Effects of inhaled nitric oxide on right ventricular function in endotoxin shock

RIGHT VENTRICULAR DYSFUNCTION IN SEPSIS: AN UPDATED NARRATIVE REVIEW

ABSTRACT

Sepsis is a multisystem disease process, which constitutes a significant public health challenge and is associated with high morbidity and mortality. Among other systems, sepsis is known to affect the cardiovascular system, which may manifest as myocardial injury, arrhythmias, refractory shock, and/or septic cardiomyopathy. Septic cardiomyopathy is defined as the reversible systolic and/or diastolic dysfunction of one or both ventricles. Left ventricle dysfunction has been extensively studied in the past, and its prognostic role in patients with sepsis is well documented. However, there is relatively scarce literature on right ventricle (RV) dysfunction and its role. Given the importance of timely detection of septic cardiomyopathy and its bearing on prognosis of patients, the role of RV dysfunction has come into renewed focus. Hence, through this review, we sought to describe the pathophysiology of RV dysfunction in sepsis and what have we learnt so far about its multifactorial nature. We also elucidate the roles of different biomarkers for its detection and prognosis, along with appropriate management of such patient population.

The Sick and the Weak: Neuropathies/Myopathies in the Critically Ill

Critical illness polyneuropathies (CIP) and myopathies (CIM) are common complications of critical illness. Several weakness syndromes are summarized under the term intensive care unit-acquired weakness (ICUAW). We propose a classification of different ICUAW forms (CIM, CIP, sepsis-induced, steroid-denervation myopathy) and pathophysiological mechanisms from clinical and animal model data. Triggers include sepsis, mechanical ventilation, muscle unloading, steroid treatment, or denervation. Some ICUAW forms require stringent diagnostic features; CIM is marked by membrane hypoexcitability, severe atrophy, preferential myosin loss, ultrastructural alterations, and inadequate autophagy activation while myopathies in pure sepsis do not reproduce marked myosin loss. Reduced membrane excitability results from depolarization and ion channel dysfunction. Mitochondrial dysfunction contributes to energy-dependent processes. Ubiquitin proteasome and calpain activation trigger muscle proteolysis and atrophy while protein synthesis is impaired. Myosin loss is more pronounced than actin loss in CIM. Protein quality control is altered by inadequate autophagy. Ca(2+) dysregulation is present through altered Ca(2+) homeostasis. We highlight clinical hallmarks, trigger factors, and potential mechanisms from human studies and animal models that allow separation of risk factors that may trigger distinct mechanisms contributing to weakness. During critical illness, altered inflammatory (cytokines) and metabolic pathways deteriorate muscle function. ICUAW prevention/treatment is limited, e.g., tight glycemic control, delaying nutrition, and early mobilization. Future challenges include identification of primary/secondary events during the time course of critical illness, the interplay between membrane excitability, bioenergetic failure and differential proteolysis, and finding new therapeutic targets by help of tailored animal models.

Right Ventricle Failure in Sepsis: A Case Report

Sepsis could produce myocardial depression and typically it affects the left ventricle (LV). Sepsis could also affect right ventricle (RV), in addition to the interdependence with LV. RV pressure may be elevated secondary to pulmonary vasoconstriction, leading to RV dysfunction. Unlike LV, RV is poorly prepared to compensate for acute overload. Aggressive volume replacement may be vital to maintain RV function, but excess hydration can cause RV dilation, decreased LV diastolic filling, and reduced cardiac output. In patients having signs of inadequate cardiac output even after initial volume resuscitation, RV function should be assessed with echocardiogram. If RV dysfunction is noted, then fluid therapy should be guided by CVP measurements. If cardiac output increases with increasing CVP, maintaining higher filling pressures on the right side is indicated. On the other hand, increasing CVP with worsening of the cardiac output could worsen the RV dysfunction. In addition to the fluid management, treatment of other reversible causes like acidosis and hypoxia is also a key.

The right ventricle in sepsis

Right ventricular dysfunction is common in sepsis and septic shock because of decreased myocardial contractility and elevated pulmonary vascular resistance despite a concomitant decrease in systemic vascular resistance. The mainstay of treatment for acute right heart failure includes treating the underlying cause of sepsis and reversing circulatory shock to maintain tissue perfusion and oxygen delivery. Decreasing pulmonary vascular resistance with selective pulmonary vasodilators is a reasonable approach to improving cardiac output in septic patients with right ventricular dysfunction. Treatment for right ventricular dysfunction in the setting of sepsis should concentrate on fluid repletion, monitoring for signs of RV overload, and correction of reversible causes of elevated pulmonary vascular resistance, such as hypoxia, acidosis, and lung hyperinflation.

