| Abstract |
The term angiogenesis is used to describe the formation of new capillaries from existing ones. In response to various stimuli acting both on luminal and abluminal surface of the existing vessels, the basic membrane and the extracellular matrix underlying the endothelium are dissolved allowing the endothelial cells to migrate into extravascular space. There they proliferate and form sprouts that subsequently acquire newly formed basic membrane and are encircled by pericytes. The sprouts become functional on anatomic connection to the already existing segments of capillary network. Specific angiogenic growth factors have been well documented to initiate and promote this extremely complex process. Vascular Endothelial Growth factor (VEGF) stimulates the migration and proliferation of endothelial cells and smooth muscle cells as well as the production of proteases that are essential for the dissolution of the basic membrane of the vessels. Basic Fibroblast Growth Factor (bFGF) is a potent mitogenic factor for endothelial cells, smooth muscle cells and fibroblasts. It also upregulates VEGF and NO. Transforming Growth Factor β1 (ΤGF- β1), mainly produced by activated endothelial cells, promotes angiogenesis by recruiting pericytes to complete and stabilize the newly formed capillaries.
It has been well documented in numerous animal and human studies that increased muscle activity consists a powerful stimulus for the formation of new capillaries in both skeletal muscles and heart muscle in order to meet their increased metabolic demands. Agiogenesis in response to increased muscle activity has been attributed to either metabolic stimuli, such as tissue hypoxia, or mechanical forces. The latter can be classified as (a) hemodynamic, acting on the luminal surface of
ΑΕΡΙΣΜΟΣ - ΑΓΓΕΙΟΓΕΝΕΣΗ ΔΙΑΦΡΑΓΜΑΤΟΣ - ΓΕΝΙΚΗ ΑΝΑΙΣΘΗΣΙΑ
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the vessels i.e. wall tension and shear stress, due to increased blood flow and (b) extravascular, acting on the abluminal surface, as a result of muscular fiber contraction and relaxation. Mechanical forces associated with shortening and relaxation of the myocytes are imparted to the microvascular network via the extracellular matrix and connective tissue elements. Any mechanical injury caused to the endothelium by the above stimuli is sufficient to initiate the process of angiogenesis, through the realease of the angiogenic factors.
The diaphragm, the main respiratory muscle that is longlife continuously and rhythmically contracting demonstrates characteristics of the skeletal muscles. It has been shown in animal and human studies that it exhibits the same angiogenic response to increased workload as locomotive muscles. Siafakas et al demonstrated increased VEGF and bFGF mRNA diaphragmatic levels in rats after 1 hour of increased ventilation due to hypercapnia and hypoxaemia, in contrast to animals with paralysed diaphragms. Increased mRNA expression of the VEGF was also found in the diaphragm of COPD and obese patients, compared to normal subjects, due to consequent increased respiratory workload.
The aim of the present study was to assess the gene expression of the three major angiogenic factors (VEGF, bFGF and TGF-β1) in the human diaphragm before and after the application of different modes of mechanical ventilation. The study investigated the mRNA levels of the above angiogenic factors in the diaphragm of patients receiving general anaesthesia in relation to (a) active diaphragmatic contractions during spontaneous breathing with partial support and (b) passive diaphragmatic movement during controlled mechanical ventilation (CMV) with and without muscle relaxation.
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SUMMARY
Women ASA physical status I-II scheduled for elective lower abdominal surgery (total hysterectomy) were enrolled in the study. All patients underwent combined general and thoracic epidural anaesthesia and were randomized in three groups according to the mode of ventilation intraoperatively. Group A received CMV with simultaneous administration of a neuromuscular blocking agent. Group B received also CMV but without neuromuscular blocking agent. Group C maintained spontaneous breathing on pressure support ventilation (PSV) mode. Diaphragmatic specimens (1-1.5cm3) were obtained from each patient at two different time points, 30 min after the induction of anaesthesia (t1) and 90 min subsequently (t2). The mRNA levels of VEGF, bFGF and TGFβ1 were measured using the Quantitative Real-Time Polymerase Chain Reaction (Real Time qPCR). Normalized mRNA levels of each angiogenic factor were statistically compared in groups A, B and C between the two time points, using the Student’s t-test and Mann-Whitney U test (2-tailed).
Τhe present study showed increased diaphragmatic mRNA levels of VEGF (3.7-fold), bFGF and TFG-β1 (2.1-fold), at time t2 compared to t1, in patients receiving controlled mechanical ventilation without muscle relaxation (group B). Ιn contrast, the gene expression of the above factors did not exhibit significant changes, either during controlled mechanical ventilation with muscle relaxation, or pressure support ventilation (groups A & C, respectively).
The increased angiogenic response in patients receiving CMV without muscle relaxation can be attributed to the increased mechanical stress that the diaphragm experienced during the positive pressure mechanical ventilation. The mechanical forces from the repetitive muscle fiber contraction and relaxation applied on the diaphragmatic capillaries, in parallel with respiratory cycles, created a powerful stimulus for diaphragmatic angiogenesis, at least at gene level. On the contrary, the administration of a neuromuscular blocking agent in patients who also received CMV diminished the
ΑΕΡΙΣΜΟΣ - ΑΓΓΕΙΟΓΕΝΕΣΗ ΔΙΑΦΡΑΓΜΑΤΟΣ - ΓΕΝΙΚΗ ΑΝΑΙΣΘΗΣΙΑ
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muscle tone and subsequently the mechanical stress of the diaphragm and there was no signal for increased angiogenesis. In patients who breathed spontaneously, there was no apparent change in mechanical stimulus applied on the diaphragm, before and following PS ventilation. Our results cannot be attributed to other factors, since our patients did not exhibit haemodynamic instability, hypoxaemia or acidosis that would possibly affect the process of angiogenesis.
Ιn conclusion, the present study demonstrated that controlled mechanical ventilation without neuromuscular blockade stimulated the angiogenic response, at least at gene level, contrary to controlled mechanical ventilation with simultaneous neuromuscular blockade as well as spontaneous breathing with PS. The different effect on angiogenesis was attributed to the different mechanical stress that the diaphragm experienced during the three different modes of ventilation. The absence of mechanical stress (unloading) of the diaphragm during controlled mechanical ventilation has been proved as the main factor leading to the so-called Ventilator Induced Diaphragmatic Dysfunction (VIDD), an entity including both muscular atrophy and contractile dysfunction.
Our results shed light into the effect of the mode of mechanical ventilation on the angiogenesis of the diaphragm. Further studies are needed to investigate any potential relation between angiogenesis and diaphragmatic function during and after discontinuation of mechanical ventilation. Possibly, angiogenesis be a potential facror that modulates the impact of different modes of either controlled or assisted mechanical ventilation on diaphragmatic biology and function. Whether increased angiogenic response of a mechanically overloaded, non-paralysed diaphragm may be a preventive measure against mechanical ventilation-induced diaphragmatic dysfunction and muscle atrophy is a question to be answered in future studies. New therapeutic strategies may be
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SUMMARY
developed under this consideration, aiming at the faster and safer weaning of critically ill patients from mechanical ventilation.
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Η επίδραση του τύπου του αερισμού στην αγγειογένεση του διαφράγματος σε ασθενείς υπό γενική αναισθησία
