Question 11(p.2)

College Answer

Candidates were expected to know the different hormonal responses to hypovolaemia The possible approach to this question can be either by explaining the hormonal response in terms of time sequence or by different hormonal systems. Good answers to this question included how different hormonal responses are activated and
mediated. The common omissions were secretion of erythropoietin within 24 hours of haemorrhage, role of macula densa and juxtaglomercular apparatus, interactions between baroreceptors and sympathetic nervous system with the secretion of ADH, cortisol, glucagon and catecholamines.
Syllabus: C1g 2b
Reference: Kam 1st edition 156, 212
Guyton 11th edition 342,287-280

Discussion

• Staging of blood loss is in terms of % of circulating volume lost:
  • Four classes (ATLS): <15% of volume, 15–30%, 30–40% and > 40%
  • Loss of 1000ml of blood (20% of the total circulating volume) in a 70kg subject  represents most of the stressed volume.
  • Loss of 2000ml of blood = 40% of the circulating volume = severe haemorrhagic shock
• Hypovolemia and cardiovascular compensation
  • Baroreflex activation and chemoreceptor activation.
  • Decreased vagal stimulus; thus increased heart rate
  • Sympathetic activation, which has multiple effects:
    • Increased peripheral vascular resistanc
    • Redistribution of blood flow away from the cutaneous and splanchnic circulation
    • Stimulation of systemic catecholamine release from adrenal glands, which produces an increased systemic effect in addition to the peripheral sympathetic nervous system effecs
    • Stimulation of vasopressin release via the projections from the nucleus of the solitary tract to the hypothalamus
    • Stimulation of renin release by sympathetic stimulation of the juxtaglomerular cells, and due to lower renal perfusion
• Transcapillary fluid redistribution and isovolaemic anaemia
  • ​​​​​​​Vasoconstriction of arterioles results in reduced capillary hydrostatic pressure
  • This results in a change in the Starling relationship in the microcirculation
  • The result is a movement of free water out of the interstitial space and into the intravascular space
  • This dilutes the blood volume and decreases the haematocrit, decreasing the haemoglobin concentration of the blood, but restoring some of the circulating volume
• Renal fluid/electrolyte conservation and haemopoiesis
  • ​​​​​​​​​​​​​​Renin secretion causes:
    • Vasoconstriction (by angiotensin)
    • Increased sodium retention (by aldosterone)
  • Vasopressin release causes:
    • Vasoconstriction (by V1 receptors)
    • Increased water retention (by V2 receptors)
  • Venous hypotension decreases atrial natriuretic peptide secretion,
    which causes:
    • Decreased renal blood flow
    • Decreased urinary water and sodium excretion
  • The net effect is decreased urine output and increased retention of sodium and water
  • Erythropoiesis  is stimulated by EPO release from the kidney, stimulated by decreased oxygen delivery 

B A Foex  Systemic responses to trauma British Medical Bulletin 1999, 55 (No 4) 726-743

Barcroft H, Edholm OG, McMichael J, Sharpey-Schafer EP. Posthaemorrhagic fainting study by cardiac output and forearm flow. Lancet 1944; i 489-91

Nelson DP, King CE, Dodd SL, Schumacker PT, Cain SM: Systemic and intestinal limits of O2 extraction in the dog.
J Appl Physiol 1987, 63:387-394

Shock. 1994 Mar;1(3):188-95. A quantitative analysis of transcapillary refill in severe hemorrhagic hypotension in dogs. Prist R, Rocha-e-Silva M, Scalabrini A, Coelho IJ, França ES, Meneghetti C, Braga GA.

Drucker WR, Chadwick CD, Gann DS. Transcapillary refill in hemorrhage and shock. Arch Surg. 1981 Oct;116(10):1344-53.

Kirkman E, Little R. Central control of cardiovascular responses to injury. In: Rothwell N, Berkenbosch F (Eds) Brain Control of Responses to Trauma. Cambridge: Cambridge University Press, 1994, 202-38

Jacobsen J, Søfelt S, Sheikh S, Warberg J, Secher NH. Cardiovascular and endocrine responses to haemorrhage in the pig. Acta Physiol Scand. 1990 Feb;138(2):167-73.

Anderson ID, Little RA, Irving MH. An effect of trauma on human cardiovascular control: baroreflex suppression. Trauma 1990; 30: 974—81

Guly, H. R., et al. "Vital signs and estimated blood loss in patients with major trauma: testing the validity of the ATLS classification of hypovolaemic shock." Resuscitation 82.5 (2011): 556-559.