2016 Second Sitting SAQs

As stated in the question, the distribution in plasma (not body) was expected. Candidates are reminded to include units [mmol/l]. Many candidates spent considerable effort describing the roles of calcium and particularly its role in excitation contraction coupling – which was not asked and hence scored no marks. It was expected that candidates were able to identify the roles of parathormone (PTH), 1,25 OH vitamin D and calcitonin as the major hormonal regulators. Better answers were able to describe the physiology and integration of these hormones at the gut, kidney and bone. Calcitonin and its transient and (probable) minor opposite role were also identified in these answers. Better answers were also able to identify the permissive roles of growth hormone, cortisol and thyroxine.

Good answers included a description of Starling forces acting at the glomerular basement membrane. A description of the local and systemic factors influencing each component was expected. It was expected candidates would discuss autoregulation of GFR & RBF, tubuloglomerular feedback, and integrated responses the body uses to keep GFR steady. Confusion about the nature of induced effects on afferent or efferent arteriolar dilation and constriction limited marks for some candidates. Many failed to mention the effects of mesangial surface area, Bowmans space pressure or serum protein content. Candidates were expected to outline the methods of GFR estimation. Better responses described the rationale behind the use and limitations. Creatinine clearance, inulin and nuclear medicine techniques all scored marks. Some candidates made no attempt at this section and missed the opportunity to score marks. Estimates of CrCl/GFR [eGFR by formulae such as Cockcroft Gault, and serum Cr] are not measurement of GFR.

Asthma drugs are typically categorised according to mechanism of action. A reasonable alternative is to categorise by clinical use, e.g. short acting, long acting, preventer, rescue etc. A lot of emphasis in marking was placed on an understanding of the beta-adrenergic pathway, its secondary messenger system and how this medicates smooth muscle relaxation. Candidates whose answers had structure as well those who described the wide range of drugs used to treat asthma scored well.

Pressure in a system is generated by the interaction between flow and resistance. A structured approach to defining and describing the many factors that influence fluid flow and resistance was required to score well. Poiseuille’s law describes the determinants of resistance to laminar fluid flow and provides a useful answer structure. It is also necessary to describe factors that determine flow. This includes factors that determine venous return, as well right and left heart output. A standard structured answer to the pharmacology of nitric oxide enabled concise and high scoring answering of this question.

Candidates who used a structured approach of using formulae that describe resistance fluid flow scored well. Poiseuille’s law describes the determinates of resistance to laminar fluid flow and provides a useful answer structure. The most common mistakes were confusion between resistance and compliance as well as failure to describe turbulent as well as laminar flow.

An integrated answer to supply and demand of oxygen was expected, as a comparison between the right and left ventricles. Many candidates concentrated on differences not similarities. Myocardial oxygen demand was in general poorly described. About 85 - 90% of oxygen demand is for internal work (major determinants wall tension 30 - 40%, heart rate 15 - 25%, myocardial contractility 10 - 15%, basal metabolism 25%). 10 - 15% of oxygen demand for external work or pressure volume work, determined by MPAP x CO. It was expected answers would comment on the phasic nature of coronary blood flow which differs between left and right and the consequence of this to subendocardial oxygen supply during systole usually. Coronary blood flow is affected by coronary perfusion pressure (determined by aortic pressure and RV pressure) & coronary vascular resistance (determined by autoregulation, metabolic factors, humoral factors, nervous control interacting with local endothelial factors) Generally, coronary blood flow is tightly coupled to oxygen demand/consumption due to high basal oxygen consumption (8 - 10 ml/ min/100g) and high oxygen extraction ratio (75%). Better answers noted that oxygen supply can only be increased to cope with increased demand only by increased coronary blood flow.

It was expected candidates would detail the process of IgE mediated type I hypersensitivity reaction with some discussion of the mediators (Histamine / tryptase and others) and their consequences. Some detail describing time frame of response and the pre-exposure to Antigen (or a similar Antigen) was expected. Drug treatments would include oxygen and fluids as well as more specific agents such as adrenaline and steroids. Adrenaline is the mainstay of therapy and some comment on its haemodynamic role and prevention of ongoing mast cell degranulation was required. Better answers noted steroids take time to work and some also discussed the role of histamine blocking agents.

A good answer included a well labelled sketch with a description of ion channels and relative directional flow. When using sketches in an answer they should be correctly labelled, and when used as a comparison with another sketch, the differences should be clear e.g. shape, duration and voltage difference.

A statement regarding the quantum of hepatic blood flow with recognition of the contributions made by the Hepatic Artery and Portal Vein, drainage into the sinusoids before entering the hepatic vein which drains into the IVC would have been a good start. Discussion was then expected to revolve around how the liver blood flow is controlled. Answering this with respect to intrinsic and extrinsic factors along with an understanding of the semi-reciprocal relationship between hepatic arterial and portal venous blood flow would have rounded out a good answer to the first part of the question. The second part of the question required recognition that hepatic clearance is the product of hepatic blood flow and the hepatic extraction ratio and considering the impact on drugs with a high or a low extraction ratio. Many answers failed to adequately mention how hepatic blood flow was controlled/regulated thus limiting the marks available. Similarly, in the second part of the question, many candidates spent considerable time mentioning the principals of drug metabolism rather than focusing on the question asked. The concept of hepatic drug clearance as the product of blood flow and its extraction ratio was poorly appreciated.

