Cardio & Anaes Flashcards
Oxygen dissociation curve
**Go look at a diagram
Right shift vs Left shift
Right shift of O2 dissociation curve
gives off oxygen easier
- Hyperthermia
- Mild hypercarbia
- Mild acidosis
- Increase in 2,3 DPG
Left shift of O2 dissociation curve
doesn’t give off oxygen easily
- Hypothermia
- Low carbon dioxide
- Alkalosis
- Decrease in 2,3 DPG
Formula for CO
Cardiac output = stroke volume x heart rate
Factors influencing stroke volume
- Preload
- Contractility
- Afterload
Discuss preload and resultant CO
- Frank-Starling’s law of the heart
- If you increase the preload, you will increase the
stroke volume up to a certain point & this will lead to
an increased cardiac output (and increased BP)
Discuss contractility and the heart muscle
o If the contractility decreases, and the afterload and diastolic volume stay the same = decreased stroke volume (decreased CO)
o If preload & afterload stay the same, and
you only increase the contractility =
increased stroke volume (increased CO)
Discuss Afterload and the CO
Afterload = impedance against which the ventricle delivers SV
- The afterload that affects the stroke volume is the resistance of the peck arteries .. not the same as
peripheral vascular resistance!
- If we increase the afterload (& nothing else changes) = decreased stroke volume (decreased CO & BP)
Discuss the correlation between CO and the heart rate
- If patients’ HR goes below 60 bpm or above 160 bpm,
the cardiac output becomes impaired & the BP will decrease
Coronary blood flow:
- Most of the coronary blood flow happens during diastole , this is determined by the coronary perfusion pressure
- If patient become tachycardic ) we reduce the time that is available for coronary perfusion to take place
- Similarly, if the aortic blood pressure drops it will negatively affect coronary perfusion
- Thus, if we want to maintain coronary perfusion > monitor the heart rate of the patient and the BP
Coronary Perfusion Pressure formula
CPP = Aorta Pressure - L ventricular pressure
Electrophysiology of the heart during anaesthesia **
- Increasing catecholamines = increasing the stress of the patient = cause a tachycardia by increasing the slope of hyperpolarization.
- surgical procedures can increase the vagal stimulation = significant bradycardia by slowing down / shortening the action potentials (decreasing the hyperpolarisation)
Describe the correlation of Hb concentration and perfusion
There is an optimal Hb whereby oxygen will optimally be given off by RBCs = this is ± 10 g/dl.
What effect does blood viscosity have on Ox delivery?
an increase in blood viscosity (which happens when RBC count incr) will impair the RBC’s ability to fit through the capillaries = impaired O2 delivery
How does the body compensate for anaemic patients
body maintains oxygen delivery by increasing the cardiac output
Effects of spinal anaesthesia on cardiovascular system
- Our elastance will decrease thus, if our preload stays the same, we will actually increase the stroke volume
- What we actually get though, is a bit of a venous dilatation / venous pooling .. thus, our stroke volume doesn’t increase as much as it theoretically should because the preload also changes
- Depending on how high the spinal moves, you expect some decrease in blood pressure (NB) if you do a saddle block, you don’t expect any change in BP
Effects of ventilation on CVS
When we start ventilating a patient (especially if they are slightly fluid behind), we can significantly decrease the preload & thus decrease the stroke volume > intrathoracic pressure increases with ventilation and blood flow into thorax may be reduced
Effects of IAA and IV and Opiate and BNZ on CVS
Some of the inhalational agents, IV agents, opiates, and benzodiazepines we use can decrease contractility
Effects of IAA and IV and Opiate and BNZ and ventilation on CVS
We now combine agents that decrease the contractility with ventilation which decreases the preload. Resultantly, you have a significant decrease in stroke volume and cardiac output (and BP).
Effects of ketamine on CVS
Unlike the other IV anaesthetic agents
Ketamine increases the sympathetic outflow > incr BP (because there is an incr SVR and an incr SV). Unfortunately, ketamine directly decreases the contractility of the heart. This is however completely overshadowed by the sympathetic effects
Effects of propofol on CVS
Propofol reduces sympathetic outflow > decr BP (because there is a decr SVR and a normal / decr HR). Furthermore, it also slightly decreases the contractility of the heart. If there is a decrease in contractility, there will be a decrease in stroke volume. In our bodies, we expect a compensatory increase in heart rate, but in this case it doesn’t occur.
Effects of thiopentone on CVS
dose-dependent hypotension
Thiopentone decreases sympathetic outflow > decr BP (because there is a decr SVR and a decr contractility). There is however an increase in HR to compensate for the decreased stroke volume caused by the decreased contractility.
Effects of etomidate on CVS
Etomidate is the most cardiovascularly stable IV agent, however not in all cases
Etomidate reduces the sympathetic outflow > decr BP (because there is a decr SVR). No other effects noted. If you have a patient in theatre who is hypovolaemic, their body will compensate by increasing the HR (sympathetic effect). Thus, if you now give that same patient Etomidate, they will become cardiovascularly unstable
Effects of IAA on CVS
They all vasopress everything. Halothane’s main effect is on the contractility, however all of them can affect the contractility in a dose-dependent way. Isoflurane, sevoflurane, and enflurane mainly have an effect on the SVR but may also affect contractility. All these agents will give you a dose-dependent drop in BP.
