Critical Care Anesthesiology

History of Critical Care Medicine and the Role of Anesthesia Intensivists

Critical care medicine and the development of the first ICU is often credited to Bjorn Ibsen, a Danish anesthesiologist, in 1952. Dr. Ibsen, who was trained in anesthesia at Massachusetts General Hospital, was consulted to help in the care of a 12-year-old girl with poliomyelitis (polio) and respiratory failure. He instituted the approach of manual positive pressure ventilation via a tracheotomy and then applied this to dozens of patients throughout the course of the polio epidemic in Copenhagen. Equally important, Dr. Ibsen implemented the approach of caring for groups of patients with respiratory failure in a dedicated location, thus creating a version of the modern ICU. In addition to this well-cited contribution to modern critical care, the Johns Hopkins neurosurgeon, Walter Dandy, created an earlier version of the modern ICU in 1923, when he opened a three-bed unit for postoperative neurosurgery patients. Max Harry Weil, another key figure in modern critical care, opened a four-bed “shock unit” in 1958 at the Los Angeles County + University of Southern California (LA+USC) Medical Center.

Since the early ICUs, modern critical care has focused on continuous physiologic monitoring and the application of advanced life-sustaining therapies, including mechanical ventilation, extracorporeal membrane oxygenation (ECMO), and continuous renal replacement therapy. One astute observer and practitioner notes that the focus has often been on syndromes—sepsis, acute respiratory distress syndrome (ARDS), acute renal failure, and delirium—rather than specific diseases. The progress in care for these syndromes and other critical care challenges has often been exciting but has also required periodic major shifts in practice, most notably with regard to sedation, mechanical ventilation in ARDS, use of the pulmonary artery catheter (PAC), intensive insulin therapy, and perhaps also goal-directed therapy.

Anesthesiologists, dating back to Dr. Ibsen, have been central to the development of modern critical care, both with the advancement of clinical practice and the development of technology and tools essential to the practice of the specialty. George Gregory, a pediatric anesthesiologist and intensivist at the University of California, San Francisco (UCSF), and colleagues used continuous positive airway pressure (CPAP) in the treatment of neonatal idiopathic respiratory distress syndrome and demonstrated a dramatic improvement in survival. John Severinghaus, who was not an intensivist but certainly a major figure in the field of anesthesiology, was the key contributor to the development of the first blood gas electrode apparatus in the late 1950s. More recently, however, there appears to be a decline in the role of the critical care anesthesiologists in the United States. In 2001, several academic leaders from the United States and Europe authored an editorial raising alarm based on the following: less than 4% of U.S. anesthesiologists have a special certificate in critical care, only 12% of the members of the Society of Critical Care Medicine (SCCM) are anesthesiologists, and relatively few anesthesia critical care diplomates graduate each year from fellowships. By contrast, in Europe, anesthesia intensivists continued to maintain a central role in the field of critical care medicine. In the nearly two decades since this article, it is unclear if much has changed. The number of accredited fellowship programs in anesthesia critical care has declined, and the number of anesthesia critical care diplomates certified by the American Board of Anesthesiology remains low in comparison to other anesthesia subspecialties like pain medicine and pediatric anesthesiology. It is unclear why critical care is a less desirable career path but key factors likely include limited exposure to high quality critical care rotations, lack of faculty mentors, and more favorable financial incentives in anesthesia-based fields. Despite these challenges, the Society of Critical Care Anesthesiologists continues to play a primary advocacy, educational, and mentoring role and memberships have increased over the past several years.

The future roles of anesthesia intensivists in the United States are likely to expand due to the increasing complexity of perioperative medicine, coupled with an aging population in need for critical care. It is also interesting to note a recent proposal by several academic anesthesia leaders from both trauma and critical care of a new paradigm: acute care anesthesiology. Modeled in part on acute care surgery, this new pathway and fellowship would include training in prehospital and emergency care, trauma, and critical care. Specifics of such a training pathway remain unclear but there are compelling reasons to consider this approach as another means of enhancing the anesthesiologist’s role in resuscitation and emergency care.

