Effective hemodynamic performance in critical illness depends not only on cardiac function or vascular tone in isolation, but on how well these two systems work together—a relationship known as ventriculoarterial (VA) coupling.
Arterioventricular coupling describes the dynamic interaction between ventricular contractility and arterial load. At its core, it reflects how efficiently the heart converts myocardial energy into forward blood flow against the impedance of the arterial system. When coupling is optimized, oxygen delivery is achieved with minimal energetic cost. When disrupted—as commonly occurs in shock—patients may exhibit impaired perfusion despite apparently “normal” blood pressure readings.
The sections that follow break this concept into its two foundational components:
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Cardiac Function — the heart’s ability to generate flow.
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Arterial Vascular Function — how arterial compliance and resistance shape afterload and influence cardiac efficiency.
Click on the tabs below to learn more about each component. Then, return to this page and scroll down to learn more about how cardiac function and arterial vascular function coalesce in optimizing perfusion through VA coupling.
The Concept of Ventricular to Arterial Coupling
Cardiac function is traditionally taught as if the heart were an isolated organ. In reality, the heart is both anatomically and functionally connected to an arterial system. Every heartbeat therefore represents an interaction between the ventricle and the arteries.
During systole, the ventricle ejects blood into the arterial circulation. This ejected volume adds to the blood that is already present in the arteries, stretching the vessel walls and generating arterial pressure. In essence, the heart transfers mechanical energy to the arterial system.
For the cardiovascular system to operate efficiently, this energy transfer must occur smoothly and with minimal energy loss. In other words, the force exerted by the ventricle and the opposition made by the arterial system must be optimized.
In cardiovascular physiology, this interaction is described by the concept of ventricular to arterial (VA) coupling. VA coupling is defined by the relationship between ventricular elastance and arterial elastance. The concept of elastance describes how much pressure changes for a given change in volume.
For pedagogical purposes, we will explain arterial elastance and ventricular elastance separately, then explore how their relationship (VA coupling) determines the efficiency of the cardiac pump, and how that efficiency relates to myocardial oxygen consumption.
Arterial Elastance (Ea)
In simple terms, arterial elastance (Ea) reflects how much arterial pressure increases when the ventricle ejects a given volume of blood into the arterial system.
Traditionally, arterial elastance is considered to be primarily influenced by:
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Systemic vascular resistance - largely a reflection of arteriolar tone. This is a major component of afterload.
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Heart rate, because it determines how much time the arterial system has to empty between beats. At higher heart rates, the cardiac cycle is shorter, leaving less time for blood to run off into the microcirculation. As a result, arterial pressure remains higher and the ventricle ejects into an already pressurized/loaded arterial system.
To a lesser extent, Ea is also influenced by the pulsatile components of afterload, such as:
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Total arterial compliance
Because it integrates several determinants of arterial load, Ea is often used as a practical surrogate of global afterload.
In the example shown in the figure:
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The dark brown curve represents a vessel with high arterial elastance. In this situation, small increases in volume produce large increases in pressure. This occurs when the arterial system is stiff.
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The orange curve represents low arterial elastance. In this state, even large increases in volume result in relatively small changes in pressure.
Ventricular Elastance
Just as the arterial system can be defined by its elastance, the ventricle can also be characterized by how much pressure it can generate for a given change in volume. This property is known as ventricular elastance.
The relationship between ventricular pressures and volumes can be elegantly visualized using the pressure–volume (PV) loop. From this single diagram, several key variables can be identified, including:
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the pressure at which the aortic valve opens
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the end-diastolic volume (EDV)
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the end-systolic volume (ESV)
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the stroke volume
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and the ejection fraction of the ventricle.
However, unlike the arterial system, the heart does not behave as a structure with a fixed elastance. Instead, the ventricle functions as a time-varying elastance pump, meaning that the relationship between pressures and volumes changes continuously throughout the cardiac cycle. In the PV loop, this can be understood as changes in the slope of the pressure–volume relationship, which becomes steeper as the ventricle contracts and flatter as it relaxes.
During diastole, ventricular elastance is minimal, allowing the ventricle to remain highly compliant and accommodate incoming blood from the atria. In the PV loop, this is reflected by a relatively flat slope, meaning that large changes in volume produce only small changes in pressure.
In contrast, during systole, elastance progressively increases as the ventricle contracts, and the pressure–volume relationship becomes progressively steeper. This reflects the ventricle’s increasing ability to effectively generate pressure for a given change in volume, reaching its maximum stiffness at the end of systole.
