Children deserve a safe and pain free perioperative period.

FROM THE DIVISION OF PEDIATRIC ANESTHESIOLOGY AT THE KENTUCKY CHILDREN'S HOSPITAL
LEXINGTON, KENTUCKY


Monday, March 9, 2015

Reading a Journal Article; How Do We Know That the Results Represent a Statistically Significant Finding?



Basic Statistical Analysis for Anesthesiologists

When we read the biomedical literature we depend, to some extent, on the journal editors to guide us about the veracity of the observations that are being presented. Beyond that, it is largely the responsibility of the reader to determine whether the data that is being presented has been appropriately derived and analyzed. We have spoken before about bias that may have been introduced into the investigation. Some elements of bias may be apparent from the close reading of the article. Some may not be.

The statistical analysis of observations in any given study allows the reader to determine whether the findings are significant based on accepted values routinely used. This does not determine whether the finding is clinically significant in the population that is evaluated or whether there is statistical or clinical significance in any given population that a clinician may be considering. As I have said before, very few single articles provide evidence that should change current practice. The best evidence is obtained from multiple randomized controlled trials that have been analyzed as a group.
Clinicians require a basic knowledge of the basis of a statistical analysis in order to understand the language of medical research and that will be the focus of this short piece.

The Language of Data Evaluation

Medical studies, especially randomized controlled trials, present data in many different ways but some of the information in a statistical analysis is invariable:

1.     Data from a study is presented using two distinct processes – a description of the sample that is being used for the analysis, and a determination of the conclusions that can be drawn from the data. Both of these aspects may be presented in tabular form

2.     As a general rule the characteristics of groups considered in a study should be as close to each other as possible. A significant difference between groups is a bias that may render the findings of the study meaningless.

3.     Variables are, of course, anything that can be measured or manipulated in a study for the purpose of demonstrating an effect. These characteristics may be presented as nominal, ordinal, interval, and/or ratio. Interval data may be presented as continuous or discrete.

4.     Randomization represents the attempt by the investigator to assure that bias is not introduced into the study by choosing patients that are intrinsically more or less likely to respond in a given way.

5.     Blinding attempts to remove any bias that is produced by the investigator, the observer, or other clinicians that may change the characteristics of care associated with the trial.

6.     Data that is presented in a continuous fashion numerically has characteristics, which describe the central tendency of a group of data points. This represents the tendency of an individual observation to be clustered around a central point – the mean, median, and/or the mode. Only the mode may be used with all types of data. The mean requires interval/ratio measurements while the median ordinal may be used with interval/ratio or ordinal data.

7.     In concert with a description of the central tendency or data is a description of the variability of the entire data set around a central value. This variability can be defined in several different ways, the most common being the standard deviation. Other methods may be noted based on the characteristics of the data set – the range, the variance, or the standard error of the mean.

8.     The appropriate use of these characteristics of the data, that is descriptions of the central tendency and the variability of the data are important in the interpretation of the data that is presented.  For a clinician, interpreting a data set with an indication of high variability may suggest difficulty in understanding the true clinical significance of the study.

(P<.05)

There is some probability that in examining the data derived from clinical trials we arrive at a value that is only indicative of chance. That is, the value in reality does not represent a true scientific finding but a random observation. How do we determine whether the finding is statistically significant or not?

First, investigators usually presume that there is no difference between an experimental group and a control group. This is termed the null hypothesis. In analyzing data we must prove that the individual values were drawn at random from the same population. When this requirement is met, we use statistical tests to determine the likelihood that the value that we have derived represents a statistically significant finding, that is that it is not representative of chance alone.

By using statistical analysis, if the probability is determined to be less than one in twenty, giving a P (probability) value of less than .05 (P<.05) then the null hypothesis is rejected and one can conclude that there is indeed a difference in the group. That said, even with careful analysis of data that is randomized, blinded, and analyzed, there always exists a certain probability of accepting or rejecting the null hypothesis in error, either a type 1 or a type 2 error.

Type One Error represents the probability of incorrectly rejecting the null hypothesis when, in fact the null hypothesis is true. (Remember that the null hypothesis assumes that there is no difference between two groups.) This value, the probability of making this mistake is termed the alpha and is usually not more than 5% (Alpha =0.05).

Type Two Error is the probability that one fails to reject a null hypothesis that is in fact false. This implies that there is a real difference in the experimental and the control group but analysis of the data did not demonstrate this difference. Obviously, if the original analysis rejects the null hypothesis, then no Type Two Error can be made.

