Gordon R. Bernard, in "Acute Respiratory Distress Syndrome: A Historical Perspective," July 14, 2005, in Respiratory and Critical Care Medicine, wrote that ARDS was actually described in ancient writings, although it didn't gain national attention until the ventilator was invented in the 1930s.
Since then there has been much wisdom learned about the syndrome, and much progress made in the care of patients diagnosed with it.
Other names for ARDS include noncardiogenic pulmonary edema, shock lung, white lung syndrome (due to whited out x-ray), hemorrhagic atelectasis, capillary leak syndrome, post-traumatic pulmonary insufficiency (often results after trauma), and wet lung syndrome.
According to emedicine.medscapes.com, "Adult respiratory Distress Syndrome," ARDS was actually used in a 1967 report describing the patients with sepsis, blood transfusions, and diffuse lung infiltrates who suffered from respiratory failure hours after the initial insult.
However, it wasn't until 1994 that the American-European Consensus Conference developed a clear definition of ARDS so that its pathogenesis and treatment could be further studied, and "adult" was removed from the term and "Acute" was added because it was learned the "syndrome" occurs in both adults and children.
The definition of ARDS is "an acute condition characterized by bilateral pulmonary infiltrates and severe hypoxemia in the absence of evidence for cardiogenic pulmonary edema." Or, more simply put, ARDS is pulmonary edema not caused by a failing heart. (To learn more of how to differentiate between ARDS and heart failure, click here)
Generally speaking, ARDS is the reaction of the lungs to some form of injury, and generally occurs to patients who are already hospitalized. Those who are at greatest risk, therefore, would be any one of the following conditions:
- Aspiration of stomach contents (damages alveolar/capillary membrane)
- Sepsis or shock (any cause)
- Blood transfusion
- Disseminated Intravascular Coagulation (DIC)
- Lung contusion (as in a trauma or personal injury accident to thoracic or non thoracic)
- Drug toxicity (overdose or toxic effects)
- Inhalation injury (oxygen toxicity, smoke inhalation, caustic chemicals)
- Near drowning
- Chemical inhalation
- Metabolic disorders (pancreatitis or uremia)
- Neurologic disorders (head trauma, brain tumor)
- High tidal volumes (this is why we now recommend lower volumes6-10cc/kg ideal body weight as opposed to 10-15cc kg ideal body weight that was taught in 1995)
Again, please note that ARDS, like Sepsis and DIC, do not occur spontaneously. There has to be one of the above occurring for some time for these syndromes to develop.
High volumes are believed to cause lung injury, according to "Ventilatory management of the Adult ARDS Patient," by Douglas S. Laher in AARC Times (June 2007), because healthy lung tissue is "inter-dispersed with that of damaged lung. Under these conditions, the healthy lung tissue receives a disproportionate amount of regional volume in the setting of high inflation pressures, thus causing lung injury to occur."If you have a patient with any of these conditions you must observe them closely for signs of pneumonia, sepsis, DIC and ARDS. They are all at risk. So preventative measures must be in place, you must be proactive, and you must anticipate the worst and be prepared to treat the patient accordingly. This is where order sets and ventilator and sepsis and pneuonia protocols come in handy. Extubate early and treat pneumonia and sepsis early and aggressively.
In 1967 when ARDS was first described mortality was near 95%, and a majority of those patients died due to respiratory failure. When I was in RT school in 1995 the mortality rate was 70% mainly due to measures to prevent the syndrome and improvements in treatment and improved ARDS wisdom. Now the mortality rate is less than 60% and the cause is usually due to organ failure.
So ARDS is often accompanied by multiple organ failure mainly due to lack of oxygen getting to these organs. Thus, ARDS has a high occurance of organ failure.
According to "Respiratory Disease," edited by Robert L. Wilkins and James R. Dexter, the greatest incidence of organ failure is kidney failure, which presents in 40-55% of cases, followed by heart failure in 10-23% of cases, and liver failure in 12-95% of cases, followed by gastrointestinal (7-30%) and central nervous system (7-30%)
According to the National Heart Lung and Blood Institute (nih.gov), "In ARDS, infections, injuries, or other conditions cause the lung's capillaries to leak more fluid than normal into the air sacs and interstitial spaces. This prevents the lungs from filling with air and moving enough oxygen into the bloodstream."
