2019 Second Sitting SAQs

It was expected candidates would provide details the consequences of water ingestion from its rapid absorption in the small intestine to the resultant impact on plasma osmolarity and the minimal impact of plasma volume of this volume. Some detail on the mechanisms of absorption (transcellular vs osmosis) was expected and the distribution of water across body fluid spaces. Many candidates accurately described the small drop in plasma osmolarity that is sufficient to trigger osmoreceptors with better answers providing details of the locations and mechanisms involved. The physiological consequences of inhibition of ADH, including the renal effects of decreased water permeability in distal renal tubules and collecting ducts. The volume load after distribution would be lower than the plasma volume triggers for the circulatory reflex responses.

This question was well answered by most candidates. The description of renal flow involves a brief comment of the anatomy including interlobar, arcuate, interlobular arteries, then afferent and efferent arterioles – 2 sets of capillaries and then corresponding veins and better answers made the distinction better cortical and medullary flow and went on to detail the consequence of this. Renal blood flow is autoregulated and most candidates describe well the various mechanisms around myogenic and tubuloglomerular feedback. Additional marks were gained with by discussing renal vascular resistance and how this may be varied. The impact of adrenoreceptor agonists is varied but generally sympathomimetic agents will vasoconstrict and therefore increase renovascular resistance and result in a decrease renal blood flow. The relative impact on afferent vs efferent arteriolar tone may alter glomerular perfusion pressure.

Some detail was expected on a general description that tension is variable with the length of muscle. It was expected answers would describe that there is a resting length at which tension developed on stimulation is maximal. Many candidates omitted that differences exist between muscle types with smooth muscle behaving differently. Additional credit was given for the distinction about active tension vs resting tension. It was expected a description of the potential mechanism would be included with discussion of sliding filament theory, overlapping fibres and optimal sarcomere length. Some candidates utilised a diagram effectively to convey understanding and more detail was rewarded with additional marks. The second half of the question involved describing how this relationship is particularly important in cardiac muscle and underpins the Frank Starling relationship and all the cardiac physiology that follows. Initial length of fibres is determined by the diastolic filling of the heart, so pressure developed is proportionate to the total tension developed. The developed tension increases as diastolic volume increases to a maximum (the concept of Heterometric regulation). Better answers appreciated that the physiology may be different for a while heart rather than isolated muscle fibres.

To pass this question, the following were required for each section (trachea and main bronchi): landmarks; basic structural anatomy; and important relations (major vessels; major nerves; major structures). Marks were also allocated for innervation, and blood supply and venous drainage of the trachea. Most unsuccessful answers did not address a number of these areas. Overall, the answers were better for tracheal anatomy compared to bronchial anatomy. A structured approach to anatomy questions works well and this was again the case (i.e. relations / blood supply / etc.

It was expected that answers include central venous blood volume, central venous vascular compliance, intrathoracic pressure and tricuspid valvular function. Good answers outlined how each of these factors determine CVP and whether it was increased or decreased. Many candidates incorrectly described the effect of venous return.

Many candidates confused the factors that affect closing capacity (CC) with factors which affect functional residual capacity (FRC). Some candidates confused airway closure with expiratory flow limitation secondary to dynamic airway compression. A good answer would have included the following: Small airway closure occurs because the elastic recoil of the lung overcomes the negative intrapleural pressure keeping the airway open. Thus, airway closure is more likely to occur in dependant parts of the lung where airways are smaller. Normally closing capacity is less than FRC in young adults but increases with age. Closing capacity becomes equal to FRC at age 44 in the supine position and equal to FRC at age 66 in the erect position. Closing capacity is increased in neonates because of their highly compliant chest wall and reduced ability to maintain negative intrathoracic pressures. In addition, neonates have lower lung compliance which favours alveolar closure. Closing capacity is also increased in subjects with peripheral airways disease due to the loss of radial traction keeping small airways open. The consequences of airway closure during tidal breathing include shunt and hypoxaemia, gas trapping and reduced lung compliance. In addition, cyclic closure and opening of peripheral airways may result in injury to both alveoli and bronchioles. Closing volume (CV) may be measured by the single breath nitrogen washout test or by analysis of a tracer gas such as xenon during a slow exhaled vital capacity breath to residual volume. Residual volume (RV) cannot be measured directly but is calculated as follows: the FRC is measured using one of three methods: helium dilution, nitrogen washout or body plethysmography. The expiratory reserve volume (ERV) may be measured using standard spirometry. Using the measured FRC and ERV we may calculate RV from the equation: RV = FRC – ERV. Then CC = RV + CV.