Randomization, blinding, measures of central tendency, and of variation,  variables, demonstration of the probability that the results are not solely due to chance, and errors in this determination; these are basic concepts that are required in determining whether the journal article that you are reading meets the basic standard of scientific investigation. We will talk about these elements in detail and other, more complete, offerings.

Rae Brown, M.D.
January 22, 2015




Friday, March 6, 2015

What We Know, What We Think We Know, and What We Don't Know



Hundreds of pages of scientific writing come across the desks of journal editors weekly. Some few articles are brilliant and represent new findings that will change medical practice and save countless human lives. Some represents a restating of information that has already been identified. Some can be charitably termed weak science that, if incorporated into the psyche of practitioners, does more harm that good. Some is fraud. How is an editor to know which category a particular document belongs in and whether, for any reason, it is justifiable to publish the document at hand?

In a recent gathering of the Editors in Chief of the largest medical journals these important aspects of their work were discussed in open forum. Of course, consensus was reached on nothing as would be expected. But the editors noted an increased recognition of fraud and bad science in submitted documents. The level of technological sophistication in the publishing industry allows more sensitive screening of articles submitted for publication but even with a raised awareness of studies that purport to demonstrate findings that they do not can and will slip into publication. As you might imagine, having to retract journal articles reflects badly on the editors and many expressed consternation at the current circumstance.

An example of the phenomenon of inaccurate or fraudulent information driving clinical practice occurred in the late 90s and early 2000s when a single practitioner, mostly in a single but very well respected publication, produced what appeared to be irrefutable evidence supportive of a method for providing postoperative pain control. Unfortunately many of the trials were not IRB approved, some may have been partially fraudulent, and some were entirely fraudulent. Most of these papers were eventually retracted, however, those data changed practice for some prior to being retracted and may have had an impact on the interpretation of other similar studies. What are we to make of this? Personally it has caused me to change the way that I analyze the information that is published and I would offer these soft rules that I have adopted:

1.    Be suspicious of all medical literature, especially that which purports to support great and rapid leaps in knowledge and clinical practice. Civilization moves slowly. Profound discoveries that reverse practice require time to be validated.

2.    Never change practice on the basis of one observation or one miraculous study. See above.

3.    Be suspect of any single investigator or group that publishes more than 5 -10 papers over a two-year period. It is simply impossible to publish as many scholarly articles as some seem to be churning out.  Think to yourself: Is it reasonable to believe that all of this science could possibly come out of a single group in such a short period of time?

4.    Understand that journal editors are human and that mistakes will be made. Do not interpret the fact that something was published as an assertion that what is written should be accepted to the letter. The review process post publication has eviscerated many dramatic studies

The information that we read can be accepted as inviolate fact, can be rejected out of hand or can be viewed with a jaded eye. In fact, some of the information that we accept as fact and go to some trouble to teach in 2015 will be demonstrated to be inaccurate, incomplete, or worse in 2018. Virtually nothing that I learned as a resident has stood the test of time, although it is still considered to be reasonable to use oxygen in many clinical situations. How then is one to practice medicine?  In terms of maintaining our knowledge and our sanity, following the concepts above may help. In general, following the precepts that are suggested by the guidelines from our professional organizations will likely put your practice in the mainstream. This information is reviewed and the data are graded on a regular basis. More than on the cutting edge, that is where most of us should be practicing.

Rae Brown, M.D. 


Wednesday, March 4, 2015

Critical Reading of the Medical Literature

When articles are presented at Journal Club, when you read articles in journals, or when articles are reviewed in the lay press, questions should arise in your mind about whether the study is appropriate, the data is real, and the analysis is appropriate. Every piece of medical literature is not law simply because it is published. Editors do the best job that they can but, in reality, the true value of individual articles in the medical literature can only be surmised after post publication analysis and time.
It takes many years to develop the knowledge and skill to analyze journal articles. I hope that, with our departmental journal clubs, we can move this process along so that all can be appropriately critical of the information that we are presented with.
These are some of the questions that come to mind as I peruse journals every month:
  1. Are the authors asking a question that can reasonably be answered? – Basic scientists ask this question every day. There are some things that we would like to know that, at the present time, do not lend themselves to randomized control trials. This relates to the need for total disclosure of risks and benefits to patients, to the physiologic differences between physiologic models in animals and humans, and to the lack of a profit motive to drive the asking of specific questions. Because of these limitations, there are some important questions that that will never be answered through analysis of multiple randomized controlled trials.
  2. Is the study that is designed to answer the question methodologically appropriate? –This is a broad and difficult topic but relates largely to determining whether there is bias underlying the development of the trials. This may be difficult to determine because even with the requirements for documentation surrounding the preparation and implementation of RCTs there is much that is unknown. That said, industry funding of RCTs often produces blatant or unwitting bias, both in the production of trials and in the non - publication of negative trials.
    Blinding of those carrying out trials is critical to accurate determination of an effect in a RCT. The sophistication that the investigators have in blinding those that are carrying out the trials is an important determining factor that should be considered as you read the article.