In other words, the ventricle behaves like a pump whose stiffness changes over time; soft during filling and stiff during ejection.
Because ventricular elastance changes continuously during the cardiac cycle, it cannot be described by a single pressure–volume relationship at every moment in time. However, there is one specific moment in the cycle at which ventricular elastance reaches its maximum value: end-systole. This moment therefore provides a useful reference point to characterize the intrinsic contractile state of the ventricle.
Just after being filled during diastole, and as the ventricle starts contracting, ventricular pressure rises. When ventricular pressure exceeds aortic pressure, the aortic valve opens and ejection begins. From that moment, ventricular volume decreases as blood is ejected into the arterial circulation.
Ventricular Elastance and Contractility
If we examine the relationship between the ventricular pressure and volume at the end of systole, we can describe the ventricle’s intrinsic ability to generate pressure, its contractile state.
This relationship is known as the end-systolic pressure–volume relationship (ESPVR).
Burkhoff D, Wang J. Mechanical properties of the heart and its Interaction with the vascular system: Accessed 2018.
The slope of this relationship is called end-systolic elastance (Ees):
A steeper ESPVR (higher Ees) indicates a stronger ventricle, while a flatter ESPVR (lower Ees) indicates reduced contractile strength.
Importantly, Ees is considered a relatively load-independent index of ventricular contractility. This means that Ees is not dependent (ie: slope does not change) on preload or afterload. In other words, the slope of the line reflects the intrinsic strength of the ventricle and is independent of external conditions.
Burkhoff D, Wang J. Mechanical properties of the heart and its Interaction with the vascular system: Accessed 2018.
As shown above, if we change the preload of the system without changing anything else (afterload does not change), the slope of the Ees line does not change. Likewise, as shown below, if we increase the afterload (Ea) while keeping preload the same, the Ees slope still remains unchanged. Therefore the ESPVR is considered "load independent."
Burkhoff D, Wang J. Mechanical properties of the heart and its Interaction with the vascular system: Accessed 2018.
When ventricular contractility increases:
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The ventricle can generate higher pressures for the same ventricular volume
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The ESPVR becomes steeper
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Ees increases
When ventricular contractility decreases:
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The ventricle generates less pressure for a given volume
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The ESPVR becomes flatter
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Ees decreases
In other words, Ees describes the intrinsic strength of ventricular contraction. This concept becomes particularly evident in conditions such as cardiogenic shock, when there is an acute reduction in ventricular contractile function and therefore a decrease in Ees. In this state, the weakened ventricle may be unable to generate sufficient pressure to overcome the arterial load. When this occurs, the ventricle cannot eject enough blood into the arterial circulation to maintain adequate arterial pressure and systemic perfusion.
Ventricular to Arterial Coupling
At the end of systole, just as the aortic valve begins to close, ventricular and aortic pressures become equal and forward flow from the ventricle into the arterial system stops. This moment represents the end of ventricular ejection and is the point at which the interaction between Ees and Ea can be assessed.
Under normal physiological conditions, the heart operates optimally when the relationship between Ea and Ees is balanced. As described in the literature, a normal relationship between Ea and Ees is ~0.5 - 1. Under these circumstances, the heart is able to contract against arterial loads while maintaining its intrinsic contractile reserve.
VAC = Ventricular:Arterial Coupling
However, this equilibrium can change. For example, ventricular contractility may increase (higher Ees) or arterial elastance may decrease (lower Ea). The result is a net decrease in the Ea:Ees ratio.
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Decreased Ea:Ees Ratio
In this situation, the ventricle is able to eject more blood into the arterial system, resulting in a lower end-systolic volume and therefore a higher left ventricular ejection fraction (LVEF). Consequently, the Ea/Ees ratio decreases, indicating that ventricular contractile capacity now exceeds the arterial load. This pattern can be seen during inotropic stimulation (higher Ees) or in states cosistent with distributive shock (lower Ea). In both cases, the relationship between Ea and Ees is reduced (lower Ea/Ees ratio.)
The opposite situation can also occur. Ees may decrease or Ea may increase, causing the net ratio between the two to be elevated.