Experimental animal models of muscle wasting in intensive care unit patients

The muscle wasting and loss of muscle function associated with critical illness and intensive care have significant negative consequences for weaning from the respirator, duration of hospital stay, and quality of life for long periods after hospital discharge. There is, accordingly, a significant demand for focused research aiming at improving our understanding of the mechanisms underlying the impaired neuromuscular function in intensive care unit (ICU) patients. However, the study of generalized muscle weakness in critically ill ICU patients is further complicated by the coexistence of multiple independent factors, such as different primary diseases, large variability in pharmacologic treatment, collection of muscle samples several weeks after admission to the ICU, and exposure to causative agents. This has led to the design of specific animal models mimicking ICU conditions. These models have often been used to study the mechanisms underlying the paralysis and muscle wasting associated with acute quadriplegic myopathy in ICU patients. This short review aims at presenting existing and recently introduced experimental animal models mimicking the conditions in the ICU (i.e., models designed to determine the mechanisms underlying the muscle wasting associated with ICU treatment).

A porcine model of acute quadriplegic myopathy: a feasibility study

BACKGROUND

The mechanisms underlying acute quadriplegic myopathy (AQM) are poorly understood, partly as a result of the fact that patients are generally diagnosed at a late stage of the disease. Accordingly, there is a need for relevant experimental animal models aimed at identifying underlying mechanisms.

METHODS

Pigs were mechanically ventilated and exposed to various combinations of agents, i.e. pharmacological neuromuscular blockade, corticosteroids and/or sepsis, for a period of 5 days. Electromyography and myofibrillar protein and mRNA expression were analysed using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), confocal microscopy, histochemistry and real-time polymerase chain reaction (PCR).

RESULTS

A decreased compound muscle action potential, normal motor nerve conduction velocities, and intact sensory nerve function were observed. Messenger RNA expression, determined by real-time PCR, of the myofibrillar proteins myosin and actin decreased in spinal and cranial nerve innervated muscles, suggesting that the loss of myosin observed in AQM patients is not solely the result of myofibrillar protein degradation.

CONCLUSION

The present porcine AQM model demonstrated findings largely in accordance with results previously reported in patients and offers a feasible approach to future mechanistic studies aimed at identifying underlying mechanisms and developing improved diagnostic tests and intervention strategies.

Effects of inhaled nitric oxide on pulmonary hemodynamics in a porcine model of endotoxin shock

OBJECTIVE

To evaluate the effects of inhaled nitric oxide (NO) on pulmonary circulation in a porcine endotoxin shock model.

DESIGN

Prospective, randomized trial.

SETTING

Laboratory at a large university medical center.

SUBJECTS

Twelve pathogen-free pigs weighing 15 to 31 kg.

INTERVENTIONS

After surgical preparation, all pigs received a 0.5 mg/kg endotoxin infusion over 30 mins. One hour after the start of endotoxin, NO inhalation (40 ppm) was initiated in six pigs, whereas the six remaining pigs served to control the progression of shock in this model. Consecutive changes in systemic and pulmonary hemodynamics, including characteristic resistance, vascular compliance, peripheral vascular resistance, and inductance, were continuously assessed during the experimental protocol using a four-element Windkessel model of the pulmonary circulation.

MEASUREMENTS AND MAIN RESULTS

Endotoxin insult resulted in a biphasic pulmonary artery pressure increase from 14 +/- 2 to 32 +/- 4 mm Hg. Inhaled NO reversed the resistance to blood flow in small pulmonary arteries from 596 +/- 69 to 424 +/- 36 dyne-sec/ cm5. In contrast, the vascular capacitance of the entire pulmonary circuit, which decreased from 2.4 +/- 0.2 to 0.8 +/- 0.1 mL/mm Hg throughout endotoxin challenge, remained insensitive to NO administration.

CONCLUSION

In endotoxin-induced pulmonary hypertension, inhaled NO may function as a modulator of distal pulmonary arterial tone but fails to act as a regulator of larger capacitance pulmonary vessels.

Inhaled nitric oxide prevents left ventricular impairment during endotoxemia

We evaluated the effect of long-term inhalation of nitric oxide (NO) on cardiac contractility after endotoxemia by using the end-systolic elastance of the left ventricle (LV) as a load-independent contractility index. Chronic instrumentation in 12 pigs included implantation of two pairs of endocardial dimension transducers to measure LV volume and a micromanometer to measure LV pressure. One week later, the animals were divided into a control group (n = 6) or a NO group (n = 6). All animals received intravenous Escherichia coli endotoxin (10 micrograms. kg-1. h-1) and equivalent lactated Ringer solution. NO inhalation (20 parts/million) was begun 30 min after the initiation of endotoxemia and was continued for 24 h. In both groups, tachycardia, pulmonary hypertension, and systemic hyperdynamic changes were noted. The end-systolic elastance in the control group was significantly decreased beyond 7 h. NO inhalation maintained the end-systolic elastance at baseline levels and prevented its impairment. These findings indicate that NO exerts a protective effect on LV contractility in this model of endotoxemia.

