Ease Into Prep with a USMLE^® Question of the Day

The correct answer is B. The increase in cardiac output (CO) and right atrial pressure (RAP) depicted by the intersection of the dashed lines in the figure (point Z) result from activation of the sympathetic nervous system (SNS) during exercise.

The plots defined by their points of intersection describe how CO is limited by venous return (VR) and how VR is limited by CO. The interdependence of CO and VR locks the cardiovascular system at the intersection (equilibrium) points.

• The cardiac function curve (CFC) is better known as a Starling curve. It describes the preload-dependence of left ventricular (LV) output.
• The vascular function curve (VFC) describes the effects of VR (which is dependent on CO and the ease with which blood travels through the vasculature) on RAP. RAP equates with LV preload.
• The pressure at which the VFC intersects the x-axis = mean systemic pressure (MSP). MSP reflects the "tightness" or "fullness" of the vasculature. It is determined experimentally by arresting the heart (CO = 0 L/min) and letting pressure in all parts of the system equilibrate. MSP is normally around +7 mm Hg.
• The point of intersection between a CFC and a VFC defines how much CO can be supported by any given preload (RAP), and vice versa.

SNS activation during exercise modifies both curves. In the present example, a resting equilibrium point gives a CO of 5 L/min, supported by 0 mm Hg of RAP. Exercise shifts the equilibrium point (point Z) to give a CO of ~18 L/min at an RAP of 2 mm Hg.

Several different physiologic mechanisms contribute to the increase in CO that is necessary to support exercise.

• Heart rate (HR): ? SNS activity and ? parasympathetic activity ? ? HR
• LV contractility: ? SNS activity ? ? contractility ? ? stroke volume
• Venoconstriction:
  • ? SNS activity ? venoconstriction ? ? venous capacity and ? venous pressure (note MSP shifts from 7 mm Hg to 20 mm Hg) ? ? LV preload
  • Tensing abdominal muscles and rhythmic contraction of active muscles compresses veins (extravascular compression) and forces blood back to the heart.

Note that the VFC also rotates upward (increased slope) during exercise because systemic vascular resistance (SVR) has decreased. SVR decreases due to the vasodilatory effects of metabolites being generated by exercising muscle.

A blood transfusion (choice A) increases central venous pressure and RAP, which shifts the VFC to the right. The CFC may reflexively drop downward and to the right to maintain a stable CO, but it would not shift leftward.

Heart failure (choice C) lowers the CFC (decreased contractility). The resulting decrease in blood pressure activates the renin-angiotensin-aldosterone system (RAAS), causing volume retention. The VFC shifts progressively rightward as a result, increasing MSP.

Hemorrhage (choice D) reduces blood volume, which decreases MSP and shifts the VFC to the left. Severe hemorrhage can culminate in hypovolemic shock, which impairs myocardial contractility through hypotension and inadequacy of coronary perfusion.

Spinal anesthesia (choice E) blocks SNS output and venodilates, which decreases MSP and shifts the VFC to the left. Myocardial contractility decreases also, which shifts the CFC downward.

This is a multi-step question.

What is the question asking?

We are being asked what most likely accounts for the increase in cardiac output and right atrial pressure depicted by point Z on the plot shown.

What is the first step?

You first have to consider what the two intersecting plots represent. Focus on the control data first (solid lines).

• The graph plots right atrial pressure (RAP) against cardiac output (CO).
• The first plot shows that CO increases with an increase in RAP. This is a cardiac function curve (CFC), more familiarly known as a Starling curve. It reflects the effects of preloading on myocardial performance.
• The second plot is a vascular function curve (VFC). It shows that as CO falls, RAP rises. This is because the heart draws blood from the veins to produce CO. When CO rises, RAP falls because the veins are being emptied.
• Because CO is dependent on RAP, and because RAP is dependent on CO, the intersection of these two plots (point X) defines what CO will be for any given RAP.

What is the next step?

You now have to consider how and why these two plots change under physiologic and pathophysiologic conditions.

• The CFC changes with changes in cardiac contractility (inotropy). Increases in contractility make the myocardium more efficient, so it supports a higher CO for the same RAP. The CFC shifts up and to the left. Conversely, contractility decreases cause the CFC to shift downward and to the right.
• The VFC curve changes with vascular compliance or the "tightness" of the venous compartment. Increases in vascular volume or venoconstriction raise RAP and shift the VFC up and to the right. Conversely, decreases in vascular "tightness" causes the VFC to shift down and to the left.

What is the next step?

The next step is to determine what the plots defined by their intersection point Z represent.

• The CFC has shifted upward and to the left, indicating an increase in myocardial contractility.
• The VFC has shifted up and to the right, indicating either an increase in intravascular volume or venoconstriction.

Together, these changes are characteristic of exercise (choice B).

• An exercising muscle must be supported by increased blood flow to deliver necessary nutrients (oxygen and glucose and other metabolic substrates) and carry away waste products (carbon dioxide and heat).
• Local vascular control mechanisms cause reflex vasodilation to increase blood supply to the muscle.
• Increased flow to active muscle decreases systemic vascular resistance (SVR) and causes mean arterial pressure (MAP) to fall.
• A drop in MAP triggers a baroreflex, which involves a decrease in parasympathetic nervous system tone and sympathetic nervous system activation.
• These autonomic responses increase heart rate, increase myocardial inotropy, venoconstrict, and constrict resistance vessels in non-essential circulations (GI system, genitals, kidneys) to help compensate for the drop in SVR caused by skeletal muscle vasodilation.
• An increase in myocardial inotropy causes the CFC to shift up and to the left. Venoconstriction causes the VFC to shift up and to the right. The intersection of these two points (point Z) shows how much CO increases as a result of these changes (choice B).

Can other answers be eliminated?

• You can eliminate any choices that would decrease CO, including heart failure (choice C) and hemorrhage (choice D).
• A blood transfusion (choice A) would increase RAP and shift the VFC to the right, but would not increase myocardial inotropy; indeed it would be more likely to shift the CFC downward to maintain CO at normal levels.
• Spinal anesthesia (choice E) blocks sympathetic output and would cause venodilation, decreased heart rate, and decreased myocardial contractility, all of which would decrease CO. The CFC and VFC would both shift downward.

What is the single best answer and why?

Exercise (choice B) is the best answer because it causes sympathetic activation and increased CO. Myocardial inotropy increases, shifting the CFC upward and to the left. Decreased vein capacitance shifts the VFC to the right; the new intersection (equilibrium) point Z defines the CO and RAP under these conditions.

MedEssentials (4th Ed.): pp. 243

First Aid (2019): pp. 284.2

First Aid (2018): pp. 281.1

First Aid (2017): pp. 275.1