Equine Critical Care and Analgesia - physiology Flashcards
Functions of the cardiovascular system?
Deliver oxygenated blood to body tissues = perfusion
Deliver deoxygenated/hypercapnic blood to lungs
Deliver waste products/toxins to sites of detoxification and excretion
Deliver hormones
When is the heart perfused? How much of the cardiac output does it receive?
During diastole from the coronary circulation
5-10% of the cardiac output at rest
Which symathetic receptors in the heart are responsible for increasing the contractility? How is an increased heart rate ahieved?
B1 receptors increase contractility
Heart rate increases by reducing vagal/parasympathetic tone
How does the parasympathetic system affect the heart?
Reduce heart rate, stroke volume and contractility
What causes the heart sounds?
S1 - Closure of AV valves
S2 - Closure of aortic valve
S3 - End of rapid ventricular filling
S4 - Atrial contraction
What makes up the conducting network of the heart?
SAN -> Conduction through atria -> AVN -> Bundle of His -> Purkinje fibres -> Ventricular myocytes (contracts apex first)
What predisposes horse to atrial fibrillation?
Large atria
High vagal tone
Cardiac output and blood pressure equations?
Cardiac output = Blood pressure/Systemic vascular resistance
Blood pressure = Cardiac output x Vascular resistance
Blood pressure = Heart rate x Stroke volume x Vascular resistance
What factors affect heart rate?
Autonomic influences
Exercise, stress, pain
Dysrhthmias
Electrolyte disturbance
Drugs
What factors affect stroke volume?
Preload - Effective circulating blood volume (ECBV), filling
Contractility - autonomic influences, Ca2+, structural heart disease
Afterload - vessel tone
What factors affect vascular resistance (afterload)?
Vasodilation - sepsis/endotoxins, drugs
Does good blood pressure always mean sufficient blood flow?
No
E.g. if arteries supplying a muscle belly are constricted, vascular resistance will be high, sop BP will appear high, but might not have sufficient blood able to reach tissue
How to calculate MAP?
Diastolic + 1/3(Systolic - Diastolic)
Does a bounding pulse mean good health?
No
A pulse will feel stronger if the diastolic pressure is very low and systolic is very high - the heart is trying to make up for the low diastolic pressure by putting systolic as high as possible - horse may not be doing well!
What counts as upper and lower respiratory systems?
Upper - nares to cervical trachea
Lower - thoracic trachea to lungs
Functions of respiratory system?
Delivery of O2
Removal of CO2
Non respiratory functions of the lungs:
- Trap airborne particles
- Reservoir of blood (stores about 10% of circulating volume)
- Metabolism - e.g. degrade angiotensin I
- Acid-base modulation - eliminating CO2
- Clotting modulation
- Filtration of blood - large particles e.g. tumour cells (so lungs common site for metastasis), emboli, clots are trapped and filtered
- Immune functions - macrophages in alveoli etc, IgA in mucus
- Modulation of body temperature - changes to ventilation
- Phonation
- Olfaction
What is ventilation?
= The exchange of air between the lungs and the atmosphere (prior to gas exchange happening)
What controls ventilation?
Sensory organs - peripheral chemoreceptors (carotid and aortic bodies), central chemoreceptors (ventral medullary sensing body), stretch and proprioception receptors
Respiratory control centres - brainstem and cortex
Feedback includes responses to changes in pH, PaCO2 and PO2
What stimuli can carotid bodies and aortic bodies detect?
Carotid bodies - pH, PaCO2, PaO2, temperature, hypoglycaemia
Aortic bodies - pH, PaO2, PaCO2, and also blood carrying content e.g. anaemia, carboxyHb
What do the central chemoreceptors in the ventral medulla respond to for ventilation?
pH - concentration of H+ ions
What are the 3 main things measured in the blood by the ventilatory system?
PaO2
PaCO2
pH
What is hypoxaemia? What are possible causes?
Low oxygen levels in the blood (<60mmHg in arterial blood)
Causes:
- Low FiO2 (breathing in low O2 e.g. barn fires)
- Diffusion impairment
- Hypoventilation (common cause)
- V/Q mismatch
- Hypotension (as then have poor perfusion to pulmonary tissues)
What is hypoventilation?
= Inadequate minute ventilation
Minute ventilation (amount of air we move in and out of our lungs per minute) = Tidal volume x RR
Would not be blowing off enough CO2, so this then reduces space for O2
Calculation for minute ventilation?
Minute ventilation = tidal volume x RR