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APPLIED CARDIOVASCULAR PHYSIOLOGY
Outline the important features of a left ventricular pressure-volume loop
The left ventricular pressure volume loops plot the left ventricular volume in millilitres is plotted on the x axis against the left ventricular pressure in mmHg on the y axis over the cardiac cycle. The LV pressure-volume loop can be divided into four basic stages; ventricular filling, isovolumetric contraction, ventricular ejection and isovolumetric relaxation. If we start at the beginning of ventricular filling when the mitral valve opens and the end systolic volume of the LV is about 50mls, we can call this point A. The ventricles fill and the LV pressure slowly rises to a point where the LV pressure rises above atrial pressure and the mitral valve closes, this can be termed point B which represents the end diastolic pressure and end-diastolic volume which is usually around 130mls. The line from point A to B is phase 1 and represents ventricular filling, this horizontal distance is stroke volume. Phase 2 then begins which is isovolumetric contraction where the volume in the ventricle remains the same as all valves are closed but the pressure rises rapidly. This continues until the intraventricular pressure exceeds the aortic diastolic pressure, often at about 80mmHg, and the aortic valve opens, or point C. Phase 3 then begins which is the period of ventricular ejection. During phase 3 the systolic pressure rises to about 120mmHg and ventricular volume decreases as blood is ejected. There is a fall of pressure to about 100mmHg at the end of systole to a point and when the LV pressure drops below that of the aorta and the aortic valve closes or point D. This point marks end systolic volume. Phase 4 is isovolumetric relaxation where both the aortic and mitral valve closes so there is no change in volume but the ventricular pressure returns back to its diastolic level. When the intraventricular pressure falls below the atrial pressure the mitral valve opens or point A and the whole cycle begins again.
The LV pressure-volume loop can provide some important information about the heart. This includes:
What is shock and how can it be classified?
Shock is a broad term for a clinical condition characterised by the failure to deliver and/or utilise adequate amounts of oxygen to supply the needs of the body. Shock can be classified according to the major component of the circulation that is affected;
Shock can also be classified physiologically into:
Both classification systems can be overly simplistic and shock may be caused by multiple categories within these groups.
Describe the cardiovascular changes that occur following the acute loss of 1L of blood in an adult
1L of blood loss is a significant amount, being about 20% of total blood volume in an average 70kg person. This will result in a decrease in venous return and thus a decrease in cardiac output via the Frank Starling mechanism. The body’s cardiovascular changes occur with an aim to reduce further blood loss and ensure adequate blood flow to vital organs. The following cardiovascular changes occur:
What are the cardiovascular consequences of PEEP?
Positive end expiratory pressure is the application of positive pressure to expiration so that airway pressure is artificially kept above atmospheric pressure.
Cardiovascular consequences of PEEP differ depending on the cardiac function and fluid status of the patient.
Outline the physiological responses to chronic anaemia
Anaemia is a condition in which the number or oxygen carrying capacity of red blood cells is insufficient to meet physiological needs. Oxygen carrying capacity is determined by the haemoglobin concentration multiplied by the saturation of haemoglobin multiplied by the oxygen binding capacity of haemoglobin plus the amount of dissolved oxygen in the blood. Therefore, a decrease in haemoglobin will significantly affect the oxygen carrying capacity of blood. Oxygen delivery is dependent on oxygen carrying capacity multiplied by cardiac output, therefore in order to maintain oxygen delivery in anaemia, cardiac output must increase. There are several responses that occur with chronic anaemia
List the changes to cardiovascular physiology in a healthy elderly person
Outline the cardiovascular changes associated with morbid obesity
Morbid obesity is defined as a body mass index of over 40 or a BMI of over 30 and experiencing obesity related health problems. The cardiovascular effects of obesity are inter-related and complex but can be sub-divided into cardiac, vascular and hormonal changes.