Capacity, Utilization, and Cost

ICU bed utilization and cost vary considerably among developed countries. Recognizing that the definition of an ICU bed also varies considerably, the United States, Belgium, and Germany having greater than 20 ICU beds per 100,000 population whereas the United Kingdom, the Netherlands, France, and Spain have fewer than 10 ICU beds per 100,000 population. Similarly, there are also large variations in the volume and type of ICU admissions in developed countries. Part of this can be attributed to differences in bed availability, but it also reflects different approaches to admission and triage that have been well described. A recent large prospective study of noncardiac surgery in Europe demonstrated high mortality rates and significant variability in utilization of critical cares services. Perhaps most striking was the finding that among surgical patients who died, the overwhelming majority (73%) were never admitted to an ICU. Therefore, despite concerns about over-utilization of expensive resources in some countries, this study suggests an alarming under-utilization of essential critical care services.

In the United States, in particular, there is continued growth in ICU beds and cost, whereas occupancy has remained generally flat. In 2005, there were 93,955 ICU beds with occupancy rates of 68%. Critical care costs represented 13.4% of all hospital costs and 0.66% of gross domestic product. By 2010, critical care beds had increased to 103,900 with stable occupancy rates over the previous decade. The greatest growth occurred in neonatal ICU beds with similar, but much lower, growth rates in adult and pediatric ICU beds. Critical care costs also grew to $108 billion, which represented an increase relative to gross domestic product (now 0.72%). The type of growth in ICU beds is also worth noting, as described in a recent analysis of Medicare and Medicaid data. Over 15 years, more than 72% of total ICU bed growth occurred in teaching hospitals. Multivariate analysis also demonstrated that large hospitals with high occupancy and teaching hospitals with high occupancy were most associated with next-year growth in the number of ICU beds. The investigators concluded that this may represent a type of “de facto regionalization,” which may in turn lead to higher quality of care. What is more, they suggest their results also contradict the theory that ICU bed growth and supply represents a type of “demand elasticity,” whereby increasing supply drives demand without clear benefits to patient care and the potential to change practice patterns and increase costs.

Structure and Staffing Patterns in the Intensive Care Unit

Much like the number and use of ICU beds, there is considerable variability throughout the world in the structure and staffing of ICUs. North American ICUs, when compared to the rest of the world, are more likely to have an “open” structure and be stratified into medical and surgical units and are less likely to have 24/7 intensivist presence. ICUs are sometimes described as “open” or “closed;” however, a common research approach is to categorize staffing as “low intensity” (no intensivist or only elective intensivist consult) or “high intensity” (all care directed by intensivist or mandatory intensivist consult). Much attention over the past two decades has focused on the optimal ICU structure and staffing models: specialized versus mixed, “high intensity” versus “low intensity,” optimal physician and nursing staffing ratios, nighttime intensivist staffing, use of advanced practice providers (APPs), and best ICU team structure.

The data do not demonstrate a clear outcomes benefit to specialized units but do show increased mortality for patients “boarding” on a non-primary unit in a specialized system. Many studies, including two large systematic reviews, have investigated outcomes associated with the intensity of ICU staffing. The outcomes have been mixed but generally favor improved mortality and length of stay outcomes with a “high-intensity” model. Data are more limited with regard to provider-to-patient ratios, but a recent retrospective study in the United Kingdom did show a U-shaped relationship between patient-to-intensivist ratios, with an optimal ratio of 7.5. Optimal nursing ratios are also not well described but a large, multinational observational study demonstrated that nurse-to-patient ratios higher than 1.5 are associated with lower risk of in-hospital death. The Working Group on Quality Improvement of the European Society of Intensive Care Medicine (ESICM) recommends 8 to 12 beds as optimal size.

The use of nighttime intensivists is an active area of study, with most studies demonstrating a lack of mortality benefit. It is important to note, however, that there is evidence that nighttime intensivist staffing, in one study, was associated with reduced in-hospital mortality in ICUs with low-intensity daytime staffing. Furthermore, a before-and-after prospective study of 24/7 mandatory intensivist presence showed an association with reduced hospital length of stay and complication rate, as well as improved staff satisfaction and adherence to processes of care. Based on available data, some investigators and critical care organizations (CCOs) have concluded that nighttime intensivists are costly and without significant benefit, whereas others vigorously argue for the value of a 24/7 intensivist model in high complexity, high-volume ICUs, emphasizing that the benefits derive not just from intensivist presence but rather intensivist-led changes in the systematic delivery of care.