According to Wikipedia symptoms usually occur within 24 to 48 hours after initial injury or acute illness, so if you have any patients with the above conditions they merit close watch.
Symptoms to watch for include but are not limited to:
- Shortness of breath
- Symptoms of underlying cause, such as shock, pneumonia, etc.
- Decreased blood pressure (shock)
- Organ failure (due to lack of oxygen)
- Rales/ crackles (due to fluid in lungs, pulmonary edema)
- ABG = respiratory acidosis
- Chest x-ray shows bilateral infiltrates
- High oxygen levels over a period of greater than three hours
One criteria for the diagnosis of ARDS is if the patient requires greater than 50% oxygen and the PaO2 continues to be under 100, otherwise known as refractory hypoxemia. Usually these patients need to be intubated and placed on a ventilator.
Ventilating ARDS patients can often be a conundrum, and some techniques are still experimental.
"Respiratory Disease" describes the typical course of the disease to follow the same pattern:
- Initial injury
- Apparent respiratory stability (it occurs patient has no pulmonary abnormalities and lasts from 1 to 24 hours
- Respiratory deterioration (dyspnea, tachypnea, tachycardia, cough are present and x-ray may appear normal and ABG reveal uncompensated respiratory acidosis with moderate hypoxemia with an increased P(A-a)O2
- Terminal stage (Increased fluid in interstitial spaces makes breathing severely difficult. Symptoms at this stage include tachypnea, labored breathing and cyanosis, inspiratory crackles, severe hypoxia and respiratory acidosis. Severe hypoxemia can lead to anaerobic metabolism and ultimately organ failure and death if not treated)
The early stage of ARDS is generally exudative, which, according to dictionary.com, is a discharge of fluid from the blood to the tissues. This stage lasts for up to a week.
According to emedicine.medscapes.com, some injury occurs (a precipitating event) that causes diffuse alveolar damage (DAD) and lung capillary endothelial damage.
DAD is characterized by:
- Damage to the lining of the alveolis (as in aspiration) or the capilary lining (as in sepsis)
- This causes the alveolar or capillary lining to swell (inflammation)
- The gap between the capillary and alveoli to widen
- Widespread damage to type I cells (pneumocytes),
- Fluid to leak from the capillary to the alveoli
- Causing alveolar and interstitial pulmonary edema (fluid in the lungs).
- Hyaline membranes are formed
Emedicine notes that there are two types of cells in the lining of the lungs (epithelium), which are your type I and type II cells or pneumocytes. Ninety percent are of the type I variety, and these are the most easily damaged resulting in leakage in the first stage of ARDS.
Type II are more resistant to injury, yet when they are injured this can lead to the decreased production of surfactant, which is the soap like substance in the lungs that makes it easy for the alveoli to open up. Thus, with less surfactant, the alveoli don't open easily, and this leads to increased atelectasis (collapsed alveoli) and decreased pulmonary compliance.
The later stages of ARDS is also referred to as the fibroproliferative phase, which is a complicated way of saying pulmonary fibrosis. Anything that interferes with the repair process here may lead to fibrosis of the lungs, and even further decrease in compliance.
Many patients survive the initial stages of ARDS only to succumb to the later stages. Yet the later stages also often result in permanent remodeling of the pulmonary vasculature which further complicate things.
Collapsed alveoli (atelectasis), and stiffened alveolar and capillary membranes, and fluid in the lungs, all result in areas in the lung that are ventilating but not perfusing from the alveoli to the capillary, and this is known as a shunt. This results in low oxygen in the blood (hypoxemia) that is not responsive to increased oxygen, which explains why one way to diagnose ARDS is hypoxemic hypoxia.
Hypoxemia then results in hypoxia (lack of oxygen to the tissues), which can lead to sepsis and eventually organ failure as mentioned above.
It's actually a lot more complicated than I describe here, yet I'm trying to dumb it down for simplicity sakes. After the initial injury, pro inflammatory cells (such as cytokines, leukotrines, etc) are released and anti-inflammatory cells are inhibited. This leads to inflammation that results in leaky alveolar/capillary membranes.
Due to stiff membranes, too much tidal volume can easily result in barotrauma, air in pleural spaces, and worsening ARDS, and therefore studies have shown that ARDS is most responsive to lower tidal volumes. You'll generaly want to use volumes at the lowest end of your scale (like 6cc/ kg ideal body weight as opposed to 10cc/kg ideal body weight).
