October 1997
Volume 61 |
Number 10
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| Simulators in
Anesthesiology: The Excitement Continues |
Michael L. Good, M.D.
Anesthesiologists continue to play a prominent role in the development
of systems that simulate a patient within a programmable clinical
environment and allow full interaction by the user in patient
care. There are simulators that present the patient and environment
solely on a computer screen; however, this article deals with
simulators that realistically present the patient as an instrumented
mannequin in a clinical environment.
Realistic simulators consist of the patient mannequin, a simulation
computer, interface hardware to control the mannequin responses
that send signals to real patient monitors and a user interface
for the instructor. The simulation computer contains mathematical
models of the physiology and pharmacology of the patient. Patients
with different underlying characteristics can be created by selecting
and altering various parameters of these models. As the clinicians
interact with the patient, interventions such as the administration
of injected or inhaled drugs are fed back to the computer, which
calculates realistic cardiovascular and pulmonary responses and
side effects in a dose-dependent manner.
The clinical realism of contemporary patient simulators is exciting.
Skeptics often focus on known limitations such as the cold plastic
skin, but learners actively engaged in a training exercise "suspend
disbelief" and are quickly drawn to the descending pitch
of the pulse oximeter tones, irregularities in the cardiac rhythm,
alarms indicating hypotension and a simulated surgeon demanding,
"Is there a problem up there?"
Examples of the Clinical Realism of Modern-Day Patient Simulators
Difficult airway management: One of the areas of learning
most effectively addressed by the simulator is airway management.
Contemporary patient simulators are built using adult-size mannequins.
The face, neck and upper airway anatomy of these mannequins allows
both routine and complex airway procedures to be practiced.
Learning routine airway techniques (such as bag and mask ventilation,
laryngoscopy, oral and nasal tracheal intubation and extubation)
on a patient simulator becomes more relevant clinically because
of the programmed complexities associated with intravenous drug
administration, timing of interventions and monitored physiologic
parameters. Mastering more complex airway techniques (such as
combitubes, retrograde wire intubation, needle cricothyrotomy,
transtracheal jet ventilation and tube cricothyrotomy) through
actual clinical experience alone is difficult, if not impossible.
How are airway problems simulated? Under scenario script or instructor
control, the mannequin's neck and jaw both lock, making intubation
difficult; the airway tissue and tongue are made to swell such
that tracheal intubation is nearly impossible. The script or instructor
may also prevent gas exchange with the lungs, leading to the clinical
situation of "cannot intubate, cannot ventilate." Other
airway complications such as laryngospasm, endobronchial intubation
and esophageal intubation are also created with realism.
Modern-day simulators recognize the composition of the inspired
gas, and they can physically consume oxygen and produce carbon
dioxide from the "alveolar space" according to the metabolic
rate. "Alveolar" oxygen concentration determines PaO2
and SpO2 according to the degree of simulated intrapulmonary
shunt fraction. The more effective preoxygenation, the longer
apnea is tolerated before desaturation. Arterial hypoxemia and
hypercarbia automatically provoke hemodynamic abnormalities and
cardiac dysrhythmias.
The susceptibility of the simulated patient to myocardial ischemia
is also determined by underlying mathematical models; in some
patients, therefore, hemodynamic changes accompany airway difficulties
and result in myocardial ischemia, placing even greater decision-making
and technical demands on the anesthesiologist.
Invasive hemodynamic assessment, fluid management and pharmacology:Several
simulation centers use the patient simulator for problem-based
learning about the care of critically ill patients undergoing
complex surgical procedures. At the University of Florida, Gainesville,
residency program director Michael Mahla, M.D., and Tammy Euliano,
M.D., make the problem-based learning (PBL) format a unique learning
exercise. Residents are frequently challenged to care for a (simulated)
patient with a "leaking" abdominal aortic aneurysm (AAA)
requiring emergent repair.
The mannequin has a speaker allowing a confederate to be the
patient's voice. Before induction of general anesthesia, the "patient"
complains loudly of severe back pain. Trainees unfamiliar with
care of AAA patients often respond with excessive medication and
soon after induction find the patient severely hypotensive, requiring
heroic resuscitation with fluids and drugs. The trainee eventually
learns from the simulated surgeon that the AAA has ruptured. The
trainee is given opportunities to repeat this case. Most residents
handle the preinduction pain much differently on later attempts.
How can this simulator PBL training translate to actual patient
care? Recently, the "day team" assumed care of a patient
undergoing emergent AAA repair, which had started at 4 a.m. In
the relief report, the call team resident noted, "This patient
was just like the one we had on the simulator."
Today's simulators provide sophisticated models of the cardiovascular
system. The clinician attaches lead wires to electrodes embedded
on the chest of the mannequin to obtain the electrocardiogram
(ECG) and wraps an oscillometric blood pressure cuff around the
arm of the mannequin to determine arterial blood pressure noninvasively.
