 as the name indicates that it is a circulatory shock that means there is some problem in circulation. So basically circulation is what it is blood flow. So in circulatory shock there is decreased blood flow and why this decreased blood flow is occurring it is because of decrease in blood pressure which basically occurs due to decrease in cardiac output. So this is the fundamental of circulatory shock that these aspects should occur. Only one thing you should note here that this decreased cardiac output is either absolute or relative that means the cardiac output can be normal also right. But for that particular condition cardiac output is less. So requirement is more and the cardiac pumping is not able to fulfill the requirement of blood flow and sometimes cardiac output can even be more okay. So that is why the term relative decrease in the cardiac output vis-a-vis the requirements for maintaining the blood pressure fine. Now how to define circulatory shock? Based on this concept circulatory shock is defined as decrease in tissue perfusion caused due to absolute or relative decrease in cardiac output yes that is circulatory shock. So what will lead to such condition what can be the causes of circulatory shock? Simple we will try to understand the concept so that we never forget the causes. We know that for maintaining the blood pressure what is required? See blood pressure is maintained by what? First of all you should know that blood pressure is determined by two things systolic blood pressure it is determined by the cardiac output and diastolic blood pressure which is determined by the peripheral resistance. So if there is decrease in cardiac output there will be decrease in systolic blood pressure and hence there will be decrease in blood pressure and if there is decrease in peripheral resistance also that will also go into decrease blood pressure isn't it? So any of these factors will affect the blood pressure. Now what determines the cardiac output again? Cardiac output is determined by two main things one is the heart pumping okay. So blood should flow out of the heart and other is the venous return venous return because if heart is pumping and venous return is not there then cardiac output is going to decrease only and this venous return is again in terms determined by many factors one of very important it is blood volume. So you see the causes of circulatory shock include which one? One is the pumping failure of the heart so that is known as cardiogenic anything which is being generated because of the heart. So genesis of this shock is in the heart that is known as the cardiogenic shock and what can be the causes of this? There can be myocardial infarction arrhythmia can be there. So proper pumping is not there. Second it can be due to decrease in blood volume. So venous return in the heart is not happening and this decrease in blood volume is known as hypovolemic shock okay. So hypovolemic shock again what will happen? That there will be decrease in the venous return and this is going to decrease the cardiac output and in turn which is going to decrease the blood pressure. So these are the two causes what about third? Third can be that heart is pumping properly but the blood is not able to flow out of the heart. So that is known as the obstructive shock because the blood flow should be smooth isn't it? So that is known as the obstructive shock and what can be the causes of obstructive shock? One can be pulmonary embolism that can lead to obstructive shock. So blood is not able to flow out of the circulation into the systemic circulation from the pulmonary circulation into the systemic circulation. And fourth cause of shock can be decrease in the peripheral resistance right. So in the vessels when there is too much vasodilation peripheral resistance is going to decrease and this is known as fourth cause of shock that is the anaphylactic shock. In this you see heart will be pumping normally rather because of too much vasodilation venous return is going to increase and heart will pump more but still BP is going to be less okay. So let us summarize little bit what are the causes of shock? So first cardiogenic shock cause can be due to the heart right. Cardiogenic shock what can be the causes can you give some examples? As I told you myocardial infarction can be there arrhythmias can be there okay. Then second hypovolumic shock and how can hypovolumic shock occur? That is the blood volume is less one is suppose excessive burns are there plasma loss will occur and obviously trauma. So there is direct blood loss that will also lead to hypovolumic shock. Let us move on to third one that is the obstructive shock what can be the cause? As I told you it can be pulmonary embolism and finally there can be anaphylactic shock where there is excessive vasodilation just remember this one because this is different from the other types of shock. When we will talk about the pathophysiology we will discuss this. So there is excessive vasodilation causing decrease in the peripheral resistance and hence it is causing decrease in the blood pressure and cause can be for anaphylactic shock it can be septic shock. The septic shock occurs because of the presence of the bacterial endotoxins which cause the vasodilation and they can be neurogenic shock. So that is causing excessive vasodilation where there is a sudden loss of vasomotor tone. Okay vasomotor tone maintains the diameter of the vessels especially the arterios whenever we talk about resistance we are referring to arterios. Okay sudden loss of vasomotor tone and the causes of this neurogenic shock can be general anesthesia. Deep general anesthesia will depress the vasomotor center or there can be blockage of the sympathetic nervous system as in spinal anesthesia. So that is important that if this spinal anesthesia extends to that level that it blocks the entire sympathetic nervous system then that can also lead to this neurogenic shock. Now this circulatory shock progresses in stages. That means initially the shock severity is not that much because the body compensatory mechanisms are enough to maintain the blood pressure despite whatever the cause of the shock. But later on if nothing is done if no intervention is done then it will progress into severe stages. So based on that how the body is responding and whether this shock will reverse to normal stage there are different stages of shock and these stages of shock are first one is non-progressive shock. So this kind of shock