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indicates a respiratory alkalosis or respiratory compensation for a metabolic acidosis; a raised PaCO2 (hypoventilation) indicates respiratory acidosis – note that PaCO2 does not rise to compensate for a metabolic alkalosis

       Check standard bicarbonate (the bicarbonate value adjusted to what it would have been if the PaCO2 were normal): if the standard bicarbonate is raised then there is either a metabolic alkalosis or metabolic compensation for a respiratory acidosis; if the standard bicarbonate is low then there is either a metabolic acidosis or metabolic compensation for a respiratory alkalosis

       Check base excess: if it is negative then there is a metabolic acidosis; if it is positive then there is a metabolic alkalosis

Table 5A.1 Blood gases in non‐pregnant and pregnant women

	pH	PaCO2	Standard bicarbonate	Base excess
Normal values	7.34–7.44	4.7–6.0 kPa	21–27 mmol/l	–2 to +2 mmol/l
Values in pregnancy	7.40–7.46	3.7–4.2 kPa	18–21 mmol/l	No change
	Increased	Decreased	Decreased
Respiratory acidosis	↓	↑	↑	+ve	Hypoventilation leading eventually to compensatory renal retention of bicarbonate
Respiratory alkalosis	↑	↓	↓	–ve	Hyperventilation leading to renal excretion of bicarbonate
Metabolic acidosis	↓	↓	↓	–ve	Excess metabolic acid leading to respiratory hyperventilation to compensate Raised lactate in most types of shock
Metabolic alkalosis	↑		↑	+ve	Excess metabolic alkali but no respiratory compensation compensation

Appendix 5.2 Radiology in the pregnant woman

Imaging using ionising radiation is often part of the management of seriously ill patients. Patients and healthcare workers are often concerned that the doses of radiation used may be harmful to the fetus (Table 5A.2). A chest X‐ray confers a minimal amount of radiation to the fetus and is the equivalent of 1 week of background radiation in London. If a chest X‐ray is clinically indicated as a first line investigation in chest pain or breathlessness it should be performed.

Table 5A.2 Safety of different imaging techniques

Investigation	Radiation dose (mGy)	First trimester	Breastfeeding
Chest X‐ray	<0.01	Safe	Safe
CT head scan*		Safe	Avoid
MRI head scan*		Avoid	Safe
CTPA*	<0.13	Safe	Avoid
V/Q scan		Safe	Avoid
CT abdomen*		Safe	Avoid
Ultrasound		Safe	Safe

      ^* Express and discard breastmilk for 24 hours if using contrast.

      Ultrasound, computed tomography (CT) scans of the head and chest and magnetic resonance imaging (MRI) are safe throughout pregnancy. Gadolinium contrast should be avoided.

      For women with suspected pulmonary embolism and a normal chest X‐ray, a lung perfusion scan should be requested in preference to CT pulmonary angiography (CTPA) because the radiation dose to maternal lung and breast tissue is lower.

CHAPTER 6 Shock

      Algorithm 6.1 Shock

      Learning outcomes

      After reading this chapter, you will be able to:

       Define and recognise shock

       Discuss the principles of treatment of hypovolaemic shock

       Recognise the physiological changes to the cardiovascular system in pregnancy and how they affect the presentation of hypovolaemic shock

       Identify other shock syndromes and understand their management

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

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