Page 632 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 52: Acute Lung Injury and the Acute Respiratory Distress Syndrome 451

TABLE 52-1 American European Consensus Conference Criteria for Acute Lung EPIDEMIOLOGY
Injury (ALI) and the Acute Respiratory Distress Syndrome (ARDS) ■
Clinical Variable Criteria for ALI Criteria for ARDS INCIDENCE
Inconsistent definitions for ALI and ARDS in large databases of diagnoses
Onset Acute Acute for hospital admissions or complications, variations in the application of
Hypoxemia Pa O 2 /Fi O 2 ≤300 mm Hg Pa O 2 /Fi O 2 ≤200 mm Hg the AECC definition in different studies, and variations in different at-risk
Chest radiograph Bilateral infiltrates consistent with Bilateral infiltrates consistent populations (such as sepsis, trauma, surgeries, and bone marrow trans-
pulmonary edema with pulmonary edema plants) previously hampered obtaining accurate estimates of the incidence
of ALI and ARDS. More recent studies have used data from the ARDSNet
Noncardiac cause No clinical evidence of left atrial No clinical evidence of left atrial clinical trials and regional cohort studies to arrive at estimates in the United
hypertension or, if measured, hypertension or, if measured, States in the range of 25 to 79 cases per 100,000 person-years. 1,2,25,26 A recent
pulmonary artery occlusion pulmonary artery occlusion epidemiologic study based out of Olmstead County, Minnesota, found
pressure ≤18 mm Hg pressure ≤18 mm Hg
that ARDS incidence decreased from 82 to 39 per 100,000 person-years
Reproduced with permission from Bernard GR, Reines HD, Brigham KL, et al: The American European consensus between 2001 and 2008. The authors hypothesized that improvements in
26
conference on ARDS: definitions, mechanisms, relevant outcomes and clinical trials coordination. Am J Resp Crit Care health care delivery (eg, use of lower tidal volumes and less blood transfu-
Med. 1994 Mar;149(3 Pt 1):818-824. This document was published in 1994. Certain aspects of this document may sions in at-risk patients) contributed to the reduced incidence and efforts
26
be out of date and caution should be used when applying the information in clinical practice and other usages. are underway to determine whether a bundle of ALI prevention strategies
can reduce the risk of ALI development in at-risk patients.
■ VALIDITY AND RELIABILITY OF AECC DEFINITIONS Based on these estimates of incidence, the number of patients with
Despite standardization, these definitions have not been formally vali- ALI and ARDS in the United States each year is estimated to be between
53,000 and 190,000. If one assumes a short-term mortality of ∼30% to
dated beyond their face validity. In addition, problems remain with the 40%, the annual number of deaths in the United States directly attribut-
reliability of the various components of the definitions. For example, able to ALI and ARDS is between 16,000 and 74,500. These estimates
1
interpretation of chest radiographs can be inaccurate and variable of attributable mortality are greater than from the acquired immune
among different observers. 18,19 However, formal training sessions can deficiency syndrome (AIDS), asthma, or cervical cancer. 1,2
19 criterion also suffers These estimates also highlight that more than 100,000 patients will sur-
improve this variability. Likewise, the Pa O 2 : Fi O 2
from variability since it can be influenced by the level of PEEP used vive ALI each year. As described in Chap. 15, many of these ALI survivors
in mechanical ventilation and other transient factors, such as airway will suffer from long-term physical and neuropsychological impairment,
secretions or inadequate sedation. For example, higher PEEP generally decreased quality of life, and their long-term mortality is affected. 27-31
Collectively, the incidence, case-fatality, and long-term consequences
increases Pa O 2 at a given Fi O 2 . This in turn may raise a patient’s Pa O 2 : Fi O 2
>300 so that the patient no longer meets the inclusion criteria for ALI. demonstrate the important public health impact of ALI and ARDS.
<300 may reflect the
Conversely, in the absence of PEEP a Pa O 2 : Fi O 2 ■
presence of simple basilar atelectasis rather than ALI or ARDS. In such a PRECIPITATING CAUSES

