College Answer

Multiple factors: consider the following:

Cardiovascular: vasodilated state with lower baseline BP, higher baseline HR, and higher cardiac output. Masking of initial hypovolaemia, risking foetal circulation, best monitored by foetal heart rate. Large intra-abdominal mass (uterus) puts patient at risk of supine hypotensive syndrome: need to displace uterus or position in left lateral position.

Respiratory: diagphragms pushed up, decreased FRC (need to insert ICCs higher); respiratory alkalosis (expected CO2 30, with HCO3 20): need to keep in mind when assessing blood gases and if ventilating patient. Swollen airway, larger breasts: intubation often difficult.

Gastrointestinal: decreased gastric emptying, and weakened lower oesophageal sphincter: increased risk of aspiration.

Haematological: hypercoagulable state, risk of Rh incompatibility with foetus: potential for Rhesus isoimmunisation.

Foetus: benefits from supplemental oxygen; avoid tetracyclines, quinalones, NSAIDs (premature ductal closure), etc.

Discussion

The management of the pregnant poly-trauma patient is discussed elsewhere. The answer prepared for Question 3 from the first paper of 2007 is very similar: "Outline the special considerations involved in the care of a pregnant patient involved in multi-trauma.".

In summary:

Airway changes

Bag-mask ventilation becomes more difficult:

• The nasal mucosa is engorged, which means there is greater resistance to flow
• The upper airway mucosa is oedematous
• There has been weight gain

Laryngoscopy becomes more difficult:

• Upper airway oedema
• Breast enlargement
• The Mallampatti grade changes during pregnancy, largely because of oedema of the pharynx, and due to weight gain. It gets even worse with labour.

Less time is available for intubation:

• Decreased FRC, less time to intubate
• Increased oxygen consumption, less time to intubate

Intubation is more risky

• Increased risk of aspiration, decreased stomach emptying

Respiratory changes

• The diaphragm is pushed up by 4cm
• Tidal volume increases by ~ 30-50%
• Respiratory rate increases to 15-17
• Minute volume increases by 20-50%.
• Chest wall compliance decreases 
• Lung compliance remains the same 
• FRC decreases during pregnancy, due to compression of the diaphragm by the gravid uterus.
• pH increases to 7.40-7.47
• PaCO2 decreases to 30 mmHg
• PaO2 increases to 105 mmHg
• HCO3- decreases to 20 mmol/L
• Maternal 2,3-DPG increases 
• p50 remains the same because of alkalosis 

Circulatory changes

• Cardiac output increases (from 5L/min to 7L/min)
  • Stroke volume increases (from 65ml to 80-90ml)
  • Heart rate increases (from 75 to 85-90)
  • Systemic vascular resistance decreases (down by as much as 40%) - in fact, the vascular system becomes fairly refractory to the effects of vasoconstrictors such as angiotensin and vasopressin
  • The IVC is compressed by the gravid uterus in the supine position, decreasing the preload
• Blood pressure decreases (and is lowest in the second trimester)
• Pulmonary vascular resistance decreases
• Pulmonary artery wedge pressure remains unchanged
• Blood volume is increased by 50%
• CVP remains unchanged
• Colloid oncotic pressure decreases
• Oxygen consumption increases by 20% during pregnancy

Electrolyte and endocrine changes

• Vasopressin release increases;
• Thus, there is water retention
• A hypervolemic hypoosmolar state develops
• In response to a decreased SVR, aldosterone release is increased. This is the major contributor to the 50% circulating volume expansion
• There is a relative iodine deficiency (the foetus is stealing it all)
• Cortisol secretion is increased, which has implications for all those people who still do random cortisol levels on their patients

Renal changes

• Renal blood flow increases: the renal arteries are also affected by the fall in SVRI, and this is mediated by relaxin (which influences endothelial nitric oxide production).
• GFR increases by as much as 85%
• Urea and creatinine decrease because of this
• Kidneys become enlarged; the renal pelvis dilates and there is a "physiological hydronephrosis" - more so on the right because the right ureter crosses iliac and ovarian vessels at an angle. This predisposes to pyelonephritis
• Tubular resorption of urate and glucose decreases

Gastrointestinal and nutritional changes

• Nausea and vomiting: in 50-90%. 
• Oesophageal sphincter tone is decreased (aspiration is more likely)
• There is increased intragastric pressure due to upward displacement
• Gastric emptying is delayed, and is virtually non-existant during labour
• Thiamine supplementation is important, because prolonged hyperemesis can result in vitamin deficiency.
• Abdominal compartment pressure measurements are going to be wildly inaccurate.
• There is insulin resistance, particularly later in pregnancy
• Metabolic fuel use favours lipolysis, preserving the glucose and amino acids for use by the foetus.
• Protein catabolism is decreased
• There is a peak of calcium demand in the third trimester