(This is significant, because when I was in RT school the recommended tidal volumes were 10-15cc/kg ideal body weight for patients on a ventilator)
Likewise, the overexpansion of alveoli, plus the force to reopen them, may result in what is called volutrauma, according to emedicine.medscapes.com. This triggers the release of even more pro-inflammatory cytokines and increases the inflammation and edema of the lungs even more.
Treatment includes mechanical ventilation and PEEP therapy. PEEP (positive end expiratory pressure) prevents the alveoli from collapsing all the way and lower tidal volumes. Laher notes that a low tidal volume strategy is "thought to reduce parenchymal lung injury by limiting 'stretching' of the lungs that takes place during mechanical ventilation, in which peak inspiratory pressures are routinely found to be between 30 and 35 cm H2o and static pressures in excess of 30 cm H2o."
However, he notes it is not the low tidal volumes that protects the lung, "but rather the decrease in ventilating pressures as a result of the lower tidal volumes." So the goal of mechanical ventilation is to maintain a plateau pressure less than or equal to 30, and a mean airway pressure between 20-25 cm H2o.
(It should be noted here that varying pressures from an Ambu-bag have been proven to bruise the lungs and therefore cuase Hyline Membrane Disease in neonates, and this is why the NeoPuff is recommended. One would have to wonder if varying pressures in adults might cause ARDS. Just a thought here).
Laher adds that using the lower tidal volume strategy has actually reduced mortality by 25% and ventilator lengh of stay by 2 days for the ARDS patients.
Usually higher amounts of oxygen are needed to maintain an adequate oxygen level to maintain life. The disadvantage to using high levels of oxygen is that after three hours (see this post for more) on a greater than 60% FiO2 (fraction of inspired oxygen) may lead to DAD. So, it's kind of a damned if you do damned if you don't kind of thing.
Too much oxygen causes the release of oxygen free radicals and oxidative stresses which may result in DAD, and is called oxygen toxicity. It was historically believed that high levels of oxygen for a period of days causes oxygen toxicity, yet new research shows this effect may actually start in a short of a period of time as just three hours.
Another problem with ARDS is vasoconstriction. This further increases shunting (areas where oxygen doesn't reach the blood) and ventilation/ perfusion mismatching (areas not ventilated). It also causes pulmonary hypertension, which means the right ventricle of the heart has to work overtime to pump blood through the lungs, and this can often lead to heart failure.
If diagnosed promptly, and treated aggressively, and if the patient does not progress to the secondary phase where fibrosis develops, the ARDS may resolve completely. Yet if fibrosis occurs, mortality and morbidity is increased.
- ABG to determine oxygenation status and respiratory acidosis
- Complete Blood Count and chemistry profile are usually abnormal due to stress on body
- Lactic acid to monitor for sepsis
- EKG to monitor cardiac function
- X-ray: After 24 hours of injury patchy bilateral infiltrates in both lungs that may ultimately appear as a whiteout, and no cardiomyopathy.
- PaO2/FiO2 less than 200 (does not improve with increased oxygen)
- PAO2 - PaO2 of greater than 300
- Static compliance (VT/Static pressure – PEEP) less than 25
- Hypoxic Hypoxemia (PaO2 less than 100 on greater than 60% FiO2)
- Pulmonary Capilary Wedge Pressure less than 18 (will rule out cardiogenic pulmonary edema or heart failure as the cause of pulmonary edema)
Initial treatment should focus on treating the underlying condition and to work to prevent infection and ARDS. If a patient needs respiratory support, noninvasive procedures should be trialed before intubation. If intubation is required, studies show that 73% of patient intubated nasally end up with VAP (Ventilator acquired pneumonia) as opposed to only 34% orally intubated. So oral intubations are preferred. (study noted at medscapes)
The first order of business is to treat the initial or underlying condition, such as by treating pneumonia or sepsis with an appropriate antibiotic, or by treating hypotension (shock) with vasopressors to improve cardiac function.
