What Is an Acute Lung Injury?

A variety of clinical diseases can cause ALI, and whether it is primary in the lung can be divided into intrapulmonary factors (direct injury) and extrapulmonary factors (indirect injury) (Table 1). According to the ALI / ARDS diagnostic criteria proposed by the 1994 European and American Joint Conference, the incidence of ALI and ARDS in the United States in 2005 was 79 / 100,000 and 59 / 100,000, respectively. The incidence of ARDS varies with the etiology. The incidence of ALI / ARDS can be as high as 25% to 50% when severe infections occur, massive blood transfusion can reach 40%, and multiple trauma can reach 11% to 25%. When there are 2 or 3 risk factors at the same time, the incidence of ALI / ARDS is further increased. In addition, the longer the duration of the risk factors, the higher the incidence of ALI / ARDS. When the risk factors continued for 24, 48, and 72 hours, the prevalence of ARDS was 76%, 85%, and 93%, respectively.

Acute lung injury

Acute lung injury (ALI) is the injury of alveolar epithelial cells and capillary endothelial cells caused by various direct and indirect trauma factors, causing diffuse pulmonary interstitial and alveolar edema, and acute hypoxic respiratory insufficiency . The pathophysiological characteristics are reduced lung volume, reduced lung compliance, imbalance of ventilation / blood flow, clinical manifestations of progressive hypoxemia and respiratory distress, and non-uniform exudative lesions in pulmonary imaging. , Whose development to the severe stage (oxygenation index <200) is called acute respiratory distress syndrome,

Etiology and epidemiology of acute lung injury

A variety of clinical diseases can cause ALI, and whether it is primary in the lung can be divided into intrapulmonary factors (direct injury) and extrapulmonary factors (indirect injury) (Table 1). According to the ALI / ARDS diagnostic criteria proposed by the 1994 European and American Joint Conference, the incidence of ALI and ARDS in the United States in 2005 was 79 / 100,000 and 59 / 100,000, respectively. The incidence of ARDS varies with the etiology. The incidence of ALI / ARDS can be as high as 25% to 50% when severe infections occur, massive blood transfusion can reach 40%, and multiple trauma can reach 11% to 25%. When there are 2 or 3 risk factors at the same time, the incidence of ALI / ARDS is further increased. In addition, the longer the duration of the risk factors, the higher the incidence of ALI / ARDS. When the risk factors continued for 24, 48, and 72 hours, the prevalence of ARDS was 76%, 85%, and 93%, respectively.

Although reports of ALI / ARDS mortality rates vary widely (15-72%), overall mortality rates for ARDS are still high. A meta-analysis of 72 ARDS clinical studies formally published internationally from 1994 to 2006 revealed a mortality rate of 43% for 11,426 ALI / ARDS patients. The mortality rate of 15 adult ICUs in Shanghai, China from March 2001 to March 2002 also reached 68.5%. Recently, a meta-analysis of Ritesh A et al. Showed that there is no difference in mortality between ALI caused by intrapulmonary and extrapulmonary factors.

Table 1 Causes of acute lung injury

Direct damage indirect damage

Severe sepsis of pneumonia

Gastroesophageal reflux massive transfusion

Drowning shock

Fat and amniotic fluid embolism pancreatitis

Lung contusion salicylate / narcotic overdose

Alveolar hemorrhage

Inhalation of smoke and toxic gases

Reperfusion injury (embolization)

ALI Diagnosis of acute lung injury ALI

At present, the diagnosis of ALI still uses the standards proposed by the 1994 European and American Joint Conference: (1) acute onset, there are pathogenic factors; (2) oxygenation index (arterial blood oxygen partial pressure / inhaled oxygen concentration, PaO2 / Fi02) < 300mmHg (1mmHg = 0.133kPa) does not refer to the positive end expiratory pressure (PEEP) level; (3) an orthotopic chest X-ray shows patchy shadows in both lungs; (4) pulmonary artery impaction <18mmHg, or no clinical evidence of increased left atrial pressure; (5) acute paroxysmal respiratory failure ^[1] . With the deepening of ALI research and understanding, the diagnostic criteria are still inadequate. The oxygenation index is not a value that excludes PEEP. Some patients use appropriate PEEP and the oxygen partial pressure reaches a satisfactory value, which excludes ALI. Currently ALI As the diagnosis is mostly used for research needs, it has been reported that the clinical written diagnosis uses a lower ALI, which is only 20-48%; In addition, among the diagnostic standards proposed by the European and American Joint Conference, the ALI oxygenation index (PaO2 / Fi02) <300mmHg, ARDS Oxygenation index <200mmHg, about 25% of ALI patients have an oxygenation index between 200mmHg and 300mmHg, of which 20% -50% of patients progress to ARDS within seven days, how to define such patients still confuse clinical .

