1. Introduction
A centrifugal blood pump does not deliver a fixed flow simply because its impeller rotates at a fixed speed. Instead, blood flow depends on the interaction between the pump's hydraulic characteristics and the pressure conditions imposed by the circulation. The head–flow (H–Q) curve describes this interaction: at a specified rotational speed, it relates the pressure head generated across a pump to the volume flow passing through it [1–3].
This principle is relevant to durable ventricular assist devices (VADs), temporary mechanical circulatory support, extracorporeal membrane oxygenation (ECMO), and cardiopulmonary bypass (CPB). The same fundamental pump physics applies across these settings, although the source of inflow pressure, downstream pressure, cannulation geometry, and extent of native cardiovascular pulsatility differ. The accompanying diagrams specifically illustrate a left ventricular-to-aortic centrifugal VAD, in which cyclic changes in ventricular pressure produce predictable variations in instantaneous pump flow.
2. Defining Head and Flow
2.1. Pressure head (H)
Head denotes the pressure difference that a pump must overcome between its outlet and inlet. In pressure units, the operating head is:
H = P(outlet) − P(inlet)
For an LV-to-aorta pump, a practical approximation is:
H ≈ Aortic pressure − Left ventricular pressure.
This is a differential pressure, not the absolute aortic pressure and not a measure of systemic vascular resistance in isolation. The true hydraulic load also reflects pressure losses through the inflow cannula, outflow graft, connectors, and other components of the circuit. Pressure head is frequently represented in mmHg in clinical descriptions; engineering pump curves may instead use units of fluid-column height.
The direction of the difference matters. If outlet pressure substantially exceeds inlet pressure, the pump must generate more head to maintain forward flow. When the pressure difference narrows, forward flow becomes easier to sustain at the same speed [1,2].
2.2. Pump flow (Q)
Q is the volumetric flow through the pump, conventionally expressed in L/min. For a rotary pump, it is a function of rotational speed, pressure head, and the pump-specific hydraulic characteristics. An H–Q curve is therefore a constitutive relationship, not a universal equation of the form Q = RPM/H.
Some durable LVAD consoles display estimated, rather than directly measured, flow derived from speed, motor power, and device-specific algorithms. Consequently, displayed flow is not necessarily identical to independently measured flow, particularly with changing viscosity, abnormal loading, or pump dysfunction [1,2].
3. Understanding the H–Q Curve
3.1. Shape of the curve
In the figures, the horizontal axis represents flow (Q) and the vertical axis represents pressure head (H). The downward-sloping yellow curve shows a simplified centrifugal-pump characteristic at one fixed rotational speed.
At a higher head, the operating point lies toward the upper-left portion of the curve, where forward flow is lower. At a lower head, the operating point shifts toward the lower-right portion, where flow is higher. Pump speed is held constant during this comparison.
The curve represents pump performance under defined testing conditions. Actual H–Q relationships are often nonlinear, differ between manufacturers and models, and may change with fluid properties. A flatter H–Q curve, when plotted with H vertically against Q horizontally, means a relatively large flow change for a given head change. It must not be confused with a horizontal segment implying constant flow [3,4].
3.2. Changing rotational speed
Changing impeller speed generates a different H–Q curve. Increasing speed generally increases the pressure head that can be produced at a given flow, and increases achievable flow at a given head. Lowering speed usually reduces these capabilities [1,3].
The operating point is determined by both the pump curve and the hydraulic conditions of the attached circulation. Thus, changing speed does not guarantee a proportional change in flow. If drainage is inadequate or downstream resistance is high, raising RPM may provide little additional flow and may worsen excessive negative inlet pressure.
3.3. Pump curve versus circuit curve
For a given operating speed, the pump characteristic describes the combinations of pressure head and flow that the pump can deliver. The attached system imposes its own pressure–flow requirements, often referred to as a system curve.
In a simplified rigid circuit, downstream hydraulic resistance increases the head necessary to maintain higher flow. In physiologic systems, however, upstream filling pressure, venous collapse, myocardial contraction, compliance, valve behavior, and time-varying vascular pressure all influence the operating condition.
