The Expert: Dr. Tarvez Tucker, a neurologist at Oregon Health & Science University, testifies for the plaintiff as to the patient’s brain damage and its link to hypoxia.
Giving testimony in a 2019 Las Vegas medical malpractice trial, Dr. Tarvez Tucker, a specialist in neurological critical care and traumatic brain injury at Oregon Health & Science University, responds to questions regarding the plaintiff’s brain damage and its link to hypoxia.
The discussion begins with the expert explaining the difference between objective and subjective findings. Subjective input from a patient are those descriptions of symptoms that the patient perceives. These are not necessarily verifiable as they are experienced by the patient and expressed in their own terms. Objective findings, on the other hand, are those presented by labs, X-rays, or clinical signs observed by the doctor. This discussion occurs in preparation for a review of pulse oximeter readings as well as arterial blood gas [ABG] measurement, an important objective lab test in this case.
A “pulse ox” or pulse oximeter reading is an objective measurement of the oxygen saturation of the blood, that can be observed utilizing a simple, noninvasive, and if desired, continual gathering of information using a light beamed into a fingertip from a clip that holds the instrument to that finger. This is equivalent to the arterial blood gas oxygen saturation level, but is collected in a continuous, less painful, and less risky manner
Arterial blood gas measurements contain several important parameters including those discussed in this video— oxygen saturation, pH, and base excess. These metrics are measured in the maximally oxygenated arterial blood rather than the oxygen depleted venous blood that electrolyte, enzyme, or tumor markers are gathered from.
The oxygen saturation is a relative measure of the concentration of oxygen that is dissolved or carried in the arterial bloodstream. It is a measure of how much oxygen is in the blood compared to the maximum that could be carried in that bloodstream. Measures of O2 saturation in the 40s are life threatening. Measurements in the 60s are just as concerning. Normally acceptable levels are in the 90s.
The pH is a measure of the acidity of the blood. Normal pH is slightly alkalotic at 7.4. Anything less than that is due to the accumulation of acids in the blood. This occurs when the body’s organs, including the brain, are not being perfused adequately with oxygen or blood, so that the products of cellular respiration and metabolism accumulate [lactic acid] and are then discharged into the bloodstream. This makes the blood more acidotic, lowering the pH. Bodily functions are inefficient and hindered by a low pH. The brain attempts to protect itself from acidosis with the blood-brain barrier, but these resources are eventually overwhelmed. In this case, the low oxygen saturation levels present over a prolonged period of time without being corrected, were predictive of permanent and severe brain injury. The patient had a pH of 7.31 40 minutes beyond the hypoxic event at which point the oxygenation had been corrected. The implication is that, while the correction was occurring, the pH was much lower than that, even if it was not measured. Yet even this seemingly small deficit is important to brain function and beyond this could be disastrous. A pH of below 7 is incompatible with life.
The base excess measures the amount of base that would have to be dumped into the bloodstream to bring the acidotic pH back to the 7.4 number that would allow optimal function. The higher the base excess, the worse the acidosis. It is a measure of the amount of acid being produced by the body and this is worse with hypoxia. The oxygen content of the blood changes rapidly and giving supplemental oxygen is immediately reflected in the O2 saturation. The base excess, however, takes much longer to correct, and the fact that, in this case, the base excess was so high after the oxygen had been corrected is an indication that this patient was acidotic for a significant amount of time prior to correction. This supports the claim that a patient under these circumstances would be expected to have severe and permanent brain damage due to the extent and duration of her hypoxic condition.
The testimony was important to the trial, though the case settled during deliberations.
Gary Gansar, MD, is residency-trained in general surgery. He served as Chief of Surgery and Staff at Elmwood Medical Center and on the Medical Executive Committee at Touro Infirmary and Mercy Hospital in New Orleans, LA. Dr. Gansar was Board Certified in general surgery while in active practice. He joined AMFS in 2015 as a Physician Medical Director.
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