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Increased Ea:Ees Ratio
In this case, the ventricle is less capable of generating pressure relative to the arterial load, In other words, the Ea/Ees ratio increases, reflecting a state in which the arterial load exceeds the ventricle’s contractile capacity. As a result, less blood is ejected into the arterial circulation, leaving a higher end-systolic volume. This pattern is commonly observed in conditions such as cardiogenic shock or acute heart failure, where reduced contractility leads to an increase in Ea:Ees, ventricular–arterial uncoupling and impaired forward flow. It is also seen in patients with states of hypertensive emergency, where the arterial elastance is too high for the ventricle to handle. In this case, Ea:Ees also rises and he same result occurs, but on account of changes to the arterial elastance, not specifically ventricular elastance.
Ventricular Efficiency
So how does ventricular-arterial coupling relate to efficiency?
Let's start by identifying the actions of the ventricle in terms of energy expenditure. During systole, the ventricle generates mechanical energy as it contracts and ejects blood into the arterial circulation. Not all of the energy produced by the ventricular squeeze is converted into forward blood flow. A portion of this energy is used to eject blood into the arterial system, while another portion remains stored within the ventricle and is ultimately dissipated. The work performed by the ventricle to eject blood into the arterial circulation is also known as stroke work, whereas the energy generated during contraction that does not contribute to forward flow is referred to as wasted energy. The sum of these two components is known as the pressure–volume area (PVA), which represents the total mechanical energy generated by the ventricle during a cardiac cycle.
As illustrated in the figure above, the green area represents the stroke work, while the brown area represents wasted energy. The total shaded region corresponds to the pressure–volume area.
Because only stroke work contributes to effective blood ejection, ventricular efficiency can be defined as the proportion of total energy expenditure that is converted into stroke work:
In other words, ventricular efficiency describes how effectively the ventricle converts the mechanical energy generated during contraction into forward blood flow. When ventricular contractility and arterial load are well matched (adequate ventriculo–arterial coupling), a larger proportion of this energy becomes useful stroke work and efficiency is high. Conversely, when the ventricle and arterial system become uncoupled, a greater fraction of energy is lost as wasted energy and efficiency decreases.
Why does this matter?
This concept is important because the PVA correlates linearly with myocardial oxygen consumption (MVO₂), as shown below in a famous study by Hiroyuki Suga and colleagues at Johns Hopkins University. In other words, the total mechanical energy spent by the ventricle during a cardiac cycle closely reflects the amount of oxygen required by the myocardium to produce that contraction.
Suga H, et al. Am J Physiol. 1981 Jan;240(1):H39-44.
As shown in Suga's experiment above, even when PVA theoretically approaches zero (meaning that no external stroke work is performed) myocardial oxygen consumption does not fall to zero, as demonstrated by the y-intercept. This residual oxygen consumption reflects the basal metabolic requirements of the myocardium, as well as the energy required for excitation–contraction coupling, particularly the processes involved in calcium cycling within cardiac myocytes.
The interaction between VO2 and PVA is particularly important because when a state of mechanical inefficiency persists, the relative VO2 consumed for a given PVA is higher. This is because a smaller fraction of the metabolic energy consumed by the myocardium is converted into useful stroke work. As a result, the increase VO2 demands can promote insufficient oxygen supply for the given metabolic needs of the myocardium. Subendocardial ischemia and myocardial injury can ensue, further worsening cardiac performance. In this way, a vicious cycle develops, where reduced efficiency promotes myocardial ischemia, and myocardial injury further worsening ventricular performance.
In Summary
In summary, the cardiovascular system functions as a coupled mechanical system in which the ventricle generates mechanical energy and the arterial circulation receives it. This interaction is described by ventriculo–arterial coupling, which reflects the relationship between ventricular contractility (Ees) and arterial elastance (Ea).
When these two systems are well matched, the energy generated by the ventricle is efficiently transferred to the arterial circulation as useful stroke work, allowing the heart to maintain adequate blood flow with minimal energy loss. Conversely, when ventricular contractility and arterial load become mismatched, the system becomes uncoupled: stroke work decreases and a larger fraction of the generated energy (PVA) is dissipated as wasted energy.
Since PVA correlates linearly with myocardial oxygen consumption, the total sum of energy spent by the ventricle closely reflects the metabolic cost of cardiac contraction. When coupling is optimal, a greater proportion of the energy generated by the heart is converted into forward blood flow. When coupling is impaired, the heart still expends metabolic energy, but a smaller fraction of that energy results in effective circulation, which can impair overall oxygen delivery and thus, tissue perfusion.