Improving Simulations in the Post Anesthesia Care Unit

1

Vanderbilt University

Department of Biomedical Engineering

BME 273

Improving Simulations in the Post Anesthesia Care Unit

Written by: Alyssa Z. Cherif

Group 28, Department of Biomedical Engineering

Advisors:

Dr. Matt Weinger, Ray Booker, Bobby Gibbons, Dr. Paul H. King

Submitted: April 24, 2007

ABSTRACT

The Post Anesthesia Care Unit Simulation Center was created to allow medical and nursing students to practice a variety of procedures on a simulated patient, SimMan. To efficiently simulate all possible scenarios that could arise in the PACU, it is necessary to provide a full range of pulmonary function. At the start of this project, there were only two options for lung size: fully functioning or completely collapsed. The overall goal for this project was to allow for a range in the available volume in SimMan's lungs and to have it be externally controlled; simulating an obstructive pulmonary disease. This goal was to be achieved in the most efficient and inexpensive manner possible. A prototype was built by inserting a saline bag into one of SimMan's lungs; this saline bag had a tube extend outside the body. The tube can be injected with air using a syringe, allowing for total external manipulation of available lung volume. Data was collected from the screen of the anesthesia monitor. To effectively simulate an obstructive pulmonary disease, there must be a decrease in the amount of air expired by the lung per breath. This was achieved, as shown by the steady decrease in the volume of air expired as the saline bag volume was increased, and hence available lung volume decreased. Also, the pressure needed to push air in and out of the lungs increased with the volume of air added to the saline bag; the relationship is well-represented by a third order polynomial. Overall, this prototype was extremely inexpensive to build and it successfully simulates an obstructive pulmonary disease.

INTRODUCTION

After undergoing a surgery involving anesthesia, the patient is sent to the Post Anesthesia Care Unit, or the PACU. Because general anesthesia affects the entire body, postoperative effects tend to appear, the most common being pulmonary complications.

The PACU Simulation Center was created to enable medical and nursing students to practice a variety of procedures on a simulated patient called SimMan. SimMan has several working bodily systems, including fully functioning lungs. His vital statistics can be externally controlled using a program called SimQuest. SimQuest can alter his statistics such as blood pressure and heart rate, and then display them on a monitor inside the Simulation Center. At the start of this project, there were only two options for lung size: fully functioning or completely collapsed.

To efficiently simulate all possible scenarios that could arise in the PACU, it is necessary to provide a full range of pulmonary function. The primary way in which pulmonary function is assessed is by volume measurements (Sherwood 479). The most important of these is tidal volume, which represents the amount of air entering and leaving the lungs in the span of one breath. The expiratory reserve volume is the amount of air which can be expired after normal exhalation, with maximum force of the expiratory muscles. The residual volume is the amount of air left in the lung after exhalation, included after maximally forced exhalation. Functional residual capacity is the amount of air in the lung after a normal, involuntary expiration; this is calculated as the sum of expiratory reserve volume and residual volume. The vital capacity is the largest possible volume change in a single breath in the lungs, after the patient maximally inhales and then exhales. The total lung capacity is the largest possible amount of air that can be held in the lungs and is calculated as a sum of residual volume and vital capacity. The final measurement is FEV1, forced expiratory volume in one second; this is calculated as the amount of air exhaled in the first second of a vital capacity measurement (Sherwood 478).

There are two types of respiratory dysfunctions which can be diagnosed using these volume measurements: obstructive and restrictive pulmonary diseases. A patient with an obstructive lung disease tends to experience problems with full expiration. Obstructive lung disease patients exhibit standard values of total lung capacity, and increased levels of functional residual capacity and residual volume; they also show a decrease in vital capacity and FEV1. These measurements show that although there is a normal amount of air going into the lung, the patient presents with difficulty in reaching full expiration: there is always some extra air left inside the lung (Sherwood 479).

Patients with restrictive lung diseases suffer from a loss of compliance, causing problems filling their lungs. These patients show reduced total lung capacity and vital capacity, caused by their inability to properly expand their lungs. The residual volume is typically normal, while the FEV1 is elevated.

The fourth leading cause of death in the United States is chronic obstructive pulmonary disease, or COPD[1]. The leading cause of COPD is smoking and it tends to affect women more often than men1. COPD is an irreversible obstruction of air expired by the lungs. The damage typically worsens with time, causing a continual decrease in air exhaled[2]. In certain cases, spasms cause further blockage of the airways. The severity of COPD is determined by the amount of both airway blockage and tissue damage.

COPD refers to one of two diseases, either chronic bronchitis or emphysema. Chronic bronchitis results in inflamed bronchi and an overproduction of respiratory mucus, sputum. It is the sputum that leads to the progression of the disease and the difficulty in reduction of inflammation. The second COPD, emphysema, is an enlargement of the alveoli. This enlargement damages the alveoli walls, causing tissue loss as well as a decrease in functional breathing[3].

The overall goal for this project was to allow for a range in available volume in SimMan's lungs and to have it be externally controlled, thereby simulating an obstructive pulmonary disease; this was to be achieved in the most efficient and inexpensive manner possible. To insure that the prototype built actually simulated an obstructive pulmonary disease, data was collected using the anesthesia machine monitor, as can be seen in Figure 1. Monitor of the anesthesia machine.. The screen shows several volumes including VE, VT and VTE. VE is the amount of air expired over a minute, and is measured in liters per minute. VT is the tidal volume; in this case, it is the amount of air delivered to the patient per breath. VTE is the amount of air actually expired in each breath; in an obstructive pulmonary disease, this value should be less than the tidal volume. The final measurement important to the data collection in this project was PMAX, the pressure needed to get air in and out of the lungs; this value should increase in an effectively simulated pulmonary dysfunction.