  3. Is the statistical analysis of the data derived from the trial appropriate for the question that is being considered? Neither an unpaired T test nor Chi square analyses will answer all questions. Lucky for us, most of the major journals employ PhDs in Statistics to assist. 
  4. Is the information that is being presented new information, old information, or does it contradict information that we think we know? – Surprisingly, all of these are important. Knowing that a positive result can be replicated is important because it adds weight to our ability to incorporate information into our practice. It helps to answer the question “Is this information generalizable?” If an article contradicts accepted information it implies that further study of the old information and the new information is required. Did the old trials have inherent bias that was originally missed?
  5. Does (or should) this study have an impact on my practice? – Will I do anything differently based on the results of this study? Does it validate what I am currently doing? In my opinion, changing practice based on a single study should be a rare event during a professional career.

As time passes, you will undoubtedly add to these questions, and many other interrogatives can be substituted. However you read the medical literature for the rest of your career it is important that this reading be systematic and critical.

Rae Brown, M.D.

Friday, February 20, 2015

Pediatric Anesthesia - An Opportunity to Learn, Teach, and Reflect

This blog has been on the web since 2007 and over that period nearly thirty thousand people have viewed it, read it, and/or commented on it. The purpose of this blog is to provide a voice for those involved in the perioperative care of infants and children. I hope that in the future those that come upon this space on the web will pass it on to friends and colleagues. I would also hope that many will comment on that which I offer up.

Remember: This is for learning, for teaching, for reflection; and all are welcome.

