Professional Development in Medicine

What you need to know, either to study altitude physiology or to monitor patients in the operating room or intensive care unit, is how to calculate alveolar oxygen pressure (PAO₂) and how to compare that calculated value with the measured arterial oxygen pressure (PaO₂). ‘P’ denotes pressure, of course (measured in mm Hg or torr, unless otherwise noted). Small capital ‘A’ denotes alveolar. Lowercase ‘a’ represents arterial. ‘PB’ is barometric pressure. ‘R’ is the respiratory quotient, which is simply the ratio of CO₂ produced by the body divided by the amount of O₂ consumed. ‘PH₂O’ is the vapor pressure of water. FIO₂ is the fraction of inspired O₂, with 1.0 equivalent to 100% inspired oxygen. PIO₂ is the partial pressure of inspired oxygen. This difference (PAO₂ -PaO₂), also termed AaDO₂, gives an estimate of how efficiently the lungs are oxygenating the blood. There are several physiologic causes of hypoxemia. Hypoventilation, lowered PIO₂, and lowered PB will not increase AaDO₂ . The other 3 will. ...Less

To begin with, the bird lung is invested with a series of hollow air channels, not dead-end alveoli. These air channels are connected to a network of thoracic air sacs. When the air sacs contract and relax in coordination, they force inspired air through the air channels in an almost continuous, unidirectional flow. The air channels contact the pulmonary vessels in an architectural array that looks suspiciously like a countercurrent exchange device. Countercurrent exchange also is engineered into blood infusion devices to rapidly warm intravenous fluid or blood for patients with extensive hemorrhage. Countercurrent exchange takes advantage of the concept that 2 media will exchange properties between themselves more efficiently if they are very close to each other and flowing in opposite directions. There is also crosscurrent exchange. In a crosscurrent arrangement, one material divides into multiple parallel branches, and each of these branches crosses over the other material at multiple points. Although crosscurrent exchange looks similar, it is not quite as efficient as countercurrent exchange. There was controversy about whether the bird lung used crosscurrent or countercurrent gas exchange. ...Less

The sine qua non of ventilation is arterial carbon dioxide. If you want to know about ventilation, just check the PaCO₂. If it is low or normal, ventilation is fine, regardless of any other parameter, including respiratory rate, tidal volume, or dead space ratio. However, if PaCO₂ is high, then alveolar ventilation (VA) is impaired (relative to the carbon dioxide load being presented to the lungs). In a conventional breathing circuit, dead space ends at the Y-shaped junction of the inspiratory and expiratory arms of the circuit and the endotracheal tube. On the machine side of that junction, the inspiratory and expiratory limbs see only fresh inspired or expired gas, respectively, but not both. You should know 2 other things about ventilation. One is the Bohr equation, which estimates the ratio of dead space to tidal volume. The anatomic dead space is estimated as the expired volume that coincides with half maximal nitrogen content. The second thing is the effect of gravity on the distribution of ventilation within the lung. ...Less

The pathway of oxygen through the body consists of the diffusion of oxygen across the alveolar-capillary membrane and then the peripheral tissue membranes, followed by the convective transport of oxygen in the blood. Any transport process will have its choke points and limitations. In the case of oxygen, the constraints can take 1 of 2 forms, perfusion limitation or diffusion limitation. ...Less

Any acceptable model must explain why the cerebral circulation must be robust enough to contend with different head positions yet continues to work in weightless environments (ie, during space flight) and it must explain the clinical phenomenon of air embolism, which is what happens when air gets sucked into perforated veins or sinuses when the surgical site is above the heart. 2 models of the cerebral circulation: the first is a common-sense model that most of us would draw without having thought about it much; the second is a model that makes a lot more physical sense and answers the questions above in a way that the first model cannot. The main difference between these models is that the correct one takes into account the principle of the siphon, whereas the incorrect one does not. A siphon is any arrangement of fluid-filled tubing that excludes air and is open on both ends to allow flow. It happens that the cerebral circulation functions like a siphon. ...Less

Suppose that your heart has just stopped. What would happen to your blood pressure? At least 2 things would happen that you might not predict (and I hope you won't discover them anytime soon). First, the various blood pressures in the different parts of your circulatory system would converge to the same value. Second, you might be surprised to find that your blood pressure is not zero. That's not just because of vertical (hydrostatic) gradients within the body. Because the blood volume is considerably greater than the passive circulatory system volume, the blood vessels are slightly stretched and maintain a non-zero pressure even after the heart stops. To determine the actual non-zero pressure during cardiac arrest, we only have to divide the stressed blood volume by vascular compliance. ...Less