Other than treating the underlying condition, treatment generally involves:
1. Mechanical ventilation: Usually the increased work of breathing associated with ARDS is not compatible with life, and for this reason mechanical ventilation is usually required. This will not treat the ARDS but will allow the patient's lungs to rest, buying time for medical clinicians to fix the patient's lungs and underlying condition. Since the lungs are more compliant with ARDS (static compliance low), higher pressures will be needed to ventilate. Yet since higher pressures and tidal volumes are associated with worsening outcomes (see study results at nih.gov), it is important to ventilate with lower pressures (6-10cc/kg ideal body weight and preferably the lower side) and to try to maintain a static pressure of less than 30. The ultimate goal is to adequately oxygenate and ventilate the patient until the ARDS and underlying condition is improved, at which time the patient is to be weaned off the ventilator.
2. PEEP: This reopens alveoli that have collapsed and helps to maintain a pressure in them so they stay open, and this converts areas of shunts to areas where gases can now be exchanged, and this results in improved oxygenation (improved SpO2 and SaO2). Recruitment of alveoli also increases Functional Residual Capacity (FRC) and pulmonary compliance. The goal of PEEP is to maintain a PaO2 of 60 or greater with less than 60% FiO2. The best PEEP is the highest PEEP available that does not result in a drop in SpO2 and blood pressure (which monitors cardiac output). PEEP should then be weaned until FiO2 is 40%, at which time PEEP should be weaned to normal physiological PEEP of 3-5cwp. (see guidelines for adjusting ventilator settings here).
3. Oxygen therapy: This will be required to prevent hypoxemia and to make sure tissues continue to get an adequate supply of oxygen to prevent sepsis and organ failure. Without mechanical ventilation, usually an FiO2 of 75-100% is required. A nonrebreather will provide the patient with 75% (or only 60% according to new studies)and BiPap and mechanical ventilator up to 100% FiO2. Generally high FiO2s are required. FiO2 should be weaned before PEEP as higher FiO2s over long periods of time are associated with causing lung damage. FiO2 should be lowered to 40% before PEEP is lowered. One of the first goals once a patient is on a ventilator is to start weaning FiO2, and all ventilator, weaning or extubation protocols should account for this. The goal of oxygen therapy therefore is to maintain an SpO2 of 88% and a PaO2 of 90.
4. BiPAP: Noninvasive ventilation is becoming more and more popular for patient comfort and to decrease risk of high pressures, high tidal volumes and nosocomial pneumonia. This can usually be trialed in the early stages to improve patient compliance, FRC and oxygenation, although in many cases the patient will eventually require intubation and mechanical intubation. This is much less invasive, although if the patient truly has ARDS a full face mask will be required, and the patient will not be able to take the mask off without causing a sudden drop in tissue oxygenation which will be evident by a drop in SpO2.
5. Volume Ventilation: What mode works best for ARDS patients is generally up to the discretion of the person or facility caring for the patient. Some trials have been done at (which you can see here) that show some form of volume control is still the best mode because it assures the patient will not receive too much volume. Where I work we have Servo 300A ventilators which have PRVC mode that allows us to control volume while making sure the lowest pressure is used. This ventilator also has volume support mode which can automatically be used when the patient starts breathing spontaneously. Many hospitals, however, are using the new APRV mode which is similar.
6. Pressure Ventilation: Some doctors like to trial ARDS patients on pressure control modes to guarantee a certain the plateau pressure does not exceed a certain (usually 30 cwp) pressure. However, the RT will not have control over tidal volumes. This is basically a low tidal volume high PEEP strategy. This allows for higher PEEP to be used. Most hospitals now use an APRV type modes, although there really are no studies showing one mode is better than another.
7. Inverse I:E ratios: Occasionally you'll see a physician trial the patient in a reverse inspiratory to expiratory (I:E) ratio, although when this occurs the patient will be uncomfortable, and sedatives and perhaps even paralytics may be necessary. This is generally only done when volume control or pressure control modes with PEEP and oxygenation fail to improve patient outcomes. This is where the the breath is triggered as soon as PEEP is reached and before full inspiration to prevent higher volumes.
8. High Frequency Jet Ventilation: According to Medscapes today, "Reducing Morbidity of Acute Respiratory Distress Syndrome," HFJV "provides adequate gas exchange while avoiding traumatic lung injury and end-expiration alveolar collapse seen with traditional ventilation modalities."