If the ECG lead disconnects or the blood pressure tubing kinks,
no data or artifactual data are reported on the physiologic monitor.
The simulation instructor can select from a list of cardiac dysrhythmias
in developing advanced cardiac life support (ACLS) scenarios and
verify that trainees appropriately perform chest compressions,
defibrillation and electrical cardioversion. Normal and abnormal
heart sounds are auscultated with a stethoscope in appropriate
precordial locations, and peripheral arterial pulses can be palpated
as long as the simulated patient is not hypotensive. Invasive
hemodynamics measurements are obtained in real-time using standard
monitoring instruments, including systemic arterial, central venous,
pulmonary artery and pulmonary artery occlusion ("wedge")
blood pressures as well as thermodilution cardiac output measurements.
Realistic Responses Even at Altitude
A few years ago, patient simulators were demonstrated at the
annual meeting of the Colorado Society of Anesthesiologists in
Vail, Colorado. Just prior to opening of the exhibition area,
anxious technicians summoned an anesthesiologist. The patient
simulator, which had been programmed as an elderly man with emphysema,
was hyperventilating and developing hypocarbia. The anesthesiologist
quickly established a diagnosis based on the decreased oxygen
tension at the altitudes on the mountains of Colorado. The anesthesiologist
administered supplemental oxygen through nasal cannula, which
decreased the spontaneous respiratory rate and, subsequently,
resulted in normalization of PaCO2. Then the anesthesiologist
loaded a "healthy young adult" patient profile into
the simulator, enabling a comparison of the responses to altitude
of two different patients with different degrees of intrapulmonary
shunting.
This "case" demonstrates the sophisticated pulmonary
physiology incorporated into the current generation of patient
simulators. Patient simulators can breathe spontaneously, or they
can be paralyzed requiring positive pressure ventilation. Appropriate
chest excursions accompany each respiratory cycle. Normal and
abnormal breathing sounds are ausculated with a stethoscope. Simulation
instructors and scenario developers can adjust baseline respiratory
rate and end-tidal volume. Simulators regulate the breathing pattern
(minute ventilation) of the simulated patient depending on PaCO2
and PaO2. If the simulated patient breathes into a
paper bag (or circle anesthesia breathing system with an incompetent
expiratory valve), carbon dioxide rebreathing causes hyperventilation
due to the rising PaCO2. A real pulse oximeter provides
SpO2 and waveform data from the simulator, and arterial
blood gas data can be obtained from the computer console (if a
blood gas sample is sent to the "lab!").
In addition to consuming oxygen and producing carbon dioxide,
simulators provide for uptake and elimination of anesthetic gases.
When connected to a capnograph or respiratory gas monitor, it
is virtually impossible to determine whether a real patient or
simulated patient is generating the display. The lung and chest
wall compliance and/or airway resistance of the simulator can
be adjusted to create clinically meaningful simulations of altered
respiratory mechanics such as a patient in "status asthmaticus."
Various scenarios dynamically interact with one another. For
example, excessive positive end-expiratory pressure applied to
the breathing system will cause hypotension in hypovolemic patients.
A decrease in cardiac output produces a fall in end-tidal CO2,
if metabolism and minute ventilation remain constant.
Simulators Need a Brain
Recent improvements to patient simulator systems incorporate
trauma pathophysiology; this includes fractures and sites for
chest tube insertion, moving eyelids, constricting and dilating
pupils, patient voice and limb movements allowing Glasgow coma
scales to be assessed. As yet, however, the simulated patient
has no brain! Ongoing research efforts include the development
of a simulated central nervous system, one which will generate
neurophysiologic and intracranial hemodynamic data (but as yet
no thoughts!).
Developers of simulators are determined to complete this work
before the "decade of the brain" (1990-1999) draws to
a close. Other simulator advances in development include the ability
to take the patient onto or off of cardiopulmonary bypass and
to arrest or restart the heart for cardiac surgery.
Seeing Is Believing
In a short report, it is difficult to do justice to modern-day
patient simulators. Readers are encouraged to check out the simulators
that will be on display in the patient safety section of the ASA
exhibit at the ASA Annual Meeting in San Diego on October 18-22,
1997. Anesthesiologists also may wish to attend one of the many
simulator-based continuing medical education programs now offered
by academic medical centers that have a patient simulator.
Disclosure Statement
The University of Florida owns and receives royalties for licensed
simulator technology. A portion of the royalties are distributed
to the inventing team, of which Dr. Good is a member.
Michael L. Good, M.D., is Associate Professor
of Anesthesiology, University of Florida College of Medicine, and
Chief of Staff at the Veterans Affairs Medical Center, Gainesville,
Florida.
E-mail the author.
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