Much attention has focused on the best approach to team structure and function in the ICU. As discussed earlier, “open” or “low-intensity” ICUs use hospitalists or emergency medicine physicians to care for patients in the ICU. Many academic and community-based institutions use nurse practitioners (NPs) and physician assistants (PA)—often described as advanced practice providers (APPs)—as core members of the team, largely because of the limited availability of intensivists. Studies demonstrate equivalent mortality and length of stay outcomes when compared to teams that include both residents and pulmonary/critical care fellows. A larger, more recent retrospective study of cohort data from 29 ICUs in 22 hospitals examined the association between exposure to NP/PA ICU staffing and in-hospital mortality. While the patients in NP/PA-staffed ICUs had lower severity of illness and use of mechanical ventilation, the unadjusted and risk-adjusted mortality was similar in ICUs with and without NPs/PAs. In addition to these outcomes, there is evidence of positive perceptions of NPs/PAs from other ICU providers, including physicians, nurses, and respiratory therapists. Accessibility, communications skills, and knowledge of and adherence to guidelines were recognized as core strengths of the NPs and PAs.

In addition to the benefits of nonphysician providers, there is evidence supporting the benefits of multidisciplinary teams and interprofessional practice in the care of critically ill patients. A retrospective study of medical ICU patients in Pennsylvania hospitals demonstrated the greatest mortality benefit to high-intensity intensivist staffing and daily multidisciplinary rounds. A recent review of interprofessional practice describes the robust literature supporting the value of interprofessional practice in the ICU. Pharmacists, respiratory therapists, and physical and occupational therapists are essential providers in the daily care of critically ill patients and improve short-term and longer-term outcomes.

There are guidelines and expectations that address some of the previously discussed aspects of ICU structure. The Leapfrog Group, a major nonprofit “watchdog” organization, rates hospitals on many factors including ICU staffing. The highest Leapfrog rating with regard to the ICU requires all critical care patients to be managed or actively comanaged by a board certified intensivist. The SCCM’s Task Force on Models of Critical Care concludes that an “intensivist-led, high-intensity” team is an “integral” part of care delivery in the ICU. These guidelines do not make a recommendation for either direct management of all patients (i.e., “closed” model) or mandatory consultation by an intensivist. Similarly, the SCCM’s Admission, Discharge, and Triage Task Force makes a 1B recommendation for a high-intensity staffing model with “day-to-day” management either via a fully “closed” structure or with a mandatory intensivist model; the SCCM does not recommend 24-hour intensivist staffing if the ICU operates within a high-intensity structure.

Management and Quality Improvement

The quality of an ICU depends to a significant degree on the management structure, as well as the organizational approach to clinical practice and process, and quality improvement. Management structures vary but have typically included at least a medical director and head nurse or nurse manager as a basic leadership structure. Experts and critical care societies recognize the challenges and importance of selecting and developing skilled critical care leaders. One recent review highlights the need for a leader to maintain and foster “continuity, consistency, and communication” in both clinical practice and the organizational practice. Although this may seem straightforward, the critical care leader often operates in a demanding and complex environment and must have a strong understanding of the broader health system operations and finances. Equally important are strong leadership skills, especially the ability to listen and learn, adapt to changing clinical and organizational circumstances, and constantly serve as a “catalyst for change.”

Beyond development of individual leaders, organizations are increasingly focused on how to best integrate ICUs and their leaders within a critical care structure. The SCCM’s Academic Leaders in Critical Care Medicine Task Force highlights the growing number of academic medical centers now with CCOs. They describe a road map for integration of operations to focus on patient quality, safety, and value: first using a “horizontal” approach with all ICUs operating in a single system with defined responsibility and accountability for value-based care; and then a “vertical” approach using a population health model with care provided across the continuum for all critically ill patients during and after ICU care. The Task Force also presents a follow-up plan for integration of critical care services into a single organization that explicitly includes the academic missions of research, professional development, and education.

