INTRODUCTION
Liposuction, which is widely performed in the field of plastic surgery, was recently identified as a necessary process for the harvesting of adipose-derived stem cells [
1]. In liposuction, fat tissue is crushed and removed using physical force. An inevitable consequence of liposuction is damage to the small vessels, through which shredded pieces of fat (fat emboli) can then flow; this phenomenon is known as fat embolism. If a pathologic state results, it is called fat embolism syndrome (FES) [
2]. In FES, fat emboli are not fat tissue fragments or fat cells; rather, they are lipid drops floating in the circulation [
3].
FES was first reported by Zenker [
4] in 1862. Both fulminant-acute and sub-acute FES, which are caused by an abnormal influx of fat into the blood vessels, are associated with characteristic clinical symptoms, including petechiae, confusion, and acute respiratory failure. FES can have fatal complications. Fortunately, most instances of fat embolism are subclinical [
5].
Fat, the causative agent of FES, can be divided into two types: transported fat, which is present in the blood; and stored fat, which is located outside the blood vessels. Transported fat is in the form of chylomicrons bound to apolipoprotein, whereas stored fat accumulates within adipocytes as triglycerides (TGs). TGs can be hydrolyzed to free fatty acids (FFAs) by lipase, and FFAs bound to albumin are then moved to other tissues through the blood vessels.
FFAs, a toxic substance, bind calcium ions in the blood and breaks the junctions between endothelial cells. This disrupts the endothelial cell layer surface of capillaries and increases vascular permeability, resulting in hemorrhage and edema [
5]. FFAs can attach to neutrophils, which play a significant role in inflammation and increase the expression of CD11b/CD18 β
2 integrins on neutrophils. Neutrophils produce proteases that digest alveolar endothelial cells [
6].
FES progresses rapidly, but few studies have examined its prediction, management, or prophylaxis [
7]. Therefore, it is important to improve our understanding of its pathophysiology.
The progression of subclinical fat embolism to significant FES can be explained by two hypothetical mechanisms. The first theoretical mechanism is mechanical obstruction, in which peripheral vessels are mechanically obstructed by fat emboli when adipose tissue is damaged and the end organs subsequently develop ischemia and severe dysfunction, called fulminant-acute FES [
8]. A significant quantity of fat emboli is needed to cause clinical symptoms, which progress very quickly. However, cases of FES caused by mechanical obstruction are very rare. The other proposed mechanism is the biochemical theory, according to which FFAs destroy organs through chemical interactions and cause sub-acute FES [
9]. The more frequent type, sub-acute FES, usually occurs in 12–48 hours after an injury. Peltier [
10] first described these two theories in animal experiments by injecting TGs and FFAs. However, no studies to date have examined the interaction between these two causative agents of FES.
In FES, fat emboli that enter the venous circulation are TGs (stored fat), not FFAs. Therefore, the two main theories of the etiology of FES, mechanical obstruction and biochemical mechanisms, are likely related and do not act independently. In other words, if the amount of fat entering the bloodstream is insufficient to induce fulminant-acute FES, the remaining fat will be hydrolyzed to FFAs. This results in a shift from mechanical obstruction to the biochemical phase, which leads to sub-acute FES. However, there is no evidence that fat emboli that cause mechanical obstruction are hydrolyzed to FFAs and cause FES through the toxic biochemical action of FFAs.
The present study was designed to test the hypothesis that fat emboli that mechanically obstruct alveolar capillaries are hydrolyzed to FFAs, causing sub-acute FES. To induce fat embolism, triolein, a representative stored fat, was injected to the veins of rabbits. The linear formula of triolein is (C17H33COOCH2)2CHOCOC17H33; its chemical formula is C57H104O6, its molecular weight is 885.45 g/mol, and its density is 0.913 mg/mL. We measured changes in the concentration of some blood lipids shortly after the triolein injection. In necropsies, we examined the lung tissues histologically to investigate the relationship between changes in blood FFA levels and the development and progression of FES.
DISCUSSION
Fat embolism is defined as the mechanical obstruction of blood vessels, especially the capillaries, due to the presence of small circulating fat droplets of 7–14 µm in diameter [
3,
11].
FES involves the functional deficit of one or more organs due to damage by fat emboli [
3]. Because fat emboli initiate and expand throughout the venous circulation, the lungs are primarily involved. However, any body organ or tissue can be affected when fat emboli pass through the pulmonary capillaries or the atrial septum [
12]. These findings have been described in some autopsy reports and animal studies in which fat emboli were found in the kidneys and brains after severe injury [
13,
14].
FES usually results from high-energy trauma in association with a long-bone or hip fracture [
15]. The incidence of FES is high in orthopedic trauma patients, but it can occur in cases of septicemia, sickle cell anemia, pancreatitis, diabetes mellitus, fatty liver, long-term steroid use, and extensive burns [
3,
16,
17]. FES also occurs frequently after cosmetic procedures such as liposuction and fat injection [
11,
18-
22]. The fat emboli caused by fractures are derived from the bone marrow fat, while those caused by liposuction are subcutaneous fat in origin; however, both sources contain the same kind of stored fat.