This question was best answered by considering form and then function. Detailing red cell size, that it is a biconcave disc, contains haemoglobin A (Hb F in-utero), has a central Fe moiety and demonstrates positive cooperativity in binding oxygen would be a good start. Additionally, noting that the RBC has a flexible membrane with shape maintained by structural proteins and that it lacks a nucleus, organelles and mitochondria, but contains carbonic anhydrase would pass this question. A complete answer would mention the3 shunts that come off the anaerobic glycolytic pathway (RBC’s only means of ATP generation), namely the production of 2-3 DPG via the RapoportLuebering shunt, generation of NADPH by the hexose monophosphate shunt (protects RBC from oxidative damage) and the reduction of metHb back to Hb by way of the NADH. Many answers lacked sufficient information to pass this question. Many answers included lengthy discussions about the production of RBC’s, the Oxyhaemoglobin dissociation curve or calculated the oxygen content of blood. RBC metabolic adaptations (e.g. 2, 3-DPG, NADPH production by the HMP shunt/ G6phosphatase to regenerate glutathione and metHb reductase) were rarely mentioned, as were vasodilatory mediators released by RBCs.

The best answers used tables and key pharmacological headings for comparisons, and avoided long sentences/ paragraphs. An answer that correctly considered the following sections would be awarded a very good pass: Presentation, pharmacodynamics, mechanism of action, organ effects, side effects and pharmacokinetics. Many candidates failed to identify agents as natural / synthetic catecholamines. Few answers correctly mentioned the available preparations of these drugs or considered the structure activity relationships. Only 3 candidates commented that dobutamine is a racemic mixture. Intracellular second messenger pathways were often incorrectly recounted or not mentioned at all. Pharmacodynamic effects on all organ systems, and all CVS parameters (HR, inotropy, PVR, SVR, SBP/DBP/MAP, regional circulations) should be considered. Metabolic fate and clinical dosage ranges were frequently incorrectly quoted.

A well labelled diagram drawn clearly to demonstrate the salient features of the aortic and radial arterial pulses was expected. This would include the different systolic pressure, the absence of a dicrotic notch in the radial pulse (instead a diastolic hump), the narrowness and the delay of the radial pulse, garnered many marks. Marks were lost for insufficient explanation such as the distance needed to travel for the pressure wave accounting for the delay, the sharper rise and decline of the radial pulse due to loss of the WIndkessel effect and the different compliance and the loss of the dicrotic notch due to summation and damping out of high frequency components of the pressure wave.

It was expected answers would include an explanation of the Monro-Kellie Doctrine. Many candidates gave insufficient details of compensatory mechanisms especially regarding decreased total cerebral blood volume (primarily venous) in response to increased intracranial pressure. Most candidates had all the information but had difficulty synthesising the information to write a cohesive answer. Factors affecting ICP could be divided into factors affecting CBV, factors affecting CSF and factors affecting brain tissue. Under factors affecting CBV the effect of blood gases, autoregulation, temperature, metabolism, drugs and venous obstruction could have been detailed.

The equations for nett fluid flux and for nett filtration pressure were incorrect in many answers. Better answers presented the equations and discussed each of the elements as relevant to the pulmonary capillary bed, including difference from systemic capillary beds. Mention of the role of lymphatics and of the effect of surfactant, left atrial pressure, gravity and posture gained marks, also.

A satisfactory answer required use of terms radiation, convection, conduction etc. in the manner defined in the texts, rather than the layman’s use of the terms. Better answers displayed understanding of the meaning, relative importance and mechanism of methods of heat loss. The second part of the question required application of these concepts in patients with artificial airway and sedatives, particularly change in control of vascular tone and voluntary behavioural control.

Most answers displayed a good knowledge of the Vaughan Williams classification, classes I to IV and the relevant electrophysiological characteristics of the classes. Answers should also have included mention of other antiarrhythmics, such as digoxin, magnesium and adenosine. The second part of the question required comment about K ion blockade and its effects. It was helpful to mention prolongation of QT and risk of torsade. Most answers omitted reference to its being a racemic mixture, with different actions of the isomers.

Good answers to this question were those that included a definition of diffusion; an outline of Fick’s Law of Diffusion and then a further description how each of the variables in the this Law affect the diffusion of oxygen and carbon dioxide in the lung; and an outline of the other factors that affect diffusion not covered by the above. Most candidates included Fick's Law in their answers and at least briefly expanded on the associated variables. Few candidates defined the process of diffusion. The other common omissions were the factors that affect diffusion that aren't directly encompassed in Fick's Law, such as cardiac output, capillary transit time, carbonic anhydrase (conversion of HCO3 to CO2) and combination of oxygen with haemoglobin.

etter answers included details of the significant structures related to the cervical and mediastinal trachea and bronchi. The lobar branches and bronchopulmonary segments requiring naming to attract full marks. Many answers lacked sufficient detail or contained inaccuracies regarding vertebral levels and key structural relations. Some candidates discussed the general anatomy of the airway, including the larynx, structure of the airways, blood supply and innervation. This did not attract marks.