High quality clinical practice and appropriate use of expensive and oftentimes limited resources benefit from a clear set of administrative and clinical guidelines. The SCCM recently revised the Admission, Discharge, and Triage Guidelines, which provide useful guidance for institutions developing policies and protocols for patient flow and quality assurance. Experts and professional societies agree that specific protocols of care—applied to practices like central line insertion and management of mechanical ventilation, and broader challenges like the use of the ICU for potentially inappropriate care—can help provide high quality, high value care in a collaborative model. Checklists and bundles often form the foundation for standardization of care and function as a mechanism to implement evidence-based practices and guidelines. The SCCM’s ICU Liberation project, for example, provides the ABCDEF Bundle as a daily, standardized approach to the implementation of the guidelines for the Prevention and Management of Pain, Agitation/Sedation, Delirium, Immobility, and Sleep Disruption in adult ICU patients. Standardization of care is a necessary step, but quality and value also depend on a robust approach to process improvement. Each ICU or CCO must first define the quality and safety metrics to be measured. It is also important that ICUs and CCOs consider the entire scope of quality in critical care. Recent guidelines describing best practices for family-centered care in the ICU highlight the importance of including measures of family support in our definition of quality in critical care. Once defined, the organization’s quality efforts will largely be determined by its ability to collect, analyze, and disseminate data to key leaders and ICU providers and staff.

In summary, critical care is a relatively young medical specialty with a strong history of anesthesiologist involvement since its inception. The practice of critical care medicine continues to grow and evolve rapidly, in terms of medical advancement, practice structure, and integration into the broader landscape of population health. In the next sections we will discuss several of the central disease syndromes and therapeutic modalities that have come to define the modern practice of critical care medicine.

Acute Respiratory Distress Syndrome

ARDS is a final common state in acute lung injury characterized by noncardiogenic pulmonary edema, heterogeneous consolidation, decreased lung compliance, and severe hypoxemia. It is a syndrome that involves injury and increased epithelial-endothelial permeability of the alveoli. It can occur as a result of direct chemical injury, systemic inflammation including sepsis, trauma, or numerous other causes common in critically ill patients.

The 2012 Berlin definition of ARDS focused on clarifying the quantification of hypoxia, the acute timing, and the radiographic findings. Using the ratio of arterial oxygen tension to fractional inspired oxygen (PaO ₂ /FiO ₂ ), ARDS is graded mild if this ratio is 300 mm Hg or less, but greater than 200 mm Hg; moderate if 200 mm Hg or less, but greater than 100 mm Hg; and severe if 100 mm Hg or less. These measurements should be obtained with a positive end expiratory pressure (PEEP) of at least 5 cm H ₂ O. The onset must be within 7 days of the presumed inciting insult, and bilateral lung infiltrates must not be fully explainable by other causes including cardiogenic pulmonary edema, effusions, or lobar collapse ( Table 83.1 ).

“Baby lung”

Imaging studies have shown that ARDS involves heterogeneous lung consolidation and small regions of remaining aerated lung. As a result, the entire mechanical stress of ventilation is borne by the small remaining regions of aerated lung. This concept has been described as the “baby lung” by some authors, and ventilation strategies have focused on preventing mechanical trauma and optimizing aeration of the remaining functional “baby lung.”

Lung Protective Ventilation

The modern approach to mechanical ventilation in critically ill patients has been characterized by a focus on the prevention of ventilator-induced lung injury. In their landmark ARMA study of tidal volume, the ARDS Network group demonstrated significant decrease in mortality and ventilator days with the use of lung protective ventilation, 6 mL/kg of predicted body weight (PBW) with plateau pressures less than 30 cm H ₂ O compared to ventilation at 12 mL/kg of PBW. This lung protective, low-tidal-volume ventilation strategy has become standard of care for patients with ARDS and subsequent studies have associated improved outcomes even in patients without ARDS.

