Critical Care in the ED: Mechanical Ventilation, Sepsis, Neurological Hypertensive Emergencies, and Pressors in Shock
The tremendous overlap between the specialties of emergency medicine and critical care medicine is particularly apparent in the initial resuscitation of critically ill patients—a vulnerable population in which the early period of care has significant impact on outcomes.
Emergency medicine and critical care medicine share a responsibility for the care of acutely ill patients with life-threatening pathologies. The skills required of both emergency physicians (EPs) and critical care specialists to recognize, diagnose, and resuscitate such patients have resulted in many shared guidelines, recommendations, and publications. When critically ill patients enter the hospital through the ED, the care provided by EPs greatly impacts both the early and long-term outcomes. It is not uncommon for critically ill patients to spend several hours under the care of an EP while awaiting an available inpatient bed in the intensive care unit (ICU) or “step down” monitored unit.
This article provides a summary review of current guidelines, evidence-based medicine recommendations, and the results of recent trials involving ventilator management, treatment of sepsis, management of hypertension accompanying neurological emergencies, and the selection of pressors for the treatment of different shock states.
Ventilator Management
,Mechanical ventilation is frequently undertaken in the ED for patients with respiratory failure—the origin of which is not always immediately clear. Data from the National Heart, Lung, and Blood Institute’s (NHLBI) acute respiratory distress syndrome (ARDS) clinical network (https://www.ardsnet.org) and other clinical trials have established the benefit of low tidal-volume, “lung-protective” ventilation in the patient with ARDS.1,2 Numerous studies have also shown the benefit of low-tidal-volume (TV), ventilation in patients without ARDS, and its use is now the standard of care for a large range of respiratory conditions causing compromise.3
The prompt initiation of lung-protective ventilation has a significant impact on reducing ICU mortality.4 A recent retrospective review of 3.5 million ED visits showed the median length of stay for patients started on mechanical ventilation in the ED to be greater than 3 hours.5 Such a length of time on mechanical ventilation in this setting can have significant effects on the course of illness; however, it is not clear whether mechanical ventilation performed in the ED typically conforms to evidence-based standards. In one study performed in an academic center, less than one-third of patients with sepsis and respiratory failure received low-volume ventilation in the ED.6 Another study suggested that emergency medicine residents may not receive as much dedicated education on the initial management of ventilators as needed—despite the potentially unforgiving physiologic process of positive-pressure mechanical ventilation.7
The fundamental principles required to safely manage most patients in respiratory failure are not difficult to master. There are several simple evidence-based ventilator strategies for managing patients with respiratory failure. The three primary principles of initiating and providing effective mechanical ventilation are: (1) avoiding traumatic ventilation; (2) maintaining normoxia; and (3) maintaining appropriate acid-base balance. Each of these principles can be achieved in a stepwise fashion.
Step I: Establishing Lung-Protective Settings on the Ventilator
Three central parameters must be selected at the initiation of assist-control mechanical ventilation: TV, respiratory rate (RR), and positive end-expiratory pressure (PEEP). These parameters have been extensively studied, and there is excellent evidence to guide the EP in choosing the correct settings.
Tidal Volume. Although the normal human lung can accommodate about 6 L of air, in cases of respiratory failure, the surface area available for gas exchange is significantly reduced due to a pathologic process undermining entire regions of the air-blood interface. Consequently, a person whose normal lungs are suddenly required to perform the life-sustaining gas exchanges in critical illness with the much smaller lung surface is at a significant disadvantage.
The widely accepted lung-protective volumes range from 6 to 8 mL/kg of predicted body weight (PBW), a height-based calculation.8 For example, in a 6-foot tall man, 6 mL/kg of PBW amounts to a TV of 466 mL; in a 5-foot tall woman, the same amount of PBW amounts to a TV of 273 mL. Volumes may be referenced using PBW tables from the NHLBI ARDS network or by employing the following equations:
Adult men: PBW (kg) = 50 + 2.3 (height [in] – 60)
Adult women: PBW (kg) = 45.5 + 2.3 (height [in] – 60).9
Respiratory Rate. The RR should be set somewhat higher than normal because the TV per breath has been slightly reduced, and also because sick patients in a catabolic state may have larger minute ventilations than they would when healthy. As previously described, since the TV is restricted, RR is the most mobile parameter in maintaining appropriate minute ventilation. Minute ventilation (MV) is the product of RR multiplied by TV (MV = RR x TV), and this should be calculated to approximate the patient’s own efforts, which are dependent upon the clinical circumstances. For example, patients whose bodies are trying to compensate for an acidosis will require much higher rates than those who are simply obtunded and intubated for airway protection. In other words, in order to remove carbon dioxide (CO2) in an acidemic patient, a higher RR rate may be required, whereas a lower rate may be selected to compensate for alkalemia while maintaining appropriate oxygen (O2) levels in both cases.10
