-induced myocardial injury (delayed preconditioning), suggesting that in early sepsis, host adaptations to oxidative stress may protect the heart against tissue injury.
Animal Model and Rationale
The CLP model is a commonly used animal model of sepsis. It is a chronic model in which blood pressure, cardiac output, and coronary blood flow (22) are maintained. In accord with these findings, we noted in the present study a small, sepsis-induced reduction in blood pressure (Table 2), which was not sufficient to cause septic shock (24). The CLP model therefore provides experimental conditions suitable for studying sepsis-induced changes in myocardial structure and function in the absence of shock or ischemia, both of the latter of which may independently cause myocardial necrosis.
Sepsis-induced alterations in preload and afterload, as well as other neurohumoral effects, may confound the assessment of myocardial contractility in intact animals (6). In small animal models it is possible to assess myocardial contractility in vitro, using the Langendorff isolated heart technique. This method was therefore well suited to the purpose of the present study, which was to examine the temporal relationship between changes in myocardial structure and function.
Recent data show that physiologic stressors, such as sepsis, may upregulate myocardial antioxidant defenses and protect the heart against oxidative injury, such as occurs after ischemia- reperfusion (23). This phenomenon, delayed preconditioning, is a potentially important adaptation protecting the host against sepsis-induced tissue injury. In the present study we used myocardial recovery from ischemia-reperfusion as a tool with which to assess myocardial adaptations to oxidative stress.
Sepsis-induced Changes in Myocardial Contractility
In the current study, we found that CLP was associated with a
reduction in myocardial contractile function. Sepsis includes the host response to tissue injury, and infection is one cause of
this syndrome (24). In the present study we found that myocardial contractility was depressed irrespective of whether the
septic insult included peritoneal infection alone or, with a laparotomy complicated by infection, constituted a septic insult
of greater clinical relevance. These findings confirm previous
reports that have documented reductions in contractility in
animal models of sepsis (11, 25). Cardiac injury was manifest
as: (1) a decrease in the ability of the heart to develop pressure; (2) a reduction in the rate of increase and decrease in
left-ventricular pressure (+dP/dtmax and dP/dtmax, respectively); and (3) a shift in the left-ventricular Starling curves
downward and to the right as compared with the curves for
control animals (Figure 2). Abnormalities in myocardial diastolic function have been reported in a porcine model of endo-toxemia (26) and in humans (27, 28). In accord with these findings, we noted in the present study that sepsis was associated
with an increase in the different ratio (+dP/dtmax/dP/dtmax) in CLP animals (Figure 2), an observation consistent with impaired diastolic relaxation.
Effect of Myocardial Edema on Contractility
Laine and coworkers (29, 30) have shown that myocardial edema may result in a reduction in myocardial contractility in a canine model of chronic pulmonary and arterial hypertension. In the present study we observed a reduction in myocardial function in the absence of tissue edema or increased microvascular albumin flux. These findings suggest that myocardial edema is not a prerequisite for sepsis-induced myocardial dysfunction. On the basis of the standard deviation (SD) of the data collected in this study, we would have been able to detect a difference in the wet:dry ratio of 0.4 with a power of 80% with an unpaired t test. This would have allowed us to detect the change in wet:dry ratio (0.6) shown by Laine and coworkers (30) to cause a 30% alteration in cardiac systolic function.
Myocardial Structure-Function Relationships in Sepsis
Myocardial edema and histologic tissue injury have been reported in animal models of sepsis (Table 1). In rats with CLP, Gotloib and associates (10) noted substantial interstitial edema after 24 h of sepsis. Unfortunately, Gotloib and associates (10) did not provide blood pressure data, making it impossible to determine whether these changes were a manifestation of sepsis or of shock. In a similar study, Langenfeld and associates (9) described subendocardial necrosis 18 h after CLP. This was associated with a reduction in the mean arterial blood pressure (68 ± 19 mm Hg) and a 40 to 50% reduction in cardiac index. These studies (9, 10) provide evidence that tissue injury may occur in animals made septic by CLP; however, the extent to which these changes are due to systemic hypotension is unclear.
Histologic changes are a common feature of sepsis-induced myocardial injury in large animal models of sepsis (Table 1). In a normotensive canine model of gram-negative peritonitis, Solomon and colleagues (12) found that myocardial contractile depression was associated with the development of capillary edema and microcirculatory fibrin deposition. They also noted histologic evidence of neutrophil infiltration, myofibrillar loss, and mitochondrial swelling. Similar marked morphometric changes have been noted in baboons made septic by the infusion of live Escherichia coli (14). We have also observed myocardial injury, manifest as patchy cellular necrosis, mitochondrial injury, and interstitial and cellular edema in sheep with CLP (13). Although myocardial function was not assessed, these changes were seen in the absence of significant hypotension and in the presence of an increase in cardiac output.
Autopsy studies provide circumstantial evidence that histologic changes are also a feature of myocardial injury in human
sepsis. In a retrospective review of the autopsy findings in 71 patients, Fernandes and colleagues (31) noted abnormalities
in the myocardium that consisted of interstitial myocarditis
(27%), interstitial edema (28%), and muscle-fiber necrosis
(7%). A further relevant study was published by Pape and coworkers (32), who reported the autopsy findings in a group of
critically ill patients admitted after multiple trauma. Patients
who perished within 24 h of admission almost always died of
brain injury; the leading cause of death from 2 to 7 d after admission was respiratory failure; and mortality after 7 d was attributable to multiple organ failure. In Pape and coworkers'
study, histologic changes consisting of interstitial edema, cell
necrosis, and neutrophil infiltration were seen in a number of
organs. However, the distribution appeared to be organ specific, with edema particularly affecting the heart and lungs, and cell necrosis affecting the kidney, liver, and heart
especially in the patients who survived longer. As sepsis is the most
common cause of multiple organ failure among patients with
multiple trauma, it is likely that a number of these changes are
attributable to sepsis.