New developments in nitrosovasodilator therapy

Under basal conditions, nitric oxide (NO) modulates vascular tone, serves as an antithrombotic agent, and inhibits vascular smooth muscle cell proliferation. NO deficiency has been implicated in the pathophysiology of several vascular disorders, including hypertension, atherosclerosis, and restenosis, and provides a plausible biologic basis for the use of NO replacement therapy in these conditions. Treatment with conventional nitrate preparations is limited by a short therapeutic half-life, systemic absorption with potentially adverse hemodynamic effects, and drug tolerance. To overcome these limitations, novel delivery systems and novel NO donors have been developed that offer selective effects, a prolonged half-life, and a reduced incidence of tolerance.

Right ventricular overload causes the decrease in cardiac output after nitric oxide synthesis inhibition in endotoxemia

OBJECTIVE

To determine whether the decrease in cardiac output after nitric oxide synthase inhibition in endotoxemia is due to increased left ventricular afterload or right ventricular afterload.

DESIGN

Prospective, randomized, unblinded study.

SETTING

Research laboratory at an academic, university medical center.

SUBJECTS

Nonanesthetized, sedated, mechanically ventilated pigs.

INTERVENTIONS

Pigs were infused with 250 microg/kg of endotoxin over 30 mins. Normal saline was infused to maintain pulmonary artery occlusion pressure (PAOP) at a value not exceeding 1.5 times the baseline value. Left ventricular dimensions and function were studied using echocardiography. Right ventricular volumes and ejection fraction were determined via a rapid thermistor pulmonary artery catheter. We also measured mean arterial pressure (MAP), cardiac output, pulmonary arterial pressure, and calculated pulmonary and systemic resistances. Gastric tonometry was used as an index of gastric mucosal oxygenation and peripheral oxygenation. When MAP had decreased to < or =60 mm Hg or had decreased 30 mm Hg from baseline, nine animals received NG-nitro-L-arginine methyl ester (L-NAME) at 15 mg/kg to restore MAP to baseline. A second group of animals (n = 6) continued to receive normal saline, ensuring that PAOP did not exceed 1.5 times its baseline value. A third group of pigs (n = 5) did not receive endotoxin and served as the time control. In this group, a balloon was used to occlude the descending thoracic aorta and to increase MAP by approximately the same amount as in the L-NAME group.

MEASUREMENTS AND MAIN RESULTS

Endotoxin caused an increase in pulmonary arterial pressure and right ventricular volumes, and a decrease in gastric mucosal pH. Cardiac output was maintained in the animals receiving the saline infusion. By 2 hrs, pulmonary arterial pressure had decreased but was still notably higher than baseline. However, by this time, MAP had decreased to < or =60 mm Hg. L-NAME administration restored MAP to its baseline value but resulted in worsening pulmonary hypertension, increased right ventricular volumes, and decreased cardiac output, compared with the saline group. Three animals that received L-NAME died of right ventricular failure. We did not observe any evidence of left ventricular dysfunction with increased left ventricular afterload. Moreover, the restoration of MAP with L-NAME infusion did not correct gastric mucosal acidosis. No changes were noted in the time-control group. Occlusion of the thoracic aorta increased MAP but did not change cardiac output. This finding demonstrates that increases in left ventricular afterload of the magnitude seen with the infusion of L-NAME do not lead to decreases in cardiac output.

CONCLUSION

The decrease in cardiac output after nitric oxide synthase inhibition in endotoxemia is due to increased right ventricular afterload and not to left ventricular afterload.

Nitric oxide attenuates platelet-activating factor priming for elastase release in human neutrophils via a cyclic guanosine monophosphate-dependent pathway

BACKGROUND

Nitric oxide (NO) has proven benefits in treating adult respiratory distress syndrome (ARDS). The protective mechanism remains unclear, but cyclic guanosine monophosphate (cGMP)-dependent signal transduction pathways have been suggested. Our laboratory has implicated polymorphonuclear neutrophil (PMN) priming and subsequent activation in the pathogenesis of postinjury ARDS and has shown that NO inhibits superoxide anion production from activated PMNs. More recently, the pivotal role of elastase in PMN-mediated tissue injury has been emphasized. Consequently, our study hypothesis was that NO attenuates platelet-activating factor (PAF) priming for elastase release through a cGMP-dependent pathway in human PMNs.

