2015 Second Sitting SAQs

A well organised answer would provide details on domains of physiology, pharmacology and “other”, such as patient centred effects. Few answers mentioned pharmacology. Providing a definition of “mild” was often overlooked but doing so assisted a focused answer. Many answers spread beyond “mild” hypothermia or address the “causes” rather than the consequence – neither of which gained marks as they were not answering the question asked.

It was expected candidates would describe the salient features of the anatomy, distribution and content of blood flow and influences on each circulation. Answers with a clear organisation and context for the normal influences of blood flow on the functioning of each organ system scored highly. Anatomy was often sufficiently covered, but candidates often did not take advantage of that by linking the anatomical features to the functional concepts. Figures should be clearly and accurately labelled to score well. Many answers failed to demonstrate a depth of understanding of key concepts. For example tubuloglomerular feedback, relationship between hepatic arterial and portal venous flows and autoregulation within both those systems was often poorly described.

This question requested a definition AND a description of measurement (one method if correctly discussed could and did generate a pass mark) although additional marks were awarded if multiple measurement methods were mentioned or described. Detailed descriptions of the factors effecting FRC and its functions were NOT requested and scored no marks. "Fowlers method" uses 100% oxygen and nitrogen analysis to calculate anatomical dead space - NOT FRC - so scored no marks. Both Helium dilution and nitrogen washout (with 100%oxygen) enable calculation of FRC using C1V1=C2V2 where V2 = V1+FRC. Body plethysmography requires more complex calculations of P1V1= P2V2 (Boyles Law) applied twice = for the box and then the lung. Few candidates had a clear understanding of this method. Most answers did not demonstrate the depth of understanding of the measurement techniques that was required to score highly

Marks were awarded for an appropriate curve with values, an explanation of the nature of positive cooperatively and notes on those factors causing changes in the p50 or "shifts” in the curve. Most candidates were able to provide the required sigmoid shaped curve with appropriate key value points (p50, venous and arterial points). Better candidates were able to identify p50 as a measure of avidity or affinity for oxygen and commented on T- Tense and R- Relaxed states, the role and production of 2,3 DPG , binding to the beta chains (nature of the lack effect on foetal haemoglobin). Describing the mechanisms associated with factors shifting the curve and commenting on changes in oxygen content over the steep and flatter parts of the curves gained additional marks. Candidates are reminded to answer the question asked - no marks were awarded for a description of dissolved oxygen delivery. Some answers confused the Bohr and Haldane effects.

It was expected that the answer would include comments on lactate generation from glucose via pyruvate and the metabolic linkage of nicotinamide adenine dinucleotide (NAD). Lactate regenerates NAD+ (pyruvate is reduced to lactate while NADH is oxidized to NAD+ ). The citric acid cycle and the electron transport chain occur in the mitochondria of cells, and will only proceed in the presence of oxygen. One molecule of glucose produces 2 ATP anaerobically (pyruvate to lactate) vs 26 aerobically (pyruvate enters TCA cycle) .Total production is about 1500 mmols/day with blood levels resting value of 1–1.5 mmol/L to a peak of 10–15 mmol/L. Lactate can be used in 3 ways: 1. Conversion to glucose via gluconeogenesis in the liver and release back into circulation (Cori cycle). This is the fate of 80 % circulating lactate from tissues low in oxygen (e.g. exercising muscle with low pO2) or red blood cells (no mitochondria). The production from glucose in RBC’s is the Embden-Meyerhoff pathway. 2. Consumed as a fuel e.g. heart (20% of circulating lactate) 3. Mitochondria and oxygen Oxidation back to pyruvate by well-oxygenated muscle cells, heart cells, and brain cells pyruvate is then directly used to fuel the Krebs cycle (generating 28 mmols ATP) Lactate generation from muscle is increased with B1 mediated stimulation e.g. from adrenalin. This topic is well covered in Power and Kam Principles of Physiology for the Anaesthetist, 3rd Edition, although some of the details are in several different sections. Most candidates showed some understanding of the role of glucose in the production of pyruvate to lactate. However, the differential ATP production, the role of NADH availability and how oxygen and the role of mitochondria were involved was less well handled. Better answers described the normal generation of lactate in some tissues (e.g. RBC) and role of muscle and liver in metabolism back to glucose (Cori cycle) and the role of lactate as a metabolic substrate in some organs. Marks were awarded for normal production values and blood levels.

It was expected answers would include a diagram of a cross section and label the lumbar epidural space and the key landmarks namely dura, subarachnoid space, epidural space. Most candidates were able to give a schematic representation even if not being able to draw. Some candidates confused the subdural space with the epidural space. Pharmacology of bupivacaine needed to cover both pharmacokinetics and pharmacodynamics. Several candidates addressed only one of these components and so missed the opportunity to score marks.

The question was very specific for the cardiovascular system and therefore answers that described respiratory changes and airway modulation failed to score marks. This answer leant itself to a tabular format. Candidates are reminded to ensure they document the facts in the correct column i.e. obesity facts in the obesity column. The cardiovascular changes associated with term pregnancy are well described in various texts. Those associated with morbid obesity required some integration from various sources and would include a structured series of comments such as heart rate (unchanged), blood pressure (tendency for hypertension), stroke volume (increased), cardiac output (increased), blood volume (increased – although perhaps decreased on a ml/kg basis), systolic function (preserved or increased), LV wall thickness increased. The pathological changes seen with the diseases associated with obesity are difficult to tease out and better answers identified this. Morbid obesity has a specific definition and stating this aided focus of the answers.