will not progress rather it will correct itself. Non-progressive shock or also known as compensated shock. And the next stage is progressive shock. We will see each stage in detail. Next stage is progressive shock and third stage is irreversible stage of shock. So this stage cannot be reversed. Non-progressive shock means the body has taken care of itself. No further intervention is required from outside. Progressive shock means that it will halt only if intervention is given from outside. As a physician you will be given some intervention so that is progressive shock and if intervention is given on time then the shock can be corrected. Irreversible shock that despite the intervention the shock cannot be stopped and eventually it will lead to death of the patient. So these are the various stages of the shock and let us see what is going on in these different stages of shock. So coming to first one that is the non-progressive stage of shock. So it is very easy actually if you understand the mechanisms which maintain the blood pressure which you might have read in blood pressure maintenance then you will understand this non-progressive shock. So how does body respond when BP decreases? We are telling that in shock BP is decreasing isn't it? So how does body respond? Simple there is activation of the reflexes and what are those reflexes? There is baroreflex right? So baroreflex responds very fast in fact within three seconds it starts responding okay? So that tries to correct the blood pressure. Then because of decreased blood pressure there is a decrease in tissue perfusion and this decrease in tissue perfusion also leads to decrease in oxygen supply to the tissues and chemoreceptors actually detect the partial pressure of oxygen and blood. So that activity also starts. So chemoreflex also responds if BP is quite low then chemoreflex starts. So this is initiated at blood pressure of main blood pressure of 40 to 70 millimeter mercury. So these are the neural reflexes but hormonal mechanisms also will start and what are these hormonal mechanisms? Decrease BP will lead to decrease in GFR and this will be at the level of the macular denser cells the feedback processes will start which will lead to activation of renin angiotensin system. So there will be formation of angiotensin 2 okay? And this angiotensin 2 does has lot of effects on body and what are this? Angiotensin 2 causes release of aldosterone so that is another hormone which is released then it also causes release of ADH so that will also release then it causes powerful vasoconstriction throughout the body. So this is going to increase the blood pressure then at the level of the kidney it is causing the maintenance of the GFR by causing constriction of the efferent arterio. So that is another topic but for blood pressure these are the main actions of angiotensin 2 because this aldosterone is going to increase sodium and water reabsorption right? And ADH is also going to increase the water reabsorption from the kidneys. So this is helping in restoring the blood volume and when blood volume is getting restored what will happen? There will be increase in the venous return and there will be increase in the contraction of the heart okay? So what is happening? Increase in contraction of the heart it is going to increase the cardiac output which is going to maintain the blood pressure. So you see as I told you if you know about blood pressure you know the compensatory mechanisms. Apart from this there are certain intermediate mechanisms also which act see this is a long-term mechanism we study in blood pressure right? But there are intermediate mechanisms also whenever blood pressure decreases actually the hydrostatic pressure in capillaries is going to decrease right? If you know about the starlings forces suppose these are the capillaries and this is the interstitium whenever the hydrostatic pressure decreases in the capillaries then water will tend to move into the capillaries right? So this is going to increase the blood volume okay? Plus there is something known as stress relaxation. See when blood pressure increases right? These are vessels are basically lined by smooth muscles so whenever there is increase in blood pressure our vessels relax a little bit and this increases the vessel diameter thus decreasing the blood pressure. On the other hand when there will be decrease in blood pressure what will happen? There will be reverse of the stress relaxation so that is known as reverse stress relaxation of the vessels so that is another mechanism. Fine so these are the various compensatory mechanisms which occur. Now if the blood loss which has happened classically actually these stages in your books have been described in hypovolumic shock so if the blood loss which has occurred is less say approximately 10% of the total blood volume if it is lost then these compensatory mechanisms are enough to maintain the blood pressure. However if blood loss is more then these compensatory mechanisms are not enough on one hand and second they work against the body especially this one more retention of volume which is occurring it becomes harmful to the body and it initiates positive feedback that can lead to progression of shock. Here everything is negative feedback that decreased BP is being corrected and ultimately there is increase in BP but in progressive shock there are certain positive feedback mechanisms which operate. So let us see what are these positive feedback mechanisms which operate and where we can intervene so that this progressive shock is interrupted and the patient is brought back into the normal state. So first of all as I told you in non progressive shock there are neural responses and there are hormonal responses and due to hormonal responses there is increase in blood volume. Fine now see suppose if the blood loss is too much and decrease in arterial blood pressure which has occurred it is so much in the beginning that it causes decrease in the coronary blood flow. Okay for coronary blood flow it is very important that the blood pressure at the aortic level itself is high right. So suppose it causes decrease in coronary blood flow then what will happen? The functioning of the heart will reduce so if functioning of the heart reduces what will happen? Its pumping ability of the heart is going to reduce pumping of heart is reduced that is very important that decrease in coronary blood flow will cause decrease pumping ability of the heart. Now your kidney is