METHODS

PMNs isolated from human volunteers were preincubated with the NO donor 3-morpholinosydnonimine hydrochloride (SIN-1; 10(-6) to 10(-2) mol/L), cGMP (10(-3) mol/L), or the cell-permeable cGMP analog dibutyryl-cGMP (10(-3) mol/L) for 10 minutes. The cells were then primed with platelet-activating factor (PAF) 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (200 nmol/L), which was followed by N-formyl-methionyl-leucyl-phenylalanine (fMLP) activation (1 mumol/L). Elastase release was measured by the cleavage of N-methoxysuccinyl-Ala-Ala-Pro-Val p-nitroanilide (AAPV-pNA).

RESULTS

NO (through SIN-1) significantly attenuated elastase release from PAF-primed PMNs in a dose-dependent manner. Cell-permeable dibutyryl-cGMP attenuated PMN elastase release similar to NO, but cGMP (not cell-permeable) had no effect.

CONCLUSIONS

NO attenuates elastase release from PAF-primed PMNs through an intracellular cGMP-dependent signal transduction pathway. These findings may partially explain the beneficial effects of NO in the treatment of postinjury ARDS and add to our knowledge of the complex intracellular pathways involved in NO/PMN interactions.

Right ventricular failure--insights provided by a new model of chronic pulmonary hypertension

This study was designed to examine the effects of both nitric oxide and milrinone on pulmonary hemodynamics and right ventricular function using a newly established model of monocrotaline pyrrole-induced chronic pulmonary hypertension. Sixteen mongrel dogs (23-25 kg) were used. All animals underwent percutanous pulmonary artery catheterization to measure right heart hemodynamics prior to and 8 weeks after a right atrial injection of either monocrotaline pyrrole (MCTP, n=8) or placebo (CTL, n=8). Eight weeks postinjection, all hearts were instrumented with a pulmonary artery flow probe and intracavitary micromanometers. Data were collected at baseline as well as following both nitric oxide and milrinone administration. There was no significant difference in the baseline hemodynamic measurements between the two groups. Eight weeks postinjection, significant increases in the pulmonary artery pressure and pulmonary vascular resistance were observed in MCTP compared with CTL. Both nitric oxide and milrinone resulted in significant improvements in pulmonary vascular resistance, pulmonary blood flow, and right ventricular contractility. In addition, nitric oxide caused a significant improvement in pulmonary artery pressure and transpulmonary efficiency, while milrinone led to a significant increase in right ventricular hydraulic power. This study demonstrates the well-known clinical effects of nitric oxide and milrinone in improving pulmonary hypertension, which were also associated with an increase in pulmonary blood flow, transpulmonary efficiency, and right ventricular hydraulic power in the setting of monocrotaline pyrrole-induced chronic pulmonary hypertension.

Cardiopulmonary effects of combined nitric oxide inhibition and inhaled nitric oxide in porcine endotoxic shock

BACKGROUND

Nitric oxide synthase (NOS) inhibition has been shown to potentiate lipopolysaccharide (LPS) associated pulmonary hypertension, which may worsen right ventricular (RV) dysfunction and decrease cardiac output during sepsis. This study evaluates whether inhaled nitric oxide can ameliorate the adverse cardiopulmonary effects of NOS inhibition during endotoxemia.

METHODS

After an infusion of Escherichia coli LPS (200 micrograms/kg), animals were resuscitated with saline (1 mL/kg/min) and observed for 3 hours while mechanically ventilated (FIO2, 0.6; VT, 12 mL/kg; positive end-expiratory pressure, 5 cm H2O). The LPS group (n = 6) received no additional treatment. The N-nitro-L-arginine methyl ester (NAME) group (n = 5) received L-NAME, a NOS inhibitor, 50 micrograms/kg/min for the last 2 hours. The NO+NAME group (n = 6) received inhaled NO (40 ppm) and L-NAME for the last 2 hours. The control group (n = 5) received only saline without LPS. Hemodynamic data and blood gases were collected hourly for 3 hours.

RESULTS

L-NAME worsened LPS-associated pulmonary hypertension and RV dysfunction as reflected by decreased RV ejection fraction. Inhaled nitric oxide significantly decreased pulmonary hypertension and improved RV ejection fraction and stroke work index. There were no adverse systemic effects.

CONCLUSIONS

Inhaled nitric oxide reverses pulmonary hypertension seen with L-NAME treatment during endotoxemia and may be a useful adjunct to NOS inhibition in the treatment of septic shock.