Managing Medical and Obstetric Emergencies and Trauma. Группа авторов

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      Algorithm 6.1 Shock

      Learning outcomes

      After reading this chapter, you will be able to:

6.1 Introduction

      Shock is defined as a life‐threatening failure of adequate oxygen delivery to the tissues. If left untreated, shock results in sustained multiple organ dysfunction, end‐organ damage and death. It occurs when the cardiovascular response to systemic challenges such as blood loss or sepsis is inadequate.

      Decreased blood perfusion of tissues, inadequate blood oxygen saturation or increased oxygen demand from the tissues result in failure of adequate oxygen delivery.

      A reduction in cardiac output and a reduction in perfusion pressure will reduce the blood perfusion of tissues. Cardiac output is the product of stroke volume (the volume of blood pumped out of the heart with each beat) and heart rate.

      Stroke volume is dependent on preload (filling status), cardiac contractility (pumping strength) and the afterload (vascular resistance – the resistance against which the myocardium has to pump). Shock may result if any of the these components are compromised.

      During normal homeostasis, organ perfusion is regulated by local metabolic and microcirculatory factors within a set range of arterial pressures. This is called autoregulation. Beyond this range, blood flow to the organ is primarily determined by the pressure differential between the arterial and venous systems.

      The blood supply to vital organs is maintained at lower blood pressures than that to non‐vital organs. In shocked states, blood is preferentially supplied to the brain and the heart at the expense of perfusion elsewhere. Unfortunately for the fetus, the uterus does not count as one of the woman’s vital organs, hence placental blood supply is not maintained in the presence of a life‐threatening challenge to the mother. The resulting fetal compromise is an early and important indicator of maternal shock.

6.2 Aortocaval compression and supine hypotension syndrome

      In the pregnant woman in a supine position, the uterus compresses the vena cava, reducing venous return to the heart from 20 weeks’ gestation. Vena caval obstruction and aortic compression can reduce cardiac output by up to 30%. The woman may experience symptoms such as nausea, vomiting or lightheadedness. This is known as supine hypotension syndrome. The reduction in venous return impacts on placental blood flow, which lacks autoregulation.

To prevent the effects of aortocaval compression, the pregnant woman should lie in the lateral position. Although a 15° tilt to the left (Figure 6.1b) is advocated there is evidence that the effects of compression still occur. Manual displacement of the uterus off the inferior vena cava up and to the left, relieves compression more effectively (Figure 6.1a). It is important to remember to relieve aortocaval compression in the initial management of the shocked woman to not exacerbate hypotension from other causes.