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An Overview of Critical Care |
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Hemodynamic Insufficiency |
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The purpose of the cardio-respiratory system is to ensure delivery of oxygen and nutrients to the tissues and to remove waste products. This requires the presence of hemoglobin (the compound required for transport of respiratory gases), oxygen delivery to and across the alveolar capillary interface and an intact cardiovascular system to pump the hemoglobin containing fluid (blood) to the tissues. The perfusion vital organs is determined by blood pressure. Hypotension is an indication of 1) an abnormality of Heart Rate, Stroke Volume or Peripheral Resistance, & 2) failure of the others to compensate.
Shock is acute circulatory failure leading to inadequate tissue perfusion and end organ injury: it classified as being due to malfunction of 1) the Pump (cardiogenic), 2 ) the Tubing (distributive), or 3) the Fluid (hypovolemic). Cardiogenic shock results from failure of the cardiac muscle, usually ischemia, cardiac outflow obstruction (pulmonary embolism), or a heart rate that is too fast or too slow. Distributive shock occurs when there is abnormal vasodilatation, due to loss of sympathetic tone or release of vasoactive metabolites (in sepsis and anaphylaxis). Hypovolemia may be due to exogenous fluid loss, or fluid redistribution (third space loss).
The objective in treating patients with hemodynamic insufficiency is to return the blood pressure to a range that is normal for the patient, doing as little harm in the process as possible. If the heart rate is too slow, then the treatment is to speed it up with chronotropic drugs or a pacemaker. If it is inappropriately fast then the heart needs to be slowed down, either with antiarrhythmics or cardioversion.
If hemodynamic insufficiency is caused by vasoplegia (pathological vasodilation), fluid depletion or sequestration or low stroke volume, the initial intervention is usually to fluid load. If an acute ischemia event has occurred, revascularization by PTCA (angioplasty) or thrombolysis is indicated (1). If there is a significant amount of non functioning, stunned or hibernating myocardium, intra-aortic balloon counterpulsation may be utilized – perfusing the brain and coronary arteries in diastole (2). Norepinephrine may be used to raise diastolic blood pressure and thus improve coronary artery perfusion. If the patient is vasoplegic, following fluid resuscitation, a vasopressor such as dopamine, norepinephrine, epinephrine or phenylephrine is administered. If a potent inotrope is required, to improve cardiac contractility, dobutamine is the most effective drug. In patients with hypotension, cause unknown, epinephrine will re-establish a circulation, with the cost of perhaps worsening a lactic acidosis and reducing gut perfusion.
The most important endpoint of resuscitation is to see that the patient is awake and urinating without diuretics. The heart, the brain and the kidneys all have autoregulated blood flow, evidence of end organ perfusion in the brain and kidneys indicates blood pressure within the physiologic (autoregulated) range. Pressor and fluid therapy should always be targeted to the pressure that is normal for that particular patient. If there is confusion or concern about a patient’s intravascular volume, a central line is placed and CVP is measured. If a cardiac abnormality is suspected, then electrocardiography followed by echocardiography (to determine ejection fraction) is performed. It there is suspicion of significantly different muscle physiology between the right and left side of the heart, then a pulmonary artery catheter may be placed to measure stroke volume and cardiac output, and estimate left ventricular (end diastolic) filling volume. From this data a Starling curve can be constructed, and optimal filling pressure for cardiac output determined.
Sinus tachycardia is a common finding in intensive care. If it occurs as part of the compensatory mechanism for a low blood pressure in a vasodilated state, no rate intervention is required. In an otherwise hemodynamically normal patient (such as a postoperative patient), tachycardia may stress the heart and provoke ischemia. If a specific cause exists – hypovolemia, sepsis or pain; this is treated, otherwise, it is prudent to slow the heart rate below 70 with beta-blockers, particularly in patients with coronary heart disease or those who have had major vascular surgery. Evaluating the Hypotensive Patient
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Please note: these tutorials are for personal study purposes only. They are not currently peer reviewed, and no responsibility will be taken for mistakes or inaccuracies. Reproduction of information is forbidden. All material is copyrighted by the GasWorks Group. |
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