Benign Physiologic Flow Murmur from Elevated Cardiac Output
The tachycardia and murmur heard best at the apex in the left lateral decubitus position in pregnant women is caused by the physiologic 50% increase in blood volume and cardiac output that occurs during normal pregnancy, creating a benign midsystolic flow murmur. 1
Physiologic Mechanism
During pregnancy, several cardiovascular changes create the conditions for this innocent murmur:
- Blood volume increases by approximately 50% with a commensurate rise in cardiac output, peaking between mid-second and third trimesters 1
- Heart rate increases by 10-20 beats per minute above baseline, explaining the tachycardia 1
- Stroke volume increases as the primary driver of elevated cardiac output
- This increased flow velocity across normal valves generates the characteristic midsystolic murmur 1, 2, 1, 2
Clinical Characteristics
The ACC/AHA guidelines clearly describe the expected findings in normal pregnant women 1:
- Soft grade 1-2 midsystolic murmur most commonly heard along the mid to upper left sternal edge
- Hyperkinetic precordial impulse from increased contractility
- Third heart sounds present in most patients (up to 80% per research data) 3
- Bounding pulses with widened pulse pressure
- The murmur may be accentuated at the apex in the left lateral decubitus position due to positioning bringing the heart closer to the chest wall
Important Distinction: What This Is NOT
Diastolic murmurs are unusual in normal pregnancy and virtually always represent pathologic conditions requiring cardiac evaluation 2, 1, 2, 1. This is a critical pitfall to avoid—while systolic flow murmurs are expected and benign, any diastolic component demands investigation.
Why Pregnancy Specifically Causes This
The guidelines explain that midsystolic murmurs occur with elevated cardiac output states including pregnancy, thyrotoxicosis, anemia, and arteriovenous fistula 1, 2, 1, 2. In pregnancy specifically:
- The increased flow rate across normal semilunar valves creates turbulence
- Most innocent murmurs originate from aortic or pulmonic outflow tracts
- The murmur is crescendo-decrescendo in configuration, starting after S1 as ventricular pressure opens the valve 2
Clinical Implications
Pregnancy can accentuate murmurs from stenotic lesions (mitral stenosis, aortic stenosis) but may actually attenuate or make inaudible murmurs from regurgitant lesions (aortic regurgitation, mitral regurgitation, ventricular septal defect) due to decreased systemic vascular resistance 1. This is crucial for distinguishing physiologic from pathologic murmurs.
When to Investigate Further
While the described murmur is typically benign, echocardiography is indicated if 2, 1, 2:
- The murmur is grade 3 or louder (may require differentiation from valvular stenosis)
- Any diastolic component is present
- Symptoms develop (paroxysmal nocturnal dyspnea, syncope, anginal chest pain)
- Other abnormal cardiac findings are present
The key takeaway: this represents normal pregnancy physiology, not pathology, caused by the hypervolemic, hyperdynamic circulatory state that characterizes gestation.