2018 First Sitting SAQs

etter answers divided oxygen carriage into that bound to haemoglobin and that carried in the dissolved form. A reasonable amount of detail on the haemoglobin structure and its binding of oxygen was expected, including an explanation of co-operative binding and the oxygen carrying capacity of haemoglobin. Better answers mentioned Henry’s law in the description of dissolved oxygen, along with an estimation of the amount of oxygen that is normally in the dissolved form. It was expected that answers include the equation for oxygen delivery, a brief description of the components of that equation and the normal value, which a large number of candidates omitted.

Some candidates failed to provide a correct definition of dead space. An outline of anatomical, alveolar and physiological dead space was expected. The Bohr equation was commonly incorrect, and many did not comment on how to measure the components of the Bohr equation. Fowler’s method was generally well described though some plotted the axes incorrectly. The impact of increased dead space was not often well explained. Very few people stated the major impact of increased dead space is reduced minute ventilation and how this would affect CO2.

A description of filtration and reabsorption, including amounts was required. Better answers described sodium handling in a logical sequence as it progressed through the nephron including the percentages reabsorbed in each segment. In addition to the amounts reabsorbed, the mechanisms of transport across the tubular luminal and basolateral membranes into interstitial space should have been described. aldosterone = stimulates Na+ reabsorption at the collecting tubules intra renal factors such as interstitial pressure which is lowered during hypovolaemia and reduced renal perfusion, thus promoting Na+ reabsorption gradient sympathetic nervous system – influences interstitial pressure and increases renin production angiotensin II – stimulates reabsorption at proximal tubule Atrial Naturetic Peptide/Factor – inhibits Na+ reabsorption Others – dopamine, cortisol, insulin => increase Na+ reabsorption, but minor factors Syllabus – D1, 2f Reference – Textbook of medical Physiology, Guyton, Chp 28

Well-constructed answers were presented in a table to compare pharmacokinetics and pharmacodynamics with a separate paragraph to discuss the concept of context sensitive halftime. Important pharmacokinetic points included: the differences in lipid solubility, ionised fractions and onset, and differences in metabolism. Marks were awarded for a definition of context-sensitive half-time. A discussion of these two drugs’ context-sensitive half-times should have included the differences in re-distribution into other compartments and rates of elimination.

Few candidates defined a buffer making it difficult to award 25% of the marks for this question.
The three main buffers in blood should have been described: bicarbonate system, haemoglobin
and proteins. The pKa, the buffering mechanism and the capacity of the system should have
been described. The Henderson Hasselbach equation was sometimes incorrect. Marks were
only awarded for buffers in blood and unfortunately some candidates described non-blood
buffers.

An outline of the blood supply from the oesophagus down to the anus was expected. Very few candidates knew the branches of the main 3 arteries and which portion of the gastrointestinal system they supplied. Concepts related to control of blood flow and autoregulation of blood flow were not asked and therefore marks were not awarded for this information.

Most candidates confused co-oximetry with other methods of measuring oxygenation of blood. Whilst there were a number of excellent descriptions of pulse oximetry, these attracted no marks for the first two sections. Structuring the answer based on the three parts asked, would have improved answers ensuring all aspects of the question were addressed.

Many candidates provided a broad overview of functions of the placenta but lacked detail. Placental blood flow has maternal and foetal components, though most only considered the maternal circulation to the placenta and didn't mention the foetal vessels. Many were not specific as to what blood vessels were described. Many stated that uterine blood flow is not autoregulated, however went on to describe myogenic and neuro-humoral mechanisms of autoregulation

This commonly used drug should be very well-known. The question asked for an outline, hence long explanations of various aspects of pharmacology (e.g. pseudocholinesterase deficiency) were unnecessary. Headings should have included: advantages (e.g. rapid onset, rapid offset, short acting, IV or IM administration, not end organ dependent for metabolism, premixed, safe in pregnancy and neonates). The disadvantages section should have included the following headings: pharmaceutical, adverse drug reactions (including several potentially fatal ones), numerous contraindications, unpleasant side-effects and potential problems with repeat dosing. 2012B 02: 13 (59.1%) of candidates passed. For a good answer candidates were expected to mention that an agonist is a drug that elicits a maximal response on binding to a receptor. A partial agonist has intrinsic affinity with only partial efficacy and hence is unable to elicit a maximal response. A competitive drug acts at the same binding site of a receptor as an endogenous ligand (e.g. acetylcholine at the neuromuscular junction) and its action therefore is surmountable with increasing concentrations of drug and how this concept relates to suxemethonium. For the remainder of the question, Candidates were expected to mention the advantages and disadvantages of suxamethonium within Intensive Care practice. Good answers included a systematic approach and use of tables and/or well organised lists.

Some answers suffered from listing things rather than describing things as the question required. Better answers included a description of metabolic, physical and neuro-humoral factors and the relative importance of each. Many described detailed anatomy which was not necessary.

Many answers lacked structure and contained insufficient information. Better answers defined diastole and described the mechanical events in the 4 phases of diastole. A common error was the ECG events in diastole. The electrical events and coronary blood flow should have been mentioned.

Whilst most candidates defined density correctly, there was a lot of uncertainty regarding viscosity. Most candidates recognised that flow may be laminar, turbulent or transitional. Most accurately recounted Reynolds number and applied this correctly. Additionally, the Poiseuille equation was correctly stated by most candidates and correctly related to laminar flow. Few candidates recalled the equation describing turbulent flow

Many candidates who scored poorly confused anticholinesterase drugs with anticholinergic drugs. Some described pharmacokinetics when it was not asked. Similarly, treatment of organophosphate poisoning and/or cholinergic crisis was not asked for in the question.

A definition and a normal value were expected. A description of the Monro-Kellie doctrine was expected. Better answers divided into the various components of the cranium with the answer focussing on cerebral blood volume and CSF volume as the brain tissue as no capacity to change its volume.

Most candidates gave an adequate definition of osmolality and tonicity. A single concise sentence for each attracted full marks. Some candidates drew diagrams & equations, which added few marks. Some candidates confused osmolarity (mOsm/L) and osmolality (mOsm/kg). Tonicity was best defined as the number of ‘effective’ osmols (those that cannot cross the cell membrane) in a solution relative to plasma. The use of a table greatly facilitated the comparison of 0.9% saline and 5% dextrose solutions. Values for composition, osmolarity and osmolality were poorly done. Some manufacturers state calculated values and some approximate values on the bags – both were accepted. No candidate correctly pointed out the fluids respectively have 9g NaCl and 50g dextrose per litre.

There were some good answers, though invasive BP measurement was better answered than oscillometry. Many candidates provided extensive detail in one area i.e. the workings of a Wheatstone bridge, to the detriment of a balanced answer. Few seemed to have a structure consisting of "equipment, method, sources of error, advantages, disadvantages" or similar and missed providing important information as a result. Several described auscultatory non-invasive blood pressure measurement, rather than oscillometry, which although related in principle is a different process.

The best answers were systematic, using a sensor, integrator, effector approach, while also mentioning physiological variations i.e. diurnal, with ovulatory cycle etc. Few candidates raised the concept of central and peripheral compartments. The differentiation of the concepts of set point, interthreshold range and thermoneutral zone was often confused.

Most candidates understood the endocrine functions of the hypothalamus, and to some degree its interactions with the pituitary. Fewer candidates mentioned the importance of the hypothalamus as an integrator for the autonomic nervous system, or its roles in arousal/emotions. Many candidates had only a vague idea of the structure of the hypothalamus, while the best candidates were able to relate function to structure quite accurately