Answers should have included the most important aspects of the pharmacology of the most commonly used drugs e.g. class, mechanism of action, pharmacodynamics and important adverse reactions. More information on beta-agonists and corticosteroids (mainstays of management) was expected than drugs like magnesium, ketamine and other adjunctive treatments.

Answers that scored well followed the structure outlined in the question and explained the principles of each component of the question

A good answer began with a definition of plasma and then listed the components - water, albumin, globulins, fibrinogen and other proteins before mentioning the lipid content, nutrient content, wastes and electrolytes. Frequently the breakdown of the globulin component was inaccurate. A common omission was dissolved gas components. Descriptions of the calculation of oncotic pressure and GFR were not asked and hence did not attract marks. The functions of albumin may be subdivided into: Osmotic pressure, transport function, acid-base buffer, anti-oxidant, anticoagulant effect, protein store, metabolism and 'other'.

Starting with the WHO definition of pain, followed by a brief description of the nature of noxious stimuli (thermal, mechanical, chemical) then proceeding to mention the nature of the cutaneous receptors would have been a very good start to this question. Following this, a description of the various substances involved in pain (K, prostaglandins, bradykinin, serotonin, substance P) and outlining the types of nerve fibres involved in pain transmission and how they synapse in the spinal cord and cortex was expected. The presence and nature of the descending inhibitory pathways was mentioned by very few.

Most candidates were able to mention some pancreatic enzymes, though often in insufficient detail to attract full marks. The amount, type, pH, etc. of pancreatic secretions was often not included. Many candidates did not describe the stimuli for pancreatic secretion. Better answers described the cephalic, gastric and intestinal phases of pancreatic secretion.

eta-blocking drugs were generally well classified. Selectivity, membrane stabilising activity and ISA should have been mentioned. Many candidates omitted or poorly answered the ‘effects’ of 6 beta blockers. Candidates who performed well answering the pharmacokinetics of metoprolol and esmolol provided a table of the two drugs. Superficial statements such as “hepatic metabolism and renal excretion” attracted minimal marks. The mechanism of action of beta blockers was not requested.

Clearance was generally well answered. It is the volume of plasma cleared of a drug per unit time, not the mass of drug cleared. An equation was helpful in identifying the relevant components of hepatic clearance. ClHep=QH X ERHep ERHep= FU x ClInt / QH + FU x ClInt QH = hepatic blood flow ERHep = hepatic extraction ratio FU = fraction of drug unbound in plasma ClInt = hepatic enzymatic capacity Many candidates did not describe the effects of hepatic blood flow and intrinsic clearance on drugs with high and low hepatic extraction ratios. Some discussion of Phase I and II reactions was also expected.

The question sought information on the structure (anatomy), function (physiology) and vascular supply of the right and left ventricle. Good answers provided detail in each section e.g. values for ventricular pressure rather than simply stating “high- and low-pressure systems”. Many marks may be gained by a simple anatomical description & labelled PV loop for each ventricle. Many candidates focussed solely on the coronary circulation, to which only a proportion of the marks were allocated.

Answers were generally well structured. Better answers described lung and chest wall compliance and the pressures which are used to calculate compliance. Better answers displayed an understanding of dynamic, static and specific compliance and provided a reasonably comprehensive list of the physiological factors affecting chest and lung compliance.