Candidates were expected to be detail the principles required to measure carbon dioxide in expired gas. This would involve some comment on ways to sample end tidal gas (in line vs side stream) and also ways to measure carbon dioxide. It was expected candidates could provide a detailed description of infrared analysis including the apparatus design and principles such as the asymmetric nature of CO2 as a polyatomic gas allowing absorption if infrared radiation with some discussion of response times and equipment design. It was expected these principles would be related to the potential errors (e.g. other gases, collision broadening)

It was expected answers would describe in detail the role of troponin, tropomyosin and calmodulin in mediating muscle contraction. Detail on the structure (histology) of the skeletal and smooth muscle cells was often lacking. Many answers omitted the mechanism of muscle relaxation.

A definition was required that included reference to plasma concentration, total body content / dose, and the theoretical nature of the volume which can exceed the physical volume of the body. A broad answer listing patient and drug-related factors was required. Patient factors could include age, gender, muscle mass, fat mass and abnormal fluid distribution (oedema, ascites, pleural effusion). The drug factors would include tissue binding, plasma protein binding and physicochemical properties of drug (size, charge, pKa, lipid solubility, water solubility).

The most comprehensive answers were those structured as sensor-controller-effector with an explanation of each part and how homeostasis was maintained. Insufficient detail was generally provided as to how central and peripheral chemoreceptors were stimulated. A description of central control was required, rather than listing nuclei or areas. Many failed to address all three components of a control question and focused primarily on the sensors. Many answers were just too brief and did not present enough information to demonstrate understanding.

This question required synthesis and application of knowledge derived from multiple sources rather than regurgitation of a published list in a text. Many candidates failed to recognise that venous return is not the only determinant of preload. Most candidates failed to discuss determinants of venous return. Factors such as contractility, afterload or chamber filling and emptying can all impact preload. In addition to listing determinants the question required an explanation of their relationship with preload (e.g. the direction of change). A discussion about the determinants of cardiac output was not asked for as did not score marks.

It was expected candidates would identify sources of ammonia (colon from metabolism of proteins, kidney, small amounts from breakdown of red blood cells and metabolism in muscles). The liver converts all circulating ammonia to urea (the urea cycle) (2NH3+CO2 = urea +H2O). Urea then undergoes enterohepatic circulation (25%) or is excreted by kidneys (75%). Ammonia (NH3) is lipid soluble and diffuses into the interstitial cell and tubular fluid by non-ionic diffusion where is buffers H+ to become non diffusible NH4+. No candidate mentioned enterohepatic circulation and most answers had very little detail on the metabolism and excretion and lacked depth. Lactulose is a non absorbable synthetic, non-digestible disaccharide. It is an osmotic laxative fermented by gut flora producing metabolites (such as acetate) which have osmotic and peristalsis-stimulating effects, and methane causing in flatulence. Few could describe how lactulose decrease absorption of ammonia and a surprising number of people did not even state that lactulose was an osmotic laxative.

Few candidates demonstrated a good knowledge of this topic. Few answers described the EEG changes associated with the stages of sleep. Respiratory changes in sleep were more commonly known though many candidates made no reference to the change in resistance associated with reduction in upper airway tone. Confusion existed about the tidal volume changes in sleep. The question asked specifically for respiratory changes and marks were not awarded for discussion about cardiovascular or metabolic responses.

The fibrinolytic pathway is a cascade largely made up of proteolytic enzmes and other factors synthesized in the liver that circulate in inactive precursor forms. Marks were awarded for description of the principal members of the cascade and the pathway relations between them. Endothelium is also important in the fibrinolytic pathway. Regulation of the pathway to localise the site and size of clot as well as delayed onset of action of fibrinolysis is central to any description. Regulation of fibrinolysis by the coagulation cascade and a description of this area of interaction were expected. Many candidates provided a reasonable description of the fibrinolytic cascade. Marks were not awarded for description of the coagulation cascade that did not have relevance to fibrinolysis. Understanding of regulation of fibrinolysis and it’s interaction with coagulation was poorly answered. Most candidates were able to name two antifibrinolytic agents. Few were able to describe mechanism of action.

The diaphragm is the principal muscle of respiration. Important and unique features of its anatomy include a central tendon that blends with the pericardium above and the fibrous capsule of the liver below, arcuate ligaments and crura that are important points of muscle insertion. There are also three major and three minor openings that allow passage of structures between the thoracic and abdominal cavities. Candidates who had studied anatomy of the diaphragm were clearly distinguishable from those who had not. Candidates who followed a traditional template for anatomy answers scored better, providing answers that covered the breath of the topic.

Most answers gave a good account of counter currents in the kidney but few described the generation of the counter current multiplier arrangement. Many candidates did not discuss counter current mechanisms in the skin but instead, focussed on thermal control by the skin.

It was expected candidates would define key concepts, particularly 'mixed venous'. Many candidates knew some of the elements that contributed to mixed venous PCO2 but few described all of the main factors. There was little mention of tissue capillary flow as a factor affecting mixed venous CO2.