Permissive Hypercapnia

During low-tidal volume ventilation, hypercapnia often occurs. While the avoidance of ventilator-induced lung injury is beneficial, the exact degree and pleomorphic effects of hypercapnia are less clear. There have been mixed data on the effects of hypercapnia on pulmonary inflammation, cellular and immunologic function, and wound healing. Furthermore, hypercapnia may impair right ventricular function, exacerbate pulmonary hypertension, and predispose to cardiac arrhythmias. As a result, most critical care practitioners adopt a pragmatic approach specific to the patients—often described as “permissive hypercapnia”—targeting a pH greater than 7.25. Extracorporeal carbon dioxide removal devices have been in active development to facilitate the management of carbon dioxide clearance during lung injury and low-tidal volume ventilation. However, these invasive devices have yet to prove their clinical utility and effectiveness.

Ventilation Modes in Acute Respiratory Distress Syndrome

In the era of lung-protective ventilation, ventilation strategies have focused on preventing volutrauma, barotrauma, and atelectrauma. Volutrauma is injury to alveoli from excessive volume distention and is closely related to barotrauma from excessive force on the alveolar walls. Atelectrauma is related to the repeated collapse and reopening of alveoli.

Conventional modes of ventilation deliver controlled pressure or flow profiles to the airways at tidal volumes larger than dead space and at rates similar to natural respiration. Gas transport occurs due to cyclic bulk flow into and out of the alveoli. Between the traditional ventilation modes of pressure control and volume control, there has been no significant difference shown in ARDS as long as low tidal volumes are used.

Airway pressure release ventilation (APRV) is a mode of ventilation that holds a high constant inspiratory pressure with periodic brief releases to a lower pressure, with spontaneous respiratory activity superimposed on this pattern. The proposed benefits are increased aeration and reduced cycling of alveolar collapse. A review of observational studies in trauma patients has suggested that early use of APRV may reduce the incidence of ARDS; however, more rigorous studies will be needed to evaluate this hypothesis. Given the risks and unproven clinical benefits of this ventilator mode, its use remains controversial.

High-frequency oscillatory ventilation (HFOV) is another less conventional mode of ventilation that requires a special oscillator pump. HFOV delivers very low volumes, often less than 100 mL, at very high rates, typically hundreds of breaths per minute. In HFOV, because tidal volumes are less than dead space, gas transfer does not depend on bulk flow but on other mechanisms, such as “pendelluft” (swinging air) effect and enhanced diffusion. Whereas initial studies demonstrated improved oxygenation, a subsequent larger clinical trial demonstrated no benefit or increased mortality in patients with ARDS. As a result, HFOV is not recommended, except as a rescue method in refractory hypoxia.

Positive End-Expiratory Pressure and Open Lung Strategies

ARDS is characterized by extensive alveolar collapse. Repeated opening and closing of alveoli during ventilation results in atelectrauma to the lungs. The lung compliance curve is characterized by hysteresis, meaning a higher driving pressure is required to inflate the lungs during inspiration, as compared to lower driving pressure required to keep the lung open during expiration. As a result, the use of PEEP and more generally “open lung” strategies have received significant attention.

Positive End-Expiratory Pressure

PEEP is a continuous pressure held in the airways even on expiration. Increasing PEEP improves oxygenation by increasing functional residual capacity and preventing alveolar collapse on expiration, thus maintaining alveolar recruitment. Excessive PEEP can result in regional or global lung over-distention or exacerbate air trapping. Furthermore, excessive PEEP impairs right ventricular function and venous return, possibly necessitating excess intravenous fluid administration.

Early animal studies demonstrated that the use of PEEP ameliorates ventilation-induced lung injury. As such, a minimum of 5 cm H ₂ O of PEEP is recommended by ARDSnet ventilation protocol. However, large clinical studies comparing lower PEEP protocols to higher PEEP protocols failed to demonstrate improvement in clinical outcomes, although oxygenation was improved. The failure of these studies led to the more sophisticated methods of titrating PEEP usually included in open lung strategies (OLSs).

Open Lung Strategies

OLS are the combination of lung protective ventilation with PEEP optimization and possibly recruitment maneuvers. The goal is to maintain aeration of functional lung tissue, thereby preventing atelectrauma, cyclic collapse of the alveoli, and focal lung stress in the remaining compliant regions of the lung. Various PEEP titration strategies have been integrated with recruitment maneuvers into “open lung” strategies.