Shock (1) Lecture - PDF

 Hemodynamics is an important part of cardiovascular physiology dealing with the forces the heart as the pump has to develop to circulate blood through the CVS  It is the study of the movements of the blood and the forces concerned therein (dynamics of the blood circulation) A...

 Hemodynamics is an important part of cardiovascular physiology dealing with the forces the heart as the pump has to develop to circulate blood through the CVS  It is the study of the movements of the blood and the forces concerned therein (dynamics of the blood circulation) Arterial Blood Pressure (ABP)  The force exerted by the blood on the vessel wall, Blood pressure (mmHg) = Force/Area  Systolic blood pressure  Diastolic blood pressure  Mean arterial blood pressure  Mean arterial blood pressure (MAP)  The Mean Arterial Pressure refers to the average pressure of the blood circulating through a person’s arteries, during the cardiac cycle  This is the driving pressure for organ perfusion and oxygen delivery  Mean arterial blood pressure (MAP)  MAP is often derived from the systolic and diastolic blood pressures (SBP and DBP) as follows: MAP = 1/3 SBP + 2/3 DBP However, this relationship is based on the assumption that diastole accounts for two-thirds of the cardiac cycle, which occurs only when the heart rate is 60 beats/minute; a rarity in critically ill patients. Therefore, calculation of MAP is not advised in the ICU setting. According to Ohm’s law, the potential difference between the extreme points of an electric circuit (DV = V1 - V2) is defined by multiplying the current (I) by the resistance of the circuit (R).  MAP – CVP = CO x SVR  Usually CVP is very low Then: MAP = CO X SVR  Mean arterial blood pressure (MAP) A. CO: (ml of blood pumped per minute) consists of stroke volume (SV) (ml of blood ejected from the left ventricle per beat) and heart rate (HR) CO = SV X HR B. SV: is determined by:  preload (amount of blood available to eject)  afterload (resistance to ejection), and  contractility (amount of force generated by the heart)  Conditions that may lower blood pressure through diminished CO in critically ill patients include cardiac failure (myocardial infarction, arrhythmia, acute heart failure, or valvular disease) and hypovolemia (hemorrhage, intractable diarrhea, or heat stroke)  Vasodilatory conditions such as sepsis, anaphylaxis, pancreatitis, acute hepatic failure, or neurotrauma, lower blood pressure by reducing SVR  Blood pressure is the driving force behind oxygen delivery  Every organ is able to autoregulate blood flow, but this ability is generally lost at MAP values lower than 65 mm Hg  Therefore, a goal MAP of 65 mm Hg is often targeted for shock to maintain perfusion Body Size  Hemodynamic parameters are often expressed in relation to body size  The popular measure of body size for hemodynamic measurements is the body surface area (BSA)  Parameters that are adjusted for body surface area are identified by the term “index” Why not use body weight to adjust for body size? BSA was chosen for hemodynamic measurements because cardiac output is linked to metabolic rate, and the basal metabolic rate is expressed in terms of body surface area. The average-sized adult has a body surface area of 1.7 m2.  Used to evaluate cardiac performance & MAP ▪ Central Venous Pressure ▪ Pulmonary Artery Wedge Pressure ▪ Cardiac Index ▪ Stroke Index ▪ Systemic Vascular Resistance Index  Shock is defined in simple terms as: a syndrome of impaired tissue perfusion usually, but not always, accompanied by hypotension  This impairment of tissue perfusion eventually leads to:  cellular dysfunction  followed by organ damage  death if untreated The most common causes of shock are situations that result in one of the following:  a reduction of intravascular volume (hypovolemic shock)  an intrinsic or extrinsic obstruction of circulation (obstructive shock results from pulmonary embolism, pericardial tamponade)  myocardial pump failure (cardiogenic shock)  increased vascular capacitance (distributive shock, sepsis) The type of treatment required depends on the etiology  The distinctions among subtypes of shock only apply in the relatively early stages  As the syndrome evolves and compensatory mechanisms are overwhelmed, it becomes increasingly difficult to determine the subtypes because the clinical and pathophysiologic features of advanced shock are the same for all  Also, different types of shock can occur at the same time (e.g., a patient with septic shock who is also hypovolemic)  Ischemia, endogenous Inflammatory cytokine release, and the generation of oxygen radicals  When cells are subjected to a prolonged period of ischemia, anaerobic metabolism begins. This inefficient process results in a decrease of ATP stores  Lactic acid and other toxic substances build up which can alter mitochondrial function and eventually result in cell death.  In the advanced stages of shock, irreversible cellular damage leads to multiple organ system failure, also known as multiple organ dysfunction syndrome  Inflammatory cytokines are produced by the body in response to ischemia, injury, or infection.  Systemic inflammatory response syndrome (SIRS) describes any acute, overwhelming inflammatory response, independent of the cause  SIRS can occur after a wide variety of insults, including hemorrhagic shock, infection (septic shock), pancreatitis, ischemia, multitrauma and tissue injury, and immune-mediated organ injury.  SIRS is usually a late manifestation of hypovolemic forms of shock. It is uncommon in cardiogenic shock but is the hallmark of septic shock.  SIRS is clinically characterized by profound vasodilation, which impairs perfusion, and increased capillary permeability, which can lead to reduced intravascular volume  The diagnosis of shock is made by the findings of impaired tissue perfusion on physical examination, and hemodynamic and laboratory changes consistent with impaired perfusion.  Although hypotension is often described as the hallmark of shock, it is not necessarily present in all patients.  Hemodynamic monitoring is vital for the determination of the type of shock and assessment of response to interventions.  Normal Hemodynamic Values and Derived Indices cMay optimally ↑ PCWP to 16–18 mm Hg in critically ill patients. The pressure recorded during balloon inflation is similar to left atrial pressure because the occluded vessel and its distal branches that eventually form the pulmonary veins act as a long catheter that measures the blood pressures within the pulmonary veins and left atrium. Normal Hemodynamic Values and Derived Indices Normal Hemodynamic Values and Derived Indices  SVO2 is measured in patients using a pulmonary artery catheter. Initially, critically ill septic patients may present with a low SVO2 value (