The intersection of the pump and circuit characteristics defines the operating point. In a VAD connected directly to a beating ventricle, this point changes throughout each cardiac cycle even when rotational speed remains fixed.
4. H–Q Behavior During the Cardiac Cycle
4.1. Ventricular systole: lower head, higher pump flow
During native ventricular systole, LV pressure rises. Aortic pressure also changes, but the aortic-to-LV pressure difference often narrows in a supported, contracting left ventricle. For an LV-to-aorta centrifugal pump:
LV pressure rises → pump pressure head falls → the operating point shifts rightward → instantaneous pump flow increases.
This is the behavior shown in the systolic illustration: the point labeled Qs lies to the right of the diastolic point Qd. The associated flow waveform rises during ventricular contraction.
Importantly, this is flow through the VAD, not the native LV ejection volume and not necessarily total systemic cardiac output. If the aortic valve opens, native ventricular ejection also contributes to forward aortic flow. Pump and native outputs must therefore be considered separately [1,2].
4.2. Ventricular diastole: higher head, lower pump flow
During ventricular relaxation, LV pressure decreases substantially. Aortic pressure remains higher than ventricular pressure over most of diastole. Accordingly, the pressure head opposing pump flow increases:
LV pressure falls → pump pressure head rises → the operating point shifts leftward → instantaneous pump flow decreases.
The diastolic illustration places Qd to the left of Qs and shows lower pump flow between ventricular contractions. In severe unloading or insufficient filling, very low LV pressure may further reduce forward pump flow or provoke inlet suction.
The arrows on an H–Q diagram describe movement along a fixed-speed curve. The arrow labels should not be mistaken for a change in impeller speed or reversal of the overall direction of normal forward LVAD support.
4.3. Why a continuous-flow pump exhibits pulsatility
A continuous-flow device is not necessarily a constant-flow device. Even without programmed speed modulation, the pump operates against a time-varying trans-pump pressure gradient. Its instantaneous flow may therefore rise and fall in synchrony with native cardiac contraction.
Pump-flow pulsatility can be described as the difference between peak and trough flow over a cardiac cycle. Its magnitude depends on native myocardial contraction, filling pressure, arterial loading, device speed, and the slope of the pump's H–Q relationship [1,4].
This mechanical pulsatility is not equivalent to normal arterial pressure pulsatility. The aortic pulse pressure is influenced by native ejection through the aortic valve, arterial compliance, peripheral runoff, and the interaction of VAD output with the vascular system. Substantial pulsatility of estimated pump flow does not guarantee a physiologic arterial pulse.
5. Preload and Afterload
5.1. Decreased preload
Reduced ventricular filling can lower LV pressure and enlarge the outlet-to-inlet pressure difference. At a fixed speed, this can reduce average pump flow. Causes include hypovolemia, inadequate right ventricular output, impaired pulmonary venous return, or mechanical obstruction of the inflow pathway.
Marked underfilling may allow the inflow cannula to approach or contact ventricular tissue. Inlet suction may produce transient flow changes, arrhythmias, or hemodynamic instability. A fall in pump flow should therefore not automatically prompt higher pump speed. Filling conditions, right ventricular function, cannula position, and echocardiographic findings require assessment [1,2].
5.2. Increased afterload
When aortic pressure increases relative to LV pressure, the pump must work against a larger pressure head. At unchanged RPM, mean flow generally decreases. Hypertension may therefore reduce effective support even when the pump is functioning normally.
This relationship explains why pump output is afterload sensitive. The magnitude of the flow reduction depends on the specific pump curve and prevailing ventricular pressures. Neither blood pressure nor RPM alone establishes actual VAD flow.
5.3. Increased ventricular contractility or filling
Greater native systolic pressure generation can reduce systolic pump head and increase peak pump flow. Higher filling pressure may also modify the diastolic portion of the waveform. Consequently, changes in pulsatility may reflect altered preload or contractility rather than improved intrinsic pump efficiency [1,2].
Interpretation must remain contextual. A high pulsatility signal is not invariably favorable, and a low-pulsatility signal may represent profound ventricular dysfunction, inadequate filling, strong ventricular unloading, or a combination of these factors.