ALI Treatment of acute lung injury ALI

3.1 Primary disease treatment

Controlling the primary disease and curbing the systemic uncontrolled inflammatory response induced by it are necessary measures to prevent and treat ALI / ARDS.

3.2 Respiratory Support Therapy

3.2.1 Oxygen therapy The purpose of oxygen therapy for ALI / ARDS patients is to improve hypoxemia, so that the arterial blood oxygen pressure (PaO2) reaches 60 to 80 mmHg. Nasal catheters, venturi masks, pure oxygen masks, etc. can be used. Due to respiratory distress and increased breathing rate in ALI / ARDS patients, simple oxygen therapy can correct hypoxia early, but it is difficult to correct respiratory distress. According to the needs of the disease, mechanical ventilation should be given early when necessary.

3.2.2 Non-invasive ventilator: The efficacy of non-invasive mechanical ventilation (NIV) in the treatment of acute respiratory failure caused by chronic obstructive pulmonary disease and cardiogenic pulmonary edema is positive, but the effect of NIV in acute hypoxic respiratory failure There is much controversy in applications. To date, there is not enough data to show that NIV can be used as a routine treatment for acute hypoxic respiratory failure caused by ALI / ARDS. When patients with ALI / ARDS are conscious, hemodynamically stable, and can be closely monitored and tracheal intubation is possible at any time, NIV treatment can be tried to gain time for the treatment of the primary disease. Once unconsciousness, hemodynamic instability, and hypoxemia are difficult to correct, invasive mechanical ventilation should be given promptly and decisively.

3.2.3 Invasive ventilation: Due to the different etiology, the severity of the disease and the course of the disease, each ALI / ARDS patient should be treated individually. General treatment principles: Protective ventilation, namely tidal volume of 6ml / kg (ideal weight), plateau pressure (Pplat) <30cmH2O, and appropriate PEEP, are currently proven measures to reduce mortality in mechanical ventilation.

Small tidal volume: Due to alveolar collapse in ALI / ARDS patients, the lung volume is significantly reduced. Conventional tidal volume ventilation can cause lung injury. Therefore, small tidal volume ventilation is recommended. 6ml / kg is recommended. In addition, the platform pressure (Pplat) should be less than 30cmH2O. The plateau pressure can reflect the alveolar pressure, the airway plateau pressure rises, and the mortality rate increases accordingly. The result of low tidal volume ventilation is insufficient ventilation, carbon dioxide retention, and hypercapnia. Acute carbon dioxide elevation leads to a decrease in blood pH, but studies have confirmed that a certain degree of hypercapnia is safe when implementing lung protective ventilation strategies, and it is recommended that the pH is> 7.20-7.25.

When patients have hypoxemia that is difficult to correct, PEEP, manual lung retension, and prone ventilation can be given, but clinical studies suggest that none of these measures can reduce mortality.

PEEP: PEEP is an indispensable part of ALI / ARDS treatment, and its role is to restore collapsed alveoli, reduce intrapulmonary shunt, and significantly improve oxygenation. Side effects are hypotension, aggravating excessive expansion of the alveoli, leading to pneumothorax and the like. Many large-scale clinical studies have confirmed that the preventive administration of PEEP cannot prevent the occurrence of acute lung injury. How much PEEP is given is the most suitable for patients, and it is still worth exploring. A clinical study randomly divided patients into a larger PEEP (12-24cmH2O) group and a smaller PEEP (5-24cmH2O) group. The oxygenation of the high PEEP group was improved, as shown by the increased oxygenation index. There were no differences in mortality, mechanical ventilation time, or number of extrapulmonary organ failures.

There are currently two methods for giving PEEP: inflection point pressure under the static PV curve + 2cmH2O, using the FiO2-PEEP correlation table (Table 2), so that the patient's blood gas analysis PaO2> 60mmHg, FiO2 <0.6, the minimum PEEP used.