case, adding PEEP may recruit enough atelectatic lung to raise Pa O 2 : Fi O 2 ALI and ARDS can be considered to be a “final common pathway” reac-
>300, so that the patient no longer meets this criterion for ALI. 20 tion of the lung to a large variety of precipitating causes. Some authors have
Finally, exclusion of congestive heart failure (left atrial hypertension) classified these causes as representing direct (pulmonary) or indirect (extra-
also presents problems with reliability. Diagnostic criteria for left atrial pulmonary or systemic) injury to the lung 32,33 (Table 52-2). Although the
hypertension on purely clinical grounds can be inaccurate. Insertion methodology for applying these labels is not standardized and thus may
1,21
of a pulmonary artery catheter for this purpose may also be inconclu- be inconsistent between studies, this construct may have pathogenetic and
sive since the pulmonary capillary wedge pressure (Ppw) may be higher pathophysiologic underpinnings since indirect causes may have different
than 18 mm Hg due to intravascular volume loading (eg, in patients mechanisms of injury to the lung compared to direct causes. 34
with shock), rather than due to congestive heart failure. In a recent Not all patients with these precipitating conditions develop ALI/
21
clinical trial, 29% of the ALI subjects enrolled had a Ppw in excess ARDS. Indeed, the frequency of ALI/ARDS is quite variable. Depending
of 18 mm Hg; importantly, only 3% of these subjects had a depressed on the precipitating cause, and the population studied, it ranges from
cardiac index. Conversely, many patients with hydrostatic pulmonary ∼5% to 40%. 32-37 If patients have more than one of these precipitating
22
edema due to congestive heart failure and high left atrial pressures may
have normal pulmonary artery occlusion pressures by the time the cath-
eter is inserted and measurements taken. Together, the evidence dem-
23
onstrates that a hydrostatic pressure gradient is present in many ALI TABLE 52-2 Examples of Direct and Indirect Precipitating Causes of Acute Lung
patients, which contributes to the development of pulmonary edema. Injury (ALI) and the Acute Respiratory Distress Syndrome (ARDS)
The need for further refinement of the reliability and validity of defini- a
tions of ALI and ARDS prompted an international expert panel endorsed Direct Precipitating Cause Indirect Precipitating Cause
by multiple societies to convene in 2011 to revise the definition. The results Aspiration of gastric contents Acute pancreatitis
of the revised, consensus draft definition for ARDS, known as “The Berlin Bacterial pneumonia (eg, Legionnaire disease) Blood product transfusions with
Definition,” were the following: (1) the radiographic elements of the AECC transfusion-related acute lung injury (TRALI)
definition were retained, (2) the timing of ARDS was clarified to occur
within 1 week of an acute insult, (3) the Ppw criteria was removed and, Chest trauma with lung contusion Post-cardiopulmonary bypass
acknowledging that left atrial hypertension may coexist with ARDS, it Near-drowning Primary graft failure of lung transplantation
was stipulated that the etiology of pulmonary edema not be due to hydro- Pneumonia due to Pneumocystis jiroveci Severe sepsis and septic shock
static pulmonary edema exclusively, and (4) the terminology ALI was
removed and ARDS was categorized into one of three mutually exclusive Toxic inhalations (eg, smoke inhalation, Toxic ingestions (eg, aspirin, tricyclic
≤ 300 with inhaled crack cocaine [“crack” lung]) antidepressants)
categories of hypoxemia severity, being mild (200 < Pa O 2 : Fi O 2
≤ 200), and severe Viral pneumonia (eg, influenza, the severe Trauma with multiple fractures and
PEEP at a minimum of 5), moderate (100 < Pa O 2 : Fi O 2
≤ 100). In the remaining chapter, we use the traditional ALI acute respiratory syndrome [SARS]) fat-emboli syndrome
24
(Pa O 2 : Fi O 2
and ARDS terminology as the evidence which is presented is based pre- a In indirect or systemic mechanisms of lung injury, the lung injury results from deleterious effects on the
dominantly on the AECC criteria. alveolar capillary endothelium by inflammatory or other mediators delivered via the pulmonary circulation.

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