Rae Brown, M.D.
February 20, 2015

Her Work is Done

Her Work is Done
It is not enough that she was only seven months old, that her three year old brother could make her giggle, that, she appeared perfect in every way - a little angel. It is not enough that she never harmed a soul, that she was beautiful when she was a born, with rosy cheeks and pudgy thighs, that she had a sweet disposition; she hardly ever cried. It is not enough that she looks like she could be napping as she lies in her bed; eyes closed, skin fair, curly brown hair with a bow in one tiny wisp. She is the treasure that many families have hoped for, have prayed for, would love and cherish every day of her life. She is the child that would fill the void in an empty heart. Despite all of what she is and what she could be, nothing could prevent someone from hurting her, for taking her life from her, from bringing her to this point, this work that she must do.
She lies in repose in front of me with technology, nurses, and her grieving family surrounding her. At her bedside, I struggle to understand the why, the how… It always comes back to the why.
 As I review her story, nurses scurry about, “packaging” her for the operating room. I glance at her vital signs, and the drugs that are keeping her alive; multiple infusions that control her tenuous blood pressure and heart rate. I get report from the ICU nurse that has cared for her faithfully these last days. There is tranquility in the infant’s room; a peace that is unfamiliar to those that frequent intensive care units. There is calm where hours earlier there was mayhem. Earlier, fluorescent lights, monitors, and the sun poured in, painting the room and all in it with a preternatural glow. Now, as the struggle of the last days is over, the struggle to somehow save this infant, the lights have been dimmed and all speak in whispers. Now the parents weep inconsolably, sobbing as their future without their baby daughter becomes clear  Now their little one will do the work that she must do. She will give of herself to other infants, so that they might survive. That is the work that she must do.
We try to speak to the parents; to ask if they understand what will happen next. “Will she have any pain? “ her mother sobs. “Will you stay with her?” “Will you keep her warm? Her feet have been so cold for so long” We tell them that we will keep her warm and safe, that she will feel no pain and  that we will care for her as if our own. We ask if they would like to follow us downstairs to the door of the operating room, if they would like to have a prayer before we go, if there is other family that wishes to see her off. We arrange the IV lines in the bed beside her; secure the wires, the catheters, and the tracheal tube. We cover her with warm blankets and I grab one of her many toys and place it under her arm so that she might cuddle it close at her side. All is made ready; each detail checked and rechecked. This all too familiar caravan begin its slow journey. I put my hand on the mom’s shoulder. “ We will take good care of this sweet baby.”
We hang outside of the operating room doors as the parents and grandparents, aunts and uncles, neighbors, pastors and her big brother, seeing what no three year old should ever see, say their last good byes.  Mom suffers at the bedside, holding her daughter’s tiny foot gently between her fingers as if to keep her from going the rest of the way. A nurse begins to cry, and I, the jaded veteran of the health care wars, the “expert” with all the training and experience, that has seen it all before, break down, tears streaming into my surgical mask. , My thoughts drift for a second and I think about all the times before that I have made this journey, taking a cherished child to their last stand, to the work that they must do.
I place her on the operating room table, holding her one last time as we transfer the infusions and the monitors. The room is sweltering; she cannot maintain her temperature. She is prepped with dispatch, covered with the surgical blue, the instruments brought to the bedside and the work begins.
 The surgeons come to the surgical field; one sighs while holding the knife over the chest and scoring the skin in an instant. They are efficient in their dissections; they have done it many times before. They isolate the kidneys, the liver, the bowel, the heart and the lungs.  Each will go to another little person in another town in the east, south, north, and west and each, at this very moment is struggling to survive in another operating room, in another medical center. This little one is working for them.
We struggle to maintain the organs that are being harvested. More fluid, more drugs, anything to keep the blood pressure up without damaging the heart that by nightfall will beat in another chest, the liver that will save the life of a child born with a congenital disorder that, at the age of one, rendered theirs ineffective, or the tiny kidneys that will help two three year olds survive to see their children, their children’s children.
The surgeon that has come to retrieve the heart speaks to the three others that are huddled with their instruments in the tiny belly. “ We’re ready up here, whenever you are,” he whispers, without lifting his head, without so much as a flinch.
“I think we’ve got everything ready at this end, another surgeon interjects. “Ready to perfuse? “
“We are ready”, the perfusionist answers. “We are ready,” I offer peering over the surgical drape. I put my hand on her head at the head of the bed, warm for the last time.
The thoracic surgeon, addressing the room says in a calm voice. “I am going to cut the veins to vent the heart, then we will cross clamp.” This time he looks around the room as he speaks, making eye contact which each and every one – surgeons, nurses, medical students technicians, and me.
One finger lifts the heart and the Metzenbaum scissors sweep deep into the open chest. Dark blood wells up in the field. He places a small clamp across the aorta. “ Perfuse, please.” He looks across the drapes at me. “Lungs down, please and vent off”.

We disconnect the circuit that has been ventilating the child. All of the waveforms on the screen are now flat. We turn off the intravenous pumps, seven in all, that have been sustaining her. The surgeons are working quickly now to excise the organs from their place in the chest and abdomen. Soon they will leave a hollow cavity behind. One will close the incision with dispatch. We will clean her up and wrap her in a blanket for her trip to the morgue. She has finished her work. Now she rests. 

Thursday, February 19, 2015

Hypoxemia in Children

Hypoxemia in Children – Diagnosis and Treatment

How do you deal with these situations?

1.    A twenty-day-old infant with a hernia for repair. During the course of the procedure, the infant begins to desaturate. Weight is 3.5 kg. the TV is 35cc. Infant is breathing spontaneously. ETT is taped at 12 cm. Why is this patient hypoxemic?

2.    Ventilating pressures are 40 torr in an otherwise normal 15 year old. The SpO2 is 90%. How do you determine why this is happening?

3.    The patient is a 9 month old that has had a portacath placement in the operating room. In the PACU, the infant’s lower extremities are mottled and the oxygen saturation is 80%. What is the most likely cause?

4.    The patient is a former premature infant that is recovering from significant BPD. During a surgical procedure for gastrostomy placement, the oxygen saturation drops precipitously when the anesthetic is switched from sevoflurane to desflurane. Why does this happen?

5.    A fifteen-month-old infant comes to the operating room for a hernia repair. Two weeks ago, the infant had a URI and was treated for wheezing in the local emergency room. During the procedure, the infant becomes hypoxemic while breathing spontaneously through an LMA. What is going on here?

