Physiology Flashcards
What effect do anaesthesia and surgery have on the CVS?
Cardiovascular depressant effect of many of the anaesthetic agents
Stress response to surgery (neural and humoral component)
Potential blood and fluid loss during surgery
(All negatively affect systemic circulation and CO)
List the indices of cardiovascular function.
Cardiac output Stroke volume Cardiac Index Blood pressure Ventricular filling
Explain cardiac output (CO)?
CO = HR X SV
The volume of blood leaving the heart each minute
Average adults at rest: 5 L/min
CO is a measure of flow
Explain stroke volume (SV)?
Stroke volume is the volume of blood leaving the heart with each beat
It is determined by
1) Preload - venous return
2) Contractility
3) Afterload - systemic vascular resistance
Explain cardiac index (CI)?
CI = CO/BSA(body surface area)
Approximately 3,2 L/min/m2 in the average adult at rest
Adjusts the cardiac output for the patients size
Explain blood pressure (BP)?
MAP (mean arterial pressure) = CO X SVR
Explain ventricular filling?
Atrial systole normally contributes less than 15% towards ventricular filling, remainder of ventricular filling occurs passively
AF results in less ventricular filling and thus less preload
Explain the physiological control of heart rate?
Sympathetic innervation of SA node (via B1 recepeptors - increases HR)
Parasympathetic innervation of SA (via vagus nerve - slows HR)
Sympathetic and parasympathetic innervation normally balances each other out, but overall parasympathetic system predominates
SA node receives further input from baroreceptors in carotid sinus
Sudden increase in BP = increased baroreceptor firing = increased vagal stimulation = vasomotor system inhibition = fall in BP
Sudden fall in BP = decreased baroreceptor firing = decreased vagal stimulation = increased sympathetic outflow from vasomotor centre = increased HR and CO
CO is affected by which changes?
HR
Preload
Contractility
Afterload
What are the factors that decrease preload?
Hypovolaemia Haemorrhage GA Neuraxial anaesthesia IPPV Autonomic neuropathy
What are the factors that affect myocardial contractility?
Increase contractility: Increase preload Decrease afterload Sympathetic stimulation Inotropes
Decrease contractility: Decrease preload Myocardial ischaemia Cardiac failure Hypokalaemia, hypoclacaemia Acidosis Hypoxia, hypercapnia Uraemia Sepsis Drugs
What are the factors that affect afterload?
Decrease afterload: Anaemia Hyperthyroidism Vasodilators AV shunts Exercise
Increased afterload: Polycythaemia Hypothyroidism Vasoconstrictors/vasopressors Hypothermia Surgical tourniquets
Describe the physiological control of BP?
Intrinsic regulatory properties of heart (Frank-Starling)
Autonomic pathways
Hormonal mechanisms (RAAS, vasopressin, adrenaline and noradrenaline)
Effects of AA on CO?
Propofol, etomidate and thiopentone: reduce CO Ketamine: increase CO Inhalational AA: decrease CO Pancuronium: may increase CO Vecuronium: may decrease CO
List the functions of the lung?
Gas exchange Phospholipid synthesis (surfactant) Synthesis of PG's and histamine Metabolism and de-activation of certain compounds Intrinsic component of the immune system Reservoir for blood (500-900ml)
Function of breathing?
To enable O2 delivery to cells (for oxidative phosphorylation and ATP production)
To eliminate CO2
Values of oxygen consumption at rest?
Adults: 3-4 ml/kg/min
Young children: 7-9 ml/kg/min
Values for respiratory failure I and II?
Type I: PaO2 less than 8kPa, PaCO2 low/normal
Type II: PaO2 less than 8kPa, PaCO2 greater than 6,5kPa
(1kPa = 10cm H20 = 7,5mmHg)
Where is the respiratory centre housed?
Medulla Oblongata (brainstem)
What is the major controller of ventilation?
PaCO2
PaO2 less than 8kPa also contributes
Tidal volume definition and normal values?
The amount of air inhaled and exhaled during each respiratory cycle
Normally 6-10 ml/kg
Opiates and AA decrease tidal volume
Minute ventilation/volume definition and normal values?
MV = VT X RR (tidal volume x RR)
Normally: 100 m/kg/min
Classification and definition of dead space?
Anatomical dead space: volume of respiratory passages NOT involved in gas exchange (i.e. nose, trachea)
Normally 150-200 ml in adults
Physiological dead space: combination of anatomical dead space and alveolar dead space (alveoli that are ventilated but not perfused)
Formula for alveolar ventilation?
Alveolar ventilation = alveolar MV = (tidal volume - phsysiological dead space) x RR
Residual volume definition?
The volume of air that remains in the lungs after a MAXIMAL exhalatory effort
Functional residual capacity definition?
The volume of air that remains in the lungs after a NORMAL tidal exhalation
FRC = residual volume + expiratory reserve volume (volume between normal tidal exhalation and maximal exhalation)
Normally 2,5-3,5 L in the average adult
Why is function residual capacity (FRC) so important in anaesthesia?
With pre-oxygenation, the FRC can be denitrogenated (all nitogen-containing air replaced with 80-100% O2)
Serves as a reservoir during periods of apneoa during induction or intubation difficulties
What factors reduce the functional residual capacity (FRC)?
Supine position Lithotomy (legs up) Trendelenburg (head down) Abdominal distension Morbid obesity Pregnancy Restrictive lung disease LVF Upper abdominal surgery GA agents Loss of muscle tone/muscle relaxants Intubation
Forced volumes definitions?
FEV1: volume exhaled in the first second of a FVC (forced vital capacity) breath
Normally 75-80% of FVC
FEV1:FVC ratio in obstructive and restrictive lung diseases?
FEV1:FVC ratio reduced less than 70% = obstructive
FEV1:FVC ratio normal or high = restrictive
O2 delivery formula?
DO2 = CO x CaO2 (arterial O2 content) CaO2 = (Hb x 1,36 x SaO2) + (0,0031 x PaO2)
Normal PaO2?
13,3kPa
Normal PaCO2?
5,3kPa
Definition and causes of hypoxia?
Hypoxia: An intracellular mitochondrial O2 tension below the critical level to sustain oxidative phosphorylation
Hypoxaemia: arterial O2 tension less than 8kPa
Causes of hypoxia? Stagnant hypoxia (not enough blood flow) Anaemic hypoxia (not enough Hb sufficient to carry O2) Hypoxaemic hypoxia (no O2) Cytotoxic hypoxia (poisoning - unable to utilise O2 despite adequate delivery)
Causes of hypoxaemic hypoxia?
Low fiO2 Hypoventilation Diffusion abnormality V/Q mismatch Shunt Decreased in mixed venous O2 tension
