A free online resource for Intensive Care Medicine.
An unofficial Fellowship Exam (CICM Part 2) preparation resource.
Deranged Physiologyis a slowly growing archive of discussions and study notes relevant (or if not relevant, then at least interesting) to the practice of intensive care medicine. The content provides an introduction to the fundamental themes in intensive care: mechanical ventilation, vasopressors, electrolyte management, hemodynamic monitoring, dialysis, and so forth. Attention is directed at equipment in intensive care, and there are attempts to revisit interesting pharmacology and physiology. The aim of this resource is to supplement the bedside teaching of senior staff, and to consolidate resources for intensive care trainees in the initial stages of their training.
Pulse oximetry relies on tow main principles: different absorption of different light wavelengths by haemoglobin species, and the ability to isolate the pulsatile arterial signal because of pulse-related changes in optical distance. The physical principle which underlies this measurement is the Beer-Lambert law (the measured absorbance for a single compound is directly proportional to the concentration of the compound and the length of the light path through the sample).
There are several ways to classify lactic acidosis. The classical system separates lactic acidosis according to its pathology, whereas more novel systems classify it according to physiology. The atrractiveness of the incumbent system lies in its simplicity, rather than its validity. Its categories are however flawed when it comes to categorising complex pathophysiological processes, such as septic shock. In counter-argument, new classification systems are equally bad at classifying complex disorders. They seize the advantage of pathophysiological completeness and gain in explanatory power while sacrificing simplicity and memorability.
This is an organised summary of the physiological effects of hyperoxia, if one ever needs such a summary, for any reason. It is hard to structure information like this because the effects of increasing oxygen exposure are often dose-dependent and specific in each organ. Therefore, the best one can do is to organise the discussion into a table, by organ systems, and then by oxygen concentration.
Capnometry is the measurement of the concentration of CO2. Capnography refers to the graphic display of this measurement over time.
The most common methods in routine use are IR spectroscopy and colour change colourimetry. The expired CO2 measurement has value in detecting oesophageal intubation, and the waveform can be used to identify airflow limitation, but the end-tidal value itself is not pathology-specific or diagnostic, and false positives are possible.
The Severinghaus electrode is essentially a slightly modified glass electrode. The CO2 dissolved in the sample diffuses into the middle compartment of the electrode via a thin membrane. Once inside, the CO2 finds itself in an aqueous solution. For convenience, there may or may not be a bicarbonate solution added to this chamber. The reaction which takes place is an old familiar carbonic acid dissociation equilibrium. Thus, the pH of the solution in the middle chamber changes. The change in pH is completely dependent on the pCO2, provided the temperature and pressure remain constant. This results in a change in potential difference in the glass electrode; and the function of this item has already been discussed at some length in another chapter. Thus, from the change in pH, one can calculate the pCO2.
Normal PaCO2-EtCO2 difference is 2-5 mmHg. This is due to alveolar dead space, which is small in healthy adults. It may increase in disease states (eg. where alveolar dead space is increased, or where V/Q matching is affected) or as thew result of a measurement error.
An excellent companion to the study of these concepts can be found at the CCC, courtesy of LITFL: their Cardiovascular Physiology Overview benefits enormously from flash-animated diagrams. This is merely a brief point-form summary to help the time-poor exam candidate. In fact, those candidates who are poorest can completely omit this vanity topic, as it is has never appeared in the past papers.
The arterial pressure wave (which is what you see there) is a shockwave; it travels much faster than the actual blood which is ejected. It represents the impulse of left ventricular contraction, conducted though the aortic valve and vessels along a fluid column (of blood), then up a catheter, then up another fluid column (of hard tubing) and finally into your Wheatstone bridge transducer. A high fidelity pressure transducer can discern fine detail in the shape of the arterial pulse waveform, which is the subject of this chapter.
Myoglobin is a small monomeric haem protein found in skeletal muscle and myocardium. It contains one oxygen binding site (for one O2 molecule). The oxygen-myoglobin dissociation curve is hyperbolic rather than sigmoid. Myoglobin has a very high affinity for oxygen: the p50 is ~ 2.7 mmHg. Its role is to maintain the oxygen supply to exercising muscle. The total oxygen store of myoglobin in the human body is around 200-300ml, equivalent to about 7-10 seconds of muscle activity/p>
This has not appeared in the past papers. However, the great Competencies document makes mention of it. Thus, if an SAQ ever came up on this topic, it might ask the candidates to "Discuss the features which identify patients at risk of developing ARF, and discuss some of the therapies that may have prophylactic benefits." Or perhaps "Discuss the influence of ARF on the morbidity and mortality of ICU patients", which sounds like some sort of creative writing task. Question 29 from the first paper of 2005 has some vague relationship with prevention of ARF, but it is in context of AAA repair, and more closely related to rhabdomyolysis and contrast-induced nephropathy.
In spite of being a vitally important metabolite and an essential ingredient of normal multicellular life, oxygen is nonetheless not excused from being treated as a drug, with side effects and contraindications. Given that virtually all patients in the ICU end up receiving oxygen, it seems reasonable to expect the competent ICU trainee to be intimately familiar with its properties - as it would be madness to funnel litres of something into one's patients while knowing nothing about its effects.
Foetal haemoglobin (HbF) is made up of two α-subunits and two γ-subunits (instead of two β-subunits), which decreases its affinity for 2,3-DPG and increases its affinity for oxygen. Because of the efficiency increases afforded by the high foetal haematocrit and the double Bohr and Haldane effects, placental gas exchange produces a high foetal blood oxygen content.
The Bohr effect describes the decrease in the oxygen affinity of haemoglobin in the presence of low pH or high CO2. Both pH and CO2 stabilise the deoxyhaemoglobin molecule and decrease its affinity for oxygen, which facilitates the release of oxygen in the peripheral tissues. Quantitatively, the changes in pH play a greater role in changing the shape of the oxygen-haemoglobin dissociation curve than do the changes in CO2.