Medical brain notes 7

  Anesthesia and other systems If you are reading this, you are not a neurologist, gastroenterologist, hepatologist, nephrologist, or hematologist. Yet, anesthesiologists need to worry about some features and functions of the stomach, liver, kidneys, blood, and particularly the brain. Here is a short perspective on the why and how. The brain General anesthesia is, ultimately, about putting the central nervous system (CNS) to sleep. We choose this or that agent in an effort to optimize the patient’s intra-operative course, but in reality the nuances of the different agents make little difference a few days after minor surgery in a healthy patient. However, in the patient with intracranial pathology, a thorough understanding of neurophysiol-ogy and the implications of anesthesia take center stage. Because we do not know which patients have undiagnosed cerebral aneurysms or tumors, we like to apply our understanding to all patients. The brain is an amazing organ. Despite weighing only...

The stomach The stomach should be empty before we give general anesthesia because regurgi-tating or vomiting and then, because of obtunded reflexes, inhaling the stuff found in the stomach can lead to serious trouble. The aspirated particulate matter can lodge in a distal bronchus, get infected, and result in bronchopneumonia or lung abscess. A large particle can block a mainstem bronchus or the trachea with obvi-ous dire consequences. Even in the OR, a patient can be treated with the Heimlich maneuver. Given an unconscious patient and the worry about more regurgita-tion and aspiration, tools such as a bronchoscope and suction available in the OR might be better suited for retrieval of foreign matter in the trachea or upper bronchial tree. More common than particulate aspiration is the aspiration of gastric juice. If it has a pH under 2.5 and a volume of more than 0.4–1.0 mL/kg, the aspirate can cause the infamous Mendelson syndrome, a nasty chemical burn of the lungs that can be fatal...

  The liver We expect this large organ to do its biotransformation magic on many of the drugs we give. For example, the liver avidly removes propofol, which is said to have a hepatic extraction ratio (HER) of close to 1. Reduced liver blood flow will, there-fore, reduce the rate of propofol biotransformation. The rate of biotransformation of drugs with a low HER, such as thiopental, will be less affected by changes in liver blood flow. Remember that the liver normally receives about 25% of cardiac out-put, roughly 2/3 of that via the low-pressure portal system, the rest by way of the hepatic artery delivering oxygenated blood. General anesthesia tends to reduce cardiac output and, proportionally, hepatic arterial blood flow more than portal blood flow. The hepatic circulation is also richly supplied with alpha recep-tors; hence the administration of alpha active vasopressors will reduce hepatic    blood flow. Because of the enormous reserves of the liver, we rarely see th...

  The kidneys The kidneys concern us when drugs or their products of biotransformation need to be eliminated in urine. For this route out of the body, the substances need to be non-protein-bound so that they make it through the glomeruli and are then ionized so as to escape tubular reabsorption.   Impaired renal function becomes relevant with advancing years (creatinine clearance declines with age), with low cardiac output and decreased glomerular filtration, and with renal disease. The elimination of some drugs can be affected by decreased renal function. Of greatest interest to the anesthesiologist are a number of muscle relaxants such as pancuronium and doxacurium and their antagon-ist, neostigmine. Thus for patients in renal failure, we might elect atracurium or cisatracurium, muscle relaxants that undergo hydrolysis in plasma making them independent of renal excretion. Patients in renal failure present special challenges not only because they cannot eliminate drugs in uri...

  The blood Three functions of the blood demand attention: its volume, its oxygen-carrying capacity, and its ability or propensity to clot. Volume Blood volume varies with age, weight, and sex (see Vascular access and fluid management). As we know from donating blood, the average adult can easily lose 500 mL without conspicuous consequences. Indeed, healthy patients can tolerate a blood loss of 20% of their total blood volume. The body compensates for such loss by mobilizing interstitial and eventually even intracellular water to replenish the decreased intravascular volume. In the process, the hematocrit will fall gradually over a couple of days. Oxygen-carrying capacity With a loss of blood volume, the patient also loses oxygen carrying capacity. Compensatory increases in cardiac output can insure uninterrupted delivery of oxygen, even in the anemic patient. As hematocrit decreases to about 30%, fluidity of blood increases, which improves flow and thus aids in the delivery of a h...