9. ECMO: Medscapes notes that "ECMO involves blood oxygenation outside the body through a veno-arterial or veno-venous access and is reserved for severe ARDS cases. Current survival rates associated with ECMO therapy have been as high as 80%."
10. Prone position: Atelectasis usually occurs in the bases, and by placing the patient on his stomach this allows recruitment of apical alveoli. This is believed to recruit alveoli and improve oxygenation in this way. However, studies have not proven this to be of benefit.
11. Antibiotics: Thse are used to treat any underlying bacterial infection. Studies have shown that patients with ARDS have a 60% chance of developing Ventilator Acquired Pneumonia (VAP), although with ventilator bundles many hospitals have seen this incidence decreased to as low as zero. In a sense, prevention is a good policy, and as soon as you suspect infection an appropriate antibiotic should be started. This is why many hospitals have initiated sepsis and extubation protocols to reduce the risk of sepsis, VAP, and ARDS by preventing them and/ or diagnosing and treating fast and aggressively based on best practice evidence.
12. Diuretics and steroids: These are sometimes useful to help remove pulmonary secretions and lower pulmonary blood pressure. Studies show that systemic corticosteroids help reduce inflammation in ARDS patients and improve outcomes.
13. Sedatives and paralytics: Used to decrease anxiety and reduce oxygen consumption. Propofol and midazolam are commonly used. Neuromuscular blockers such as atracurium or cisatracurium relax the patient to maximize clinical efforts to control their ventilation. If a patient is receiving a neuromuscular blocker they must be given a sedative, because they may feel pain and not be able to communicate. Plus they must receive artificial ventilation because you'll be knocking out their drive to breathe.
11. Surfactant replacement therapy: This has been proven to be beneficial to neonates in with IRDS, (infant RDS) however is not beneficial (according to studies) with adult ARDS.
12. Nutritional feedings: Oral gastric tubes have been proven to be beneficial to nasal gastric tubes because they reduce the rate of infection. Long term feedings should be by gastrostomy or jejunostomy tube placement. Appropriate nutrition is essential to assure patient has proper nutrients to speed time of recovery and extubation.
12. Other therapies are always being studies, including recent studies using nitric oxide, to help increase perfusion of better ventilated areas. Partial liquid ventilation is also a new treatment.
13. Permissive Hypercapnia: Low tidal volumes may cause the patient to have a low pH (less than 7.20) and a high CO2. Instead of increasing the tidal volume to help blow off Co2, physicians either increase respiratory rate, or simply allow the CO2 to stay high while the patient recovers, and until normal tidal volumes can be given. This is allowed when the benefits supercede the disadvantages of a high CO2.
14. Recruitment Meneuver: Increase PEP above the set tidal volume with the goal of achieving maximal physiologicac stretch in as many lung units as possible. Laher notes this is believed to "sustaine inflation at maximal stretching pressures of the lung (30-45 cm H2O) for up to one minute. Unlike PEEP however, RMs are designed to initially open (or recruit) the alveoli, where PEEP is the method of maintaining patency. These methods have been proven very effective in improving oxygenation, but just like PEEP, clinical data does not support using RMs as a means of improving outcomes or mortality." However, the risks must be measured against the risks.
The ARDS Clinical Network recomments the low tidal volume strategy with the following FiO2/PEEP combinations to optimize patient care:
- FiO2 30% set PEEP at 5
- FiO2 40% set PEEP at 5-8
- FiO2 50% set PEEP at 8-19
- FiO2 60% set PEEP at 10
- FiO2 70% set PEEP at 10
- FiO2 80% set PEEP at 14
- FiO2 90% set PEEP at 14-18
- Fio2 100% set PEEP at 20-24
It should be mentioned here that no medicine has proven to reduce length of stay on a ventilator for ARDS patients, so the best strategy is to focus on low pressures and high peep strategy as mentioned above.
To prevent malnutrition, a feeding tube may need to be put in place. The patient will need to be monitored for renal failure, cardiac arrhythmias, and other complications of ARDS and ventilator therapy.
So I'm sure as new wisdom is learned treatment for ARDS will be altered. Studies are always ongoing in this regard, especially considering ARDS is listed as the most critical of all the respiratory ailments.
To learn more, check out the ARDS Network at the National Heart Lung Blood Institute by clickind here. If you have more ARDS wisdom to add, please educate us in the comments below.