6.    A twelve-year-old male has a complex femur fracture after a bicycle accident. During the course of the surgical repair, the patient has an acute drop in his SPO2, associated with hypotension. Why is this happening and what will you do?

7.    A sixteen year old with sickle cell disease has been poorly controlled and comes to the operating room for a cholecystectomy. In the holding room the oxygen saturation is 90% on room air. Why is this? What do you need to do for this patient before taking him to the operating room?

8.    The patient is a five year old with a history of chronic pneumonia and bronchiectasis. Despite no change in his acute condition, he has an SPO2 of 88% on room air. A chest x-ray shows chronic right middle lobe opacification. Is there anything that needs to be done for this child before he goes to the operating room?

9.    A four-year-old little girl was recently adopted from China. She has a large VSD and an O2 saturation of 90% on room air. Why is this child hypoxemic? What can you do to improve the child’s clinical condition?

10. An 18-year-old male was treated for testicular cancer with surgery and chemotherapy, including bleomycin. He is hypoxemic at rest. Does his history explain this finding? How will you manage this patient during the course of this procedure?

Bonus question: The child in question eight is coming to the operating room for a thoracotomy. How will your intraoperative management of this child affect his postoperative recovery. How can you manage this anesthetic to reduce the risks of respiratory failure and prolonged ventilation during the postoperative period?
Discussion

The differential diagnosis of hypoxemia in an infant or child may be different than that for an adult. This is because of different disease processes, different caliber of the airway and the impact of the anatomy of an infant on the capacity to effectively ventilate. As examples, infants do not smoke, but they may be exposed to second hand smoke. Infants that breathe spontaneously while laying on their back will be much more likely to become hypoxemic because closing capacity is within the tidal ventilation of the infant. Bronchopulmonary dysplasia, still a possibility in premature infants, produces an increase in airway irritability and in pulmonary hypertension.

Many infants and children develop hypoxemia during the perioperative period for exactly the same reasons as adults. In the management of these patients, consideration must be given to all of the possibilities with more weight given to those issues that reflect the pathophysiology at hand.

Question 1: There are several reasons for this patient to desaturate. An infant breathing spontaneously through an endotracheal tube is likely to develop atelectasis producing intrapulmonary shunt. The work of breathing through an endotracheal tube is great and infants tire quickly. This is caused by the caliber of the endotracheal tube and the difference in the respiratory muscle type that predominates in infants. Muscles of ventilation expend their energy load rapidly and the infant will become hypercarbic and hypoxemic. In addition, this infant‘s endotracheal tube is taped at 12 CM at the lip. The tip of this tube is likely in the right mainstem bronchus and the infant is ventilating only one lung. For most infants the correct depth is ten cm at the lip.

Question 2. Defining the reasons for an acute change in ventilating pressures requires a studied approach. Obstruction of the circuit from the anesthesia machine to the end of the endotracheal tube must be investigated. Obstruction distal to the ETT must be considered and may involve large and small airways. Pneumothorax can produce an acute rise in PIP. Consideration must be given to the medical condition of the patient, (Does the patient have pneumonia?) as well as physical characteristics of the circuit, (Is the ETT kinked?). Consideration of the anesthetic that is being provided as a gaseous agent must be considered as well as the possibility of anaphylaxis.  Hypoxemia usually occurs coincident with elevations in ventilating pressures.  This occurs because of incomplete expansion of the lung and subsequent atelectasis. Management of this issue requires an increase in FiO2 until a definitive diagnosis is made and the pressure is reduced.

Question 3. One must consider pneumothorax before one can diagnose and treat. Mottled lower extremities suggest low cardiac output.

Question 4. As above, infants with BPD have irritable airways and will often respond to Desflurane with bronchospasm and hypoxemia. In addition, the disease process often produces a state of chronic pulmonary hypertension that is exacerbated by agents that produce an increase in activity in the sympathetic nervous system (such as Desflurane). 

Question 5. RSV is common in the population of patients that come to the operating room. Some of the patients that have this virus in their airways are asymptomatic, but begin to develop an increase in mucous when the airway is instrumented. Because of this effect, children are at risk for atelectasis when breathing spontaneously even through an LMA.  It is often necessary to intubate these patients in order to suction the airway and provide recruitment breaths.