retaining more and more blood volume I mean it is increasing the blood volume by retaining water so with decreased pumping ability now this heart has to handle more volume so volume overload of the heart is occurring. Okay so do you think that an injured myocardium will be able to handle increased volume? No rather it will start to fail so left ventricle suppose if it pumps less if it pumps less what will happen? It is going to decrease the blood pressure further right plus there will be back pressure developing so left atrial pressure will increase and ultimately it will lead to pulmonary congestion and if pulmonary congestion occurs will the blood which is passing through the lungs will it get oxygenated properly? No right so it will lead to hypoxia so decreased blood pressure and further decrease in tissue perfusion is already affecting the tissue perfusion and this delivery of oxygen and now the blood which is passing through the lungs it is also not getting oxygenated properly so this is further going to exacerbate the delivery of oxygen to the tissues so it is going to lead to hypoxia in the tissues isn't it and decrease blood pressure you see what will happen it will again be acting on the kidney because further there will be decrease in GFR and this will retain more and more volume right so again there will be further overload so volume overload continues because of not functioning of the heart decreasing pumping of the heart and continuously decreased BP being maintained despite the volume being increased because see kidney doesn't know why BP is decreased its job is to retain volume isn't it so that's why this becomes a positive feedback and it becomes very harmful then what are the other positive feedbacks which will start see because of hypoxia there will be increase in the capillary permeability and if capillary permeability increases then the volume of the blood in the vessels is going to decrease right so again this is going to decrease blood volume then tissue ischemia is also occurring because of decreased blood pressure and hypoxia tissue hypoxia is occurring and there will be death of the cells which is occurring this will release toxins from the tissues and ultimately these toxins are going to circulate in the blood and it is going to lead to further depression of the functioning of the heart so in simple terms if I say that how progressive shock occurs simple thing which you should remember is once tissue ischemia occurs especially of the heart once that tissue ischemia occurs of the heart then these positive feedback will start operating but as a physician actually you can intervene at this stage and I will tell you from this flow chart only where you can intervene so that progressive shock doesn't deteriorate further rather it can be brought back to normal so first thing you don't allow the volume overload right so what we can give we can give diuretics okay so that how much ever heart is functioning it should be able to function then second you give certain drugs which increase the pumping ability of the heart third you have to give oxygen therapy so that hypoxia doesn't occur so oxygen therapy is the one okay fourth you prevent the release of the toxins by stabilizing the lysosomal membrane and this can be done by giving the glucocorticoids so glucocorticoids actually stabilize the lysosomal membranes so these are various ways in which we can intervene in the case of progressive shock do you think that we can give vasoconstrictors as well for shock see as I told you that in the neuro reflexes which are happening the baro reflex and chemo reflex what they are going to do they are going to increase the sympathetic activity and with increase in the sympathetic activity there is vasoconstriction so body is at its maximum vasoconstriction so that drugs which are causing vasoconstrictions will not help much rather there is one type of shock in which they can help can you say in which there is vasodilation extreme vasodilation yes these that is anaphylactic shock in that the baro reflex is not able to bring about vasoconstriction so in anaphylactic shock we can give the sympathetic drugs which cause this vasoconstriction and obviously you have to treat the cause for example in hypovolemic shock main problem is hypovolemia so in initial stages of the shock we have to give the blood and plasma transfusion similarly in cardiogenic shock we have to treat the that particular condition now let's talk about the third stage of shock so we have dealt with non-progressive shock or the compensated stage then progressive shock let's come to third stage that is the irreversible shock irreversible shock occurs when whatever treatment you give doesn't work and we are not able to bring the cardiac output to normal and the blood pressure to normal rather sometimes what happens that for a small time we can see that cardiac output and blood pressure have come to normal but after some time again there will be deterioration and the shock will progress so what is the cause of the shock main cause which is attributed to irreversible stage of the shock is decrease in the ATP reserves in the cell see we are telling that there is hypoxia occurring because of decrease in tissue perfusion and obviously in progressive stage we saw how there will be decrease in oxygenation as well so when this decrease in hypoxia occurs then the cells are not able to utilize the oxygen for generating adequate amount of ATP and rather there is lot of breakdown of existing ATP into ADP and phosphate and this ADP again is broken down to adenosine which enters into the blood vessels and causes excessive vasodilation adenosine is causing vasodilation but you may ask so what if we are correcting and providing enough intervention then why can't the shock be corrected this is because the rate of loss of ATP which has occurred and the amount of ATP which has been lost cannot be matched by the rate of the synthesis after a particular time so once like phosphate stores of the cell are depleted and ATP is like the store is very less but there are other stores as well there is creatinine phosphate so that also will be converted to ATP and that also will be utilized so once these are utilized they are very difficult to replenish so that is the reason that if shock reaches these state it cannot be corrected so hope you have understood the fundamentals of circulatory shock and rest we will discuss in the live session on question and answers on circulatory shock