METHODOLOGY

Several requirements were considered before embarking on this project. First, the prototype must be able to withstand regular SimMan operations, including cardiopulmonary resuscitation (CPR). The prototype must also fit inside SimMan's chest cavity and cause as little disturbance as possible. SimMan, like a regular human being, operates at a range of pulmonary parameters; the prototype must work within these ranges. The lung must always be airtight, so the prototype should not allow air leakage. The prototype should be efficient, inexpensive and easily repairable. Most importantly, the prototype should allow for external control of the lung volume.

When first exploring possible options, measurements were made to get base knowledge of SimMan's lungs. Knowing the available lung volume is vital when manipulating the lungs. To make the measurement, the lung was filled with water. Then, the water was poured into a graduated cylinder. The final volume of water that fully expanded the lungs was found to be 1263ml. After this volume measurement, length was then considered. The bronchus was found to be 11.4cm with an inner diameter of 15.9mm and an outer diameter of 19.1mm. These measurements were all considered while selecting the best possible method.

As mentioned, the prototype must work at the full range of lung parameters. Each parameter has a range specified by the person operating the anesthesia machine; the actual values are displayed on the ventilator screen. When SimMan's values go outside the range, alarms alert the user. Typically, SimMan's tidal volume is in the range of 400 – 600ml, although 600ml is a bit high for normal operation. The ranges of VE and VTE are 3.8 – 5.8 L/min and 140 – 1000ml, respectively. Finally, the Plimit is usually set at 40cm H2O, which can be adjusted higher or lower, depending on a given situation.

The first idea considered was a valve, to be placed either in or around the bronchus of the lung. This would easily allow variable amounts or air to enter the lung, and it would take up minimal space within the chest cavity. To implement this method, a solenoid with variable resistance would be attached to the inside of the lung bronchus; the resistance of the solenoid could be externally manipulated. Unfortunately, this method would change the lung resistance as opposed to its volume; solenoids are also generally expensive and fragile. This prototype would most likely not withstand daily SimMan activities, namely CPR, so this idea was rejected.

The second idea was a weight of some sort to be placed atop the lung, inside the chest cavity. The weight would be of variable mass; as the mass of the object changed, so would the internal volume of the lung. This method would allow for a large range of lung volumes, but it also brought up several concerns. First, if the weight was moved during operation, the lung could be completely collapsed; it would not be able to be repaired until the end of the simulation. The chances of the object moving would be high due to CPR and other chest compressions. Second, while the object could be externally controlled, it would take up a great deal of space within the chest cavity. Due to these possible complications, the weight idea was overruled.

The final idea was to insert a balloon of some sort inside the lung; this balloon would have a tube to extend outside the lung and chest cavity to allow for external manipulation. During balloon expansion, available lung volume would decrease. As long as a balloon with dimensions smaller than those of the lung could be found, the standard parameters should be achieved. Hence, this method was the optimal choice, and taken to the next level.

The first step to making the prototype was finding a balloon to fit comfortably inside the lung. Saline bags come in a variety of sizes are quite inexpensive. A 1000ml saline bag was chosen because its volume was closest to that of the lung. Due to this similarity, the prototype would be able to simulate a deflated lung, a fully expanded lung and every volume in between. Another convenience of saline bags is that they have tubing that fits directly into their own tubes.

After acquiring the saline bag and its tubing, it needed to be inserted into the lung. The saline bag has a 12.1cm width, making it too wide to be fit through the bronchus. A bronchoscope was used to examine the interior of the lung; it was discovered that the bronchus could not be removed. The final decision was to make a slit at the bottom of the lung, and slip the saline bag in through this hole. The tube receiver of the saline bag was placed at the top of the lung, at the rear. This positioning allows for the tubing that attaches to the bag to exit the chest cavity via holes previously drilled in SimMan's back.

Once the saline bag was placed inside the lung, the next concern was resealing the lung and ensuring that it stay airtight. At first, silicon glue was suggested because this type of glue typically works to seal vinyl, the material of which the lung is made. After setting for 36 hours, the two sides of the lung did not adhere. The next method tried was a heat gun. Heat was applied to the bottom of the prototype. Due to prolonged exposure without any cooling, the lung became extremely singed. This led to over-sealing and a great reduction in available lung volume. The first prototype was discontinued and used later for extra material.

Upon insertion of the saline bag into the second prototype, the heat gun was used again. This time, intermittent blasts of hot and cold air were administered to prevent over-sealing. Although this method worked, it took at exceptional amount of time and it was far from flawless. Hence, a new type of glue was sought after. This time, a flexible adhesive was used; this glue works on vinyl, fabric and plastics. This glue was completely effective and provided an appropriate amount of flexibility.

The next step was to allow access to the saline bag from the exterior of the lung. The saline bag's tubing measures a 6.35mm diameter. A hole was punctured in the rear of the lung roughly of similar size. Due to the suppleness of the vinyl lung, it tore easily. To cover up any exposed areas, additional pieces of vinyl from the primary prototype were glued around the saline bag tubing, using the same flexible adhesive. To secure these additional pieces, rubber washers were positioned on the tubing that exits the lung. The washers have a 6.35mm inner diameter, so they fit perfectly.

After the second prototype was completed, it was taken to the simulation center for testing. Once hooked up to SimMan, the anesthesia machine was turned on. At this point, the bellows began pumping air at the specified VT. The bellows constantly returned to maximum height, guaranteeing that the prototype was in fact airtight. The first trial was then run to begin collecting data.