The mechanism of FES is very complex, and two current hypotheses exist. The first is the mechanical obstruction theory proposed by Gauss [
8] in 1924. In this theory, symptoms appear due to mechanical obstruction of the pulmonary capillaries by fat emboli. In such cases, if the fat emboli obstruct more than 80% of the pulmonary capillaries, pulmonary arterial pressure increases and falls, causing acute right ventricular failure, which rapidly leads to death [
7,
12,
23]. The second is the biochemical theory introduced by Lehman and Moore [
9] in 1927. According to this theory, FFAs, which are toxic, bind with calcium ions in the blood and disrupt the junctions between endothelial cells, disrupting the endothelial cell layer of capillaries, increasing vascular permeability, and resulting in diffuse hemorrhaging and edema [
3]. In addition, neutrophils play a significant role in the cell damage that occurs during inflammation [
5,
6]. The toxic action of FFAs seriously damages the alveoli and capillaries, resulting in hemorrhage.
We propose that these two hypotheses may not be independent. If the amount of fat emboli is insufficient to initiate mechanical FES, the pathologic reaction changes from a mechanical obstruction to a biochemical process. However, no study to date has examined the relationship between these two reactions during the development of FES. In the present study, we experimentally induced FES by injecting triolein (700 mg/kg) into 25 rabbits and observed the disease progression. Eight animals died immediately after the injection, while another six developed dyspnea and confusion and died 7–60 hours after the injection. The other 11 recovered from tachypnea and survived to the end of the experiment.
The rabbits that died immediately after the injection had pale lungs and right ventricular hypertrophy. Histological examinations revealed large fat vacuoles within large pulmonary vessels, but normal lung parenchyma. These findings suggest mechanical obstruction of the large pulmonary artery by the injected triolein. Further, blockage of the blood flow through the lung caused right-side heart failure and sudden death [
10]. These findings suggest that the rabbits died of fulminant-acute FES due to the triolein injection.
The rabbits that died 7–60 hours after the injection of triolein showed diffuse hemorrhage and edematous fluid in the alveoli and hemorrhagic exudate in the pleural cavity, similar to the typical findings of FFA-induced FES [
10].
The rabbits that survived until the end of the experiment recovered from tachypnea. Histological examination of the lungs showed mixed features of normal lung tissue and hemorrhagic lesions. Many macrophages were visible under the microscope as phagocytosing red blood cells and absorbing extravasated blood filling the alveoli. These findings suggest that these rabbits were recovering from lung hemorrhage, rather than from obstruction.
The result of blood tests over time in the surviving rabbits showed that the FFA and FFA/albumin levels increased by up to 40%, whereas TG and lipase levels slightly decreased below the initial level during the first 12 hours. This suggests that the fat emboli were hydrolyzed to FFA more rapidly than expected. After 12 hours, the FFA and FFA/albumin levels rapidly decreased, whereas the TG level began to increase markedly. After 48 hours, the reduced FFA and FFA/albumin levels began to increase slightly and returned to the pre-injection level (
Fig. 7).
Altogether, eight of 25 rabbits died immediately of mechanical obstruction; six died of FES by toxic effects due to markedly increased FFA levels induced by the injected triolein; and 11 recovered from FES associated with a decreased FFA level, which increased up to 40% in the early phase of the experiment. These clinical, histological, and biochemical results are consistent with the hypothesis of this study that fat emboli, which mechanically obstruct the alveolar capillaries, are hydrolyzed to FFAs, the toxic effects of which cause sub-acute FES.
These results align well with the clinical course of FES. The rabbits that died within 1 hour functioned as models of quickly progressing fulminant-acute FES. The rabbits that died 7–60 hours after the injection were models of sub-acute FES, which usually occurs 12–48 hours after injury in the clinical setting. We stopped this study at 72 hours because the incidence of FES after this time period is very rare and none of the surviving rabbits had clinical symptoms.
In the hydrolysis process, alveolar cells can produce lipase, which is believed to hydrolyze fat emboli to FFAs if the lung alveolar capillaries are obstructed [
3,
23]. Thus, the initial decrease in lipase levels that occurred for up to 24 hours was probably due to a consumption-related decrease in existing lipase in the bloodstream during hydrolysis of the injected triolein. A possible explanation for the increasing lipase levels after 24 hours may be that lipase was produced by the lung and released into the bloodstream by reperfusion.
Another point to note in the present study is that the FFA concentrations in the rabbits that died at 7–60 hours after injection showed rapid and tremendous increases. This finding suggests that the onset of sub-acute FES was determined by how many of the fat emboli were transformed to FFAs and how rapidly this transformation took place.
In the present study, the increase in FFA by hydrolysis of fat emboli was among the important causes of tissue damage in FES. Further studies are needed to identify ways of reducing the blood FFA levels, tissue damage, and mortality rate.
In conclusion, the two hypotheses of FES, mechanical obstruction and biochemical processes, can be generally accepted. The present study is the first to describe these two hypotheses as being interrelated, rather than independent. Fat emboli initially act as mechanical obstructors, but are gradually hydrolyzed to form toxic FFAs, inducing sub-acute FES.
The present study revealed the pathophysiology of FES in more detail. These results are expected to help predict the occurrence of FES and aid in the development of more effective treatments for it.