Question 8

Compare and contrast the physiological changes in the cardiovascular system in pregnancy at term and morbid obesity (BMI > 30).

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College Answer

The question was very specific for the cardiovascular system and therefore answers that  described respiratory changes and airway modulation failed to score marks. This answer leant itself to a tabular format. Candidates are reminded to ensure they document the facts in the correct column i.e. obesity facts in the obesity column. The cardiovascular changes associated with term pregnancy are well described in various texts. Those associated with morbid obesity required some integration from various sources and would include a structured series of comments such as heart rate (unchanged), blood pressure (tendency for hypertension), stoke volume (increased), cardiac output (increased), blood volume (increased – although perhaps decreased on a ml/kg basis), systolic function (preserved or increased), LV wall thickness increased. The pathological changes seen with the diseases associated with obesity are difficult 
to tease out and better answers identified this. Morbid obesity has a specific definition and stating this aided focus of the answers. 


One must agree with the examiners, that this answer certainly inclined itself from the perpendicular and rested for support against a tabular format. Without further snark, this is actually a great question, because obesity and pregnancy are frequently conflated by people unfamiliar with the properties of either, and they are clearly very different states. Both feature cardiovascular adaptation to various stresses, but one lends itself more to vasodilation and increased cardiac output, whereas the other is all about increased afterload and making the heart's work more difficult. 

Cardiovascular Physiology:
Pregnancy vs. Morbid Obesity
Parameter  Pregnancy Morbid obesity
Heart rate Increased (from 75 to 85-90) Unchanged or slightly increased
Stroke volume Increased (from 65ml to 80-90ml) Increased by about 1.25ml per every 1kg/m2 BMI
LV structure Unchanged Increased wall thickness
Cardiac output Increased (from 5L/min to 7L/min) Increased by approximately 1L/min for every 12.5 BMI
Total body oxygen requirement Increased by about 150% Decreased, in terms of ml/kg
(as adipose tissue is less metabolically active than lean tissue)
Blood pressure Decreased Increased
Systemic vascular resistance Decreased Usually increased (RAAS activation by leptin)
Pulmonary vascular resistance Decreased Increased (by OSA, which is hard to separate from obesity itself)
Pulmonary artery wedge pressure Unchanged Increased, due to increased total body fluid volume 
Blood volume Increased by 50% Increased by RAAS activation
CVP Unchanged Increased (as is MSFP)
Colloid oncotic pressure Decreased Unchanged


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