Palliative medicine is the physician discipline within the broad therapeutic model known as palliative care. Education and training in this discipline and model of care are devoted to producing an expert physician who participates in achieving the best possible quality of life for patients facing a life-threatening illness and for their families. The goals of the palliative medicine specialist are realized through the relief of suffering and the control of symptoms throughout the course of illness, including the time of death and the period of bereavement. ...The practice of palliative medicine requires comprehensive age-appropriate assessment of the patient and family and participation in interdisciplinary, inter-professional care that can address physical, psychological, social, and spiritual needs. Palliative care helps the patient and family manage advanced illness and the prospect of death, assured that comfort will be a priority, values and decisions will be respected, spiritual and psychosocial needs will be addressed, practical support will be available, and opportunities for emotional growth and closure will exist.Training in palliative medicine extends beyond competency in clinical care. A specialist in palliative medicine will want to acquire competency in education, programme-building, research, administration, and process (quality) improvement. In addition, because the specialist serves as a consultant to other physicians, a mastery of consultation etiquette is required.The structure of a training programme in a particular country or setting will need to relate to the structure of the larger ‘house’ of medicine for which all physicians are trained. This chapter presumes that some readers want to make the case for specialist training in palliative medicine as well as advocate the development of new programmes to train specialist physicians in their own centres or countries. We imagine that this chapter, which has a US focus, adapted for a specific country, could serve as the document that could be submitted to a Minister of Health or Minister of Education in support of the development of palliative medicine training. Therefore, we start broadly with a rationale for specialist palliative medicine training as part of the professionalism of medicine and its social contract. Then we describe how training programmes are structured as they relate to the prevailing scheme for training of doctors. Finally, we give some specifics about structuring a training programme, including evaluation of specialist competencies.Less

If you're going to understand how to think about pressures within the circulatory system, though, you'll need to know a few of the not-so-obvious principles of hydrostatics. Hydrostatic pressure is affected only by the density of the fluid, its vertical height above or below a measurement point, and the acceleration due to gravity. If the larger mass of water in the ocean exerts greater pressure than the smaller mass of water in your vertical pipe, then you should see water perpetually flowing through the pipe and rising higher than sea level. Continual flow doesn't happen in this example for the same reason as described above—hydrostatic pressure is not determined by the shape of the fluid container or the total mass of fluid that it contains. Here, only the vertical distance above the measurement point is important. Now let's apply these principles to the siphon, defined as any fluid-filled conduit that excludes air. The reason for broaching this issue is that the cardiovascular system also obeys the principle of the siphon. ...Less

We've already looked at 2 types of pressure that affect physiology (atmospheric and hydrostatic pressure). Now let's consider the third: vascular pressures that result from mechanical events in the cardiovascular system. As you already know, cardiac output can be defined as the product of heart rate times stroke volume. Heart rate is self-explanatory. Stroke volume is determined by 3 factors—preload, afterload, and inotropy—and these determinants are in turn dependent on how the left ventricle handles pressure. In a pressure-volume loop, ‘afterload’ is represented by the pressure at the end of isovolumic contraction—just when the aortic valve opens (because the ventricular pressure is now higher than aortic root pressure). These loops not only are straightforward but are easier to construct just by thinking them through, rather than by memorization. ...Less

Physiologist Claude Bernard lived in a time when very little was known about the mechanisms underlying physiologic findings, and he had ample access to clues garnered from observing machines. Let's consider homeostasis (a concept championed by Bernard), an example for which an engineered machine shed light on a fundamental principle of physiology. Homeostasis is simply the tendency of the body to maintain important physiologic variables (eg, heart rate, blood pressure, PACO₂) at constant, preset values. An example is a simplified mechanical governor that could be used to regulate the rotational speed of a steam engine shaft. ‘Autoregulate’ might be a more apt word because the governor performs without external help or guidance, provided it is designed and built properly. It doesn't take much imagination to see an analogy between the mechanical governor and the autonomic nervous system. Both maintain specific variables at a constant set point through a process of feedback loops. ...Less