Table 2 FiO2-PEEP association table

FiO2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

PEEP 5 5-8 8-10 10 10-14 14 14-18 18-22

Manipulative maneuvering: Different causes of ALI / ARDS have different responses to lung maneuvering methods. It is generally believed that lung exogenous ALI / ARDS responds better than lung endogenous ALI / ARDS. The course of ALI / ARDS also affects the effect of lung maneuver, and the effect of early ALI / ARDS maneuver is better. At the same time due to the existence of pneumothorax, hypotension and other side effects, it should be performed under the guidance of an experienced physician.

Prone position ventilation: Prone position ventilation improves oxygenation, but this improvement is often transient. Prone position ventilation is generally not recommended for ALI patients. Only patients with severe ALI / ARDS who are ineffective with conventional mechanical ventilation can be considered for contraindications.

(3) Sedation and retention of spontaneous breathing: Patients with mechanical ventilation should consider the use of sedative analgesics to relieve anxiety, restlessness, pain, and reduce excessive oxygen consumption. Proper sedation and proper analgesia are the basic links to ensure the safety and comfort of patients. When sedatives are applied during mechanical ventilation, a sedation plan should be developed first, and daily wake-ups should be implemented. Long-term use of muscle relaxants should be avoided to reduce the occurrence of diaphragmatic dysfunction and VAP. In the case of stable circulation function and good man-machine coordination, it is necessary to retain spontaneous breathing during mechanical ventilation in ALI / ARDS patients.

Offline and body position: ALI / ARDS patients can be placed in a semi-recumbent position, which can reduce ventilator-related infections, reduce the chance of reflux aspiration, and increase diaphragm function. About 2/3 of the mechanical ventilation time of patients with ALI / ARDS mechanical ventilation is taken offline. Once the patient's cause of ALI / ARDS is removed, PEEP <8cmH2O and FiO2 <50%, stable hemodynamics and spontaneous breathing , You can enter the programmatic offline.

4.3 Fluid management: Increased capillary permeability and increased pulmonary water are the pathophysiological manifestations of ALI / ARDS. Animal experiments have confirmed that reducing lung water helps improve oxygenation and lung compliance. A study of different ALI / ARDS fluid management strategies completed by ARDSnet in 2006 showed no significant mortality in the restricted and unrestricted fluid management groups The patients in the restricted fluid management group had significantly improved oxygenation index, significantly reduced lung injury scores, and significantly shorter ICU hospital stay. Hypoproteinemia often exists in critically ill patients, and plasma colloid osmotic pressure is involved in the mechanism of edema. Therefore, for severe hypoproteinemia (ALB <30g / L), albumin combined with diuretics should be used to ensure negative liquid balance. Sometimes ALI / ARDS is often part of severe septic shock. If the patient is in shock, fluid resuscitation should be performed early. Once circulation is stable and tissue perfusion is good, fluid management strategies should be implemented.

4.4 Glucocorticoids: A large number of clinical studies have confirmed that glucocorticoids can neither prevent the occurrence of ALI nor have a therapeutic effect on early ALI. The ARDSnet study observes the therapeutic effect of glucocorticoids on advanced ARDS (7-24 d), and the results show that glucocorticoid treatment does not reduce the 60-day mortality rate, but can improve hypoxemia and pulmonary compliance, and shorten the Shock duration and mechanical ventilation time, but subgroup analysis showed that application of glucocorticoids with ARDS> 14 days significantly increased mortality. Current research shows that glucocorticoid therapy should not be used routinely for ALI patients. However, for patients with ALI due to allergies, early empiric therapy with glucocorticoids may be effective. In addition, in patients with septic shock complicated by ALI, if adrenal insufficiency is combined, an alternative dose of glucocorticoid may be considered.

4.5 Other treatments: At present, new treatments continue to appear in the clinic. Some are to suppress inflammatory factors and reduce inflammatory responses, some are to improve oxygenation, correct hypoxia, and some are to promote the absorption of fluid in the lungs. These methods include drugs and methods such as ketoconazole, pentoxifylline, risophylline, antioxidants, colony cell stimulating factors, surfactants, liquid ventilation, nitric oxide (NO), activated protein C, etc., which need to be further studied The research was conducted in large clinical trials to verify.