Thus, although there are considerable data demonstrating that myocardial tissue edema and injury occur in association with sepsis, to our knowledge, no studies have sought to determine whether such changes are a prerequisite for the development of contractile dysfunction. The present study is unique in that it provides evidence that sepsis-induced myocardial contractile dysfunction is not secondary to structural changes, which may themselves alter myocardial contractility by: (1) directly damaging important cellular components essential for normal contractile function (such as mitochondria); or (2) causing myocardial edema (30). Power calculations, based on the SD of the tissue injury scores in the present study, suggest that we would have detected with a greater than 80% probability the levels of tissue injury observed previously in crystalloid-resuscitated sheep (8). These findings are consistent with data published by McDonough and coworkers (11), who measured myocardial contractility in rats at 2 to 7 d after CLP. Although vascular permeability was not measured and tissue morphometry was not done in McDonough and coworkers' study, they found that myocardial contractility was reduced in the absence of myocardial edema (assessed by tissue wet:dry weight ratios).
Two possible explanations are offered for the absence of histologic changes in the present study. First, the onset of shock may lead to visceral hypoperfusion and ischemic changes in the myocardium (33). This is an established mechanism of myocardial injury and may have caused the histologic changes seen in the more severe small-animal models of sepsis discussed earlier (9) (Table 1). The data presented in some of these studies are difficult to interpret, since blood pressure data are not reported (10). In the present study, animals were normotensive after 24 h of sepsis, so that myocardial depression cannot be attributed to shock. Second, although rats are commonly used in animal models of sepsis, they are resistant to endotoxin as compared with other species such as sheep, dogs, or humans. If endotoxin does play a role in the pathogenesis of sepsis, the absence of histologic injury seen in this study may have been due to the endotoxin resistance of this species. Previous studies reported in the literature would support this view, since in endotoxin-sensitive animals, histologic evidence of tissue injury is seen in the absence of systemic hypotension (Table 1).
Effect of Ischemia-Reperfusion
Twenty-four hours after exposure to a physiologic stressor such as ischemia, endotoxin, or cytokines, there is upregulation of myocardial antioxidant defenses that is manifested in experimental models as myocardial protection following ischemia- reperfusion ("delayed preconditioning," [34]). Although this response has been studied in animal models of endotoxemia and cytokine exposure, few studies have examined the effect of focal bacterial sepsis on myocardial susceptibility to oxidant stress. In one such study, McDonough and coworkers (25) found a nonstatistically significant improvement in contractile recovery (compared with that in time matched controls) after ischemia-reperfusion in rats made septic by the injection of E. coli into the dorsal subcutaneous space. In the present study, using a model of polymicrobial intraabdominal sepsis, we also observed a trend toward improved contractility in septic animals (as compared with baseline contractility; Figure 5). This was associated with a statistically significant effect of sepsis on postischemic resting tension (i.e., ischemic contracture; Figure 4). This observation is consistent with myocardial protection, since previous studies have shown that ischemic contracture and irreversible myocardial damage correlates closely with the time course of myocardial high-energy phosphate depletion (35). In a recent study done with the Langendorff isolated heart technique, Davidson and associates (36) observed that antecedent sepsis (CLP) induced tolerance to the effects of further myocardial oxyradical (hydrogen peroxide) exposure, resulting in improved myocardial functional and biochemical integrity. These animal data therefore suggest that the myocardium is protected against oxidant-mediated tissue injury in established sepsis. Such a compensatory mechanism may play an important role in preventing tissue injury in the context of ongoing sepsis, and hence in the absence of tissue injury observed in the present study. Further studies are warranted to determine whether delayed preconditioning is seen in animal models (i.e., sheep) in which histologic changes are commonly associated with normotensive sepsis.
Summary
The objective of the present study was to determine whether tissue injury or edema are prerequisites for the development of myocardial dysfunction in sepsis. To our knowledge, previous studies of the mechanism of myocardial dysfunction in sepsis have not attempted to define the temporal relationship between the development of contractile dysfunction and morphometric injury. The present study demonstrated that the reduction in myocardial contractility associated with sepsis may occur in the absence of alterations in myocardial structure, such as interstitial edema or ultrastructural changes in subcellular organelles. Further, this model yields data suggesting that the myocardium may be relatively resistant to tissue injury, as a result of a phenomenon resembling "delayed preconditioning." These findings support the view that myocardial depression in normotensive sepsis results from subtle injury to subcellular macromolecules vital for normal contractile function, or alternatively stems from changes in specific signal-transduction pathways responsible for the modulation of contractile function.
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Footnotes |
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Dr. Sibbald was supported by Grant MIIID3 from the Ontario Heart and Stroke Foundation.
Dr. Piper was supported by a Neil Hamilton Fairley fellowship from the Australian National Health and Medical Research Council.
Correspondence and requests for reprints should be addressed to Dr. W. J. Sibbald, London Health Sciences Centre, Victoria Campus, 375 South Street, London, ON, N6A 4G5 Canada.
(Received in original form October 23, 1996 and in revised form May 9, 1997).
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References |
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REFERENCES
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