Physiology

Fluid

• Total body water = intracellular fluid + extracellular fluid.
• Extracellular fluid = plasma + interstitial fluid + transcellular fluid (gastrointestinal tract, urinary tract, synovial fluid and cerebrospinal fluid).
• Percentage of bodyweight comprized of various fluid compartments:
  • Total body water:
    • Adult: 60%.
    • Neonate: 73%.
  • Intracellular fluid volume:
    • Adult: 30%.
    • Neonate: 29%.
  • Extracellular fluid volume:
    • Adult: 30%.
    • Neonate: 44%.
  • Plasma volume:
    • Adult: 5%.
    • Neonate: 7%.

Acid-base physiology

• See Anesthetic monitoring: acid-base and electrolyte.
• pH of the blood is determined by the concentration of hydrogen ions in plasma = [H+].
• Normal blood pH (venous) – 7.35 – 7.5.
• Acid-base homeostasis is primarily maintained through buffering of carbonic acid (H2CO3) and bicarbonate (HCO3-); as illustrated by the equation: H⁺ + HCO₃^- ⟷ H₂CO₃ ⟷ H₂0 + CO₂
• The equation can be manipulated to produce: [H+] is proportional to [CO2] / [HCO3-]  this means the pH changes as a result of a change in [CO2] or [HCO3-].
• [CO2] is controlled through respiration and [HCO3-] is controlled by excretion (primarily renal) or altered due to buffering other acids, eg lactic acid.
• pH changes can therefore be classified as respiratory or metabolic.
• The body can compensate for acid-base changes using the same respiratory or metabolic mechanisms eg in metabolic acidosis (decreased [HCO3-]) the body compensates by actively reducing [CO2] by increasing respiration. These compensatory mechanisms are summarised in this table Acid-base changes: compensatory mechanisms.
• pH can also decrease due to the addition of another acid eg lactic acid produced due to tissue hypoxia which is then the buffered by HCO3- resulting in a relative decrease in [HCO3-].
• The ‘strong ion difference’ (SID) is an alternative model to explain acid-base control. Body fluid must be electrically neutral ie all positive ions must balance all negative ions. The difference between the cations and anions which fully dissociate (strong ions) is termed the SID and is summarised in this diagram .
• The simplified SID equation is SID = [Na+] + [K+] – [Cl-].
• Any difference in [Na+], [K+] or [Cl-] will be effectively compensated by a change in [H+] to maintain electroneutrality. For example a decrease in [Na+] results in an increase in [H+] and a decrease in pH.
• Limited differences in the practical implications regardless of underlying theory applied.

Assessing need for fluid therapy

Different fluid options available

Formulating fluid therapy plan

Neonates

Adult cows

Practical points

Home made preparations

Further Reading

Publications

Refereed Papers

• Recent references from PubMed and VetMedResource.
• Hallowell G & Remnant J (2016) Fluid therapy in calves. In Practice 38 (9), 439-449 PubMed.
• Constable P D (2014) Acid-Base assessment. Vet Clin North Am Food Anim Pract 30 (2), 295-316.
• Roussel A J (2014) Fluid therapy in mature cattle. Vet Clin North Am Food Anim Pract 30 (2), 429-439 PubMed.
• Smith G W & Berchtold J (2014) Fluid therapy in calves. Vet Clin North Am Food Anim Pract 30 (2), 409-427.
• Trefz F M, Lorch A, Feist M, Sauter-Louis C & Lorenz I (2012) Construction and validation of a decision tree for treating metabolic acidosis in calves with neonatal diarrhea. BMC Vet Res 8 (1), 238 PubMed.
• Lorenz I (2009) D-Lactic acidosis in calves. Vet J 179 (2), 197-203 PubMed.
• Grove-White D (2007) Practical intravenous fluid therapy in the diarrhoeic calf. In Practice 29 (7), 404-408 VetMedResource.
• Bleul U T & Schwantag S C (2007) Effects of hypertonic sodium bicarbonate solution on electrolyte concentrations and enzyme activities in newborn calves with respiratory and metabolic acidosis. Am J Vet Res 68 (8), 850-857 PubMed.
• Constable P (2003) Fluid and electrolyte therapy in ruminants. Veterinary Clinics of North America: Food Animal Practice 19 (3), 557-597 PubMed.
• Constable P D, Walker P G, Morin D E & Foreman J H (1998) Clinical and laboratory assessment of hydration status of neonatal calves with diarrhea. JAVMA 212 (7), 991-996 PubMed.
• Grove-White D & White D (1993) Diagnosis and treatment of metabolic acidosis in calves: a field study. Vet Rec 133 (20), 499-501 PubMed.
• Naylor J M (1989) A retrospective study of the relationship between clinical signs and severity of acidosis in diarrheic calves. Can Vet J 30 (7), 577-580 PubMed.

Other sources of information

• Constable D, Kenneth W, Hinchcliff Stanley H Done, Grünberg W & Radostits O M (2017) Veterinary Medicine: A Textbook of the Diseases of Cattle, Horses, Sheep, Pigs and Goats. Elsevier, USA.
• Vogels Z (2010) Calf Fluid Therapy Made Simple. ACV Proceedings.