Outline the physiological responses to the rapid intravenous administration of 1 litre
of 0.9 % saline to a 70 kg euvolaemic person.
Answering this question required the integration of information from areas of cardiovascular, body fluid and renal physiology which proved difficult for most candidates. Both breadth and depth was expected so as to score well. This question is best answered using a time-based approach. For example, upon the rapid infusion of a litre of normal saline there will be a brief period of hypervolemia, increase in arterial blood pressure and an associated physiological reflex response to these changes(e.g. baroreceptors, atrial stretch receptors, etc.). There will also be an associated increase in renal perfusion and stimulation of intrarenal receptors (e.g. juxtaglomerular apparatus).
Candidates were expected to outline these changes, their effector responses (e.g. autonomic nervous system reflexes and humoral changes) and their physiological consequences. A more prolonged redistribution phase of the administered saline then occurs. This saline redistributes throughout the extracellular fluid space. Candidates were expected to briefly describe this effect as well as the subsequent management of the sodium and water load by the kidney. Most candidates spoke about the pressure effects, and only some compared these with the volume effects. The effect of redistribution and other effects were not considered by the majority of the candidates.
Final volume expansion:
Change in osmolality:
Change in plasma oncotic pressure:
Change in biochemistry:
Hahn, Robert G., and David S. Warner. "Volume kinetics for infusion fluids." The Journal of the American Society of Anesthesiologists 113.2 (2010): 470-481.
Reddi, Benjamin AJ. "Why is saline so acidic (and does it really matter?)." International journal of medical sciences 10.6 (2013): 747.
Hahn, Robert G. "Clinical pharmacology of infusion fluids." Acta medica Lituanica 19.3 (2012): 210-212.
Griffel, Martin I., and Brian S. Kaufman. "Pharmacology of colloids and crystalloids." Critical care clinics 8.2 (1992): 235-253.