Low Stroke Volume Problems with Reception (filling)

Problems with reception are: Inadequate venous return diastolic dysfunction & cardiac inflow obstruction

Fluid loss is caused by either absolute hypovolemia (e.g.blood loss) or relative hypovolemia (“third spacing”).

1) Inadequate venous return: due to hypovolemia. Inadequate preload or filling pressure occurs as a result of intravascular fluid depletion, which may be due to volume loss, such as bleeding or dehydration, or fluid redistribution – third space losses, such as occurs in bowel obstruction or capillary leak syndrome. Often severe fluid depletion exists before patients become hypotensive, due to the potency of the compensatory mechanism. In this state patients are vulnerable to interventions which may unmask fluid depletion, such as the administration of vasodilators, including anesthetic agents, and aggressive manual ventilation. Patients admitted to intensive care in compensated shock may become severely hypotensive following the administration of even very small doses of propofol or thiopental. There are many warning signs of under-resuscitation: a lingering tachycardia, cold peripheries or a pulse oximeter that is not reading, oliguria, low CVP, a large base excess on blood gas analysis, a lactic acidosis.

Diastolic Dysfunction = stiff heart, requiring higher filling pressure to achieve normal volume.

2) Diastolic Dysfunction: loss of left ventricular compliance impairs it’s ability to receive blood. This disorder most commonly results from systolic dysfunction, and as a consequence of myocardial fibrosis – for example due to ischemia or hypertension. Diastolic dysfunction is characterized by the requirement of higher filling pressures to achieve normal filling volumes, while the heart is less compliant and receptive to blood. Aggressive volume loading of patients with diastolic dysfunction frequently results in backward heart failure, causing acute pulmonary edema.

Cardiac inflow obstruction is caused by a pericardial (tamponade) or intrathoracic process (PEEP), or a lesion within the heart itself (mitral stenosis).

3) Cardiac inflow obstruction: occurs either due to a constriction around the heart, a pericardial or intrathoracic process, or a lesion within the heart itself. Pericardial injuries include pericardial effusion or hematoma constrictive pericarditis – an acute crisis associated with a pericardial injury is called tamponade. Tamponade is diagnosed as a tetrad of shock, clear lung fields, inaudible or muffled heart sounds, and an increase in the jugular venous pulse waveform on inspiration.

An often forgotten but extremely common cause of hypotension is excessive intrathoracic pressure. This can be transmitted from within the alveolar space – as with positive end expiratory pressure (PEEP) and gas trapping in airway obstruction (auto-PEEP), or within the pleural space – Pneumothorax, hemothorax or, if the patient is in extremis, tension Pneumothorax. Intracardiac lesions may also cause inflow obstruction; these include mitral and tricuspid stenosis or thrombosis, and atrial myxoma. 

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