Question 6. A pulmonary embolus caused by a thrombus or by fat is common with long bone fractures, especially during the time that they are being manipulated. V/Q mismatch produced by small fat globules may be accompanied by hypoxemia, which may or may not be associated with hypotension or a significant change in ETCO2. Large emboli may manifest with precipitous reductions in cardiac output causing secondary reduction in ETCO2 and decreases in perfusion pressure and SPO2.

Question 7. Acute chest syndrome represents widespread sickling within the microcirculation of the lung. Patients that have these episodes develop pulmonary infarcts, hypoxemia, and eventually pulmonary hypertension. This patient will show evidence of multiple past infarcts on a CT scan of the chest; thus the hypoxemia that is discovered in the holding room. Preparing patients such as these for safe general anesthesia will require eliminating the possibility of an acute infection and oxygen supplementation to reduce the risk of a sickle cell crisis during the perioperative period.

For patients that require chest procedures, judicious use of fluids to reduce the risk of postoperative respiratory failure is key. This must be balanced against the need to maintain sufficient intravascular volume reducing the risk of dehydration and a sub sequent crisis.

Question 8. Chronic atelectasis produces a respiratory shunt and subsequent hypoxemia. Often this anatomic finding has been present for such a long period of time that the lung segment is and never will be functional. Post obstructive pneumonias form in some of these segments. Surgical exenteration may be required in order to reduce the risk of further chest pathology or the development of widespread septicemia.

Perioperative treatment includes aggressive pulmonary toilet to remove foreign matter from as much of the lung as possible. In addition treatment of acute pneumonias based on bronchial washings reduces the risk of sepsis in the operating room or during the recovery phase.

Question 9. This child has a wide-open VSD. Because of the high flow through this defect the lungs are chronically flooded increasing the amount of interstitial water to a level that is greater than can be filtered and carried away. Interstitial water prevents effective movement of oxygen across the alveolar membrane leading to hypoxemia. In addition, the high flow causes anatomic changes in the muscular walls of pulmonary arteries producing increases in pulmonary artery pressures over time and eventually reversing flow to a rt. to lt. shunt. This is a late finding and is rarely encountered.

Early treatment includes the use of diuretics to reduce pulmonary interstitial edema and eventual pulmonary banding to substantially reduce pulmonary blood flow. As the heart grows, it becomes more appropriate to patch graft the VSD and remove the pulmonary band.

Question 10. Bleomycin is an antibiotic antitumor agent effective against germ cell tumors. Pulmonary toxicity has been reported with this compound since the 60s with attention being given to the effect of bolus dosing, age and total dose of the drug. In addition, exposure to oxygen has often been cited as a mechanism of toxicity thought the primary cause of this has yet to be elucidated. Recent studies have focused more attention on the renal function of the patient as a reason for elevations in the levels of the drug that can produce pulmonary interstitial fibrosis. In this patient, his young age and the fact that in the modern era drug is given as an infusion, and after verification of renal function, his hypoxemia is likely not secondary to bleomycin toxicity.

Bonus Question: Appropriate management of intraoperative fluids has been shown to affect significantly the length of stay as well as the risk for postoperative respiratory failure for patients having surgical procedures in the chest. While appropriate resuscitation is often required, excessive fluid administration is ill advised.

General Approach to Hypoxemia

If the patient’s oxygen saturation has decreased:

  1. Ask the question “Is the patient at risk for injury?” if the answer is yes, then you must increase the flow of oxygen to the patient. This gives the clinician time to think and should virtually always be the first order of business.
  2. Ask the question” Am I actually delivering oxygen to the patient?” Could this be a hypoxic mixture, a line swap, loss of pipeline flow with empty tanks?
  3. Am I adequately ventilating the patient? Is the ETCO2 waveform present and of normal configuration? Is the ventilating pressure reasonable? Does the patient have bronchospasm?
  4. Is the endotracheal tube taped appropriately? Right main stem intubation is the most common cause of hypoxemia in other wise healthy patients.
  5. Has the thorax been invaded – central line placement, retroperitoneal dissection, and laparoscopic cases? Is the risk of a pneumothorax elevated for any reason?
  6. Has there been a catastrophic decrease in the cardiac output such as with a dissection of the aorta, a pericardial tamponade or profound anemia?
  7. Is this a procedure that predisposes to hypoxemia – one lung ventilation?
  8. Finally, after every other avenue has been exhausted, could this be a spurious reading? Occasionally caused by the compression of a blood pressure cuff, the administration of dye, or probe failure?
 



 Rae Brown, M.D.
February 19, 2015