6. Interpreting the Pressure and Flow Tracings
The source material juxtaposes three signals: aortic pressure, LV pressure, and estimated VAD flow. The principal relationships are:
- Systole: LV pressure approaches aortic pressure, reducing pump head and typically increasing VAD flow.
- Diastole: LV pressure falls while aortic pressure remains relatively elevated, increasing pump head and typically decreasing VAD flow.
- Stable RPM: the instantaneous operating point traverses one pump-specific H–Q curve rather than remaining at a single flow.
- Altered loading: preload and afterload can shift both mean flow and waveform shape, even when the pump speed does not change.
These are conceptual relationships rather than exact proportionalities. In patients with profound ventricular dysfunction, minimal LV pressure generation, an unusual loading state, aortic valve disease, or active pump speed modulation, waveform morphology may differ significantly [1,2,5].
7. Application Beyond Durable LVADs
7.1. ECMO and temporary extracorporeal centrifugal pumps
Centrifugal ECMO pumps obey the same pressure-head and speed–flow principles, but their operating pressures are determined by the extracorporeal circuit. In venovenous ECMO, flow depends on venous drainage and return conditions. In venoarterial ECMO, venous drainage, arterial return pressure, cannula dimensions, and oxygenator or tubing resistance all contribute.
At fixed RPM, insufficient drainage can decrease actual flow despite increasingly negative inlet pressure. Simply increasing speed may aggravate drainage collapse or circuit chatter. Increased downstream resistance, such as an obstructed oxygenator or kinked return line, can also reduce flow [6,7].
These circuits often incorporate a direct flow probe and may have relatively little cardiac-synchronous pump-flow variation, depending on cannulation and pressure transmission. Therefore, the distinctive systolic–diastolic pattern shown for an LV-to-aorta VAD must not be applied indiscriminately to VA-ECMO, VV-ECMO, or CPB.
7.2. Cardiopulmonary bypass
During CPB, the centrifugal pump is similarly preload dependent and afterload sensitive. Inlet reservoir level, venous return, arterial cannula resistance, and systemic arterial pressure affect delivered flow. Under adequate reservoir conditions, flow can be maintained by adjustment of pump speed, but RPM alone does not verify systemic perfusion.
A calibrated circuit flow measurement, pressure monitoring, venous return assessment, and perfusion targets remain essential. A lower flow may result from inadequate inlet supply, increased outlet resistance, or a mechanical circuit problem rather than intrinsic pump failure.
8. Clinical Interpretation and Limitations
The H–Q framework is useful because it separates pump speed, the pressure gradient across the pump, and actual blood flow. These variables are linked but not interchangeable.
It also establishes several important limitations. Pump-specific curves are required for quantitative predictions; a schematic curve is insufficient for selecting a patient-specific speed. Pressure gradients may include substantial cannula and graft losses. Pump flow displayed by a durable device may be an algorithmic estimate, whereas extracorporeal systems often provide independently measured circuit flow.
Further, flow through an LVAD does not necessarily equal effective systemic output. Aortic regurgitation can create a recirculating loop between the aorta and LV, allowing apparent pump flow to overestimate useful forward systemic perfusion [5]. Similarly, circuit recirculation in VV-ECMO limits the inference of effective extracorporeal gas exchange from pump flow alone.
Finally, waveform changes should be integrated with blood pressure, echocardiography, ventricular dimensions, right ventricular performance, device parameters, lactate, end-organ perfusion, and relevant circuit measurements rather than interpreted in isolation.
9. Key Clinical Principles
- Head is the outlet-to-inlet pressure difference, not pump speed or blood pressure alone.
- At fixed speed, greater head generally means less flow along a centrifugal pump's H–Q curve.
- In LV-to-aorta support, systole generally increases instantaneous pump flow as the aortic-to-LV pressure difference narrows.
- Diastole generally decreases instantaneous pump flow as ventricular pressure falls and pump head rises.
- Continuous-flow support can still exhibit cardiac-synchronous flow pulsatility through patient–pump interaction.
- Preload, afterload, and mechanical circuit resistance affect flow, even without changing RPM.
- The LVAD cardiac-cycle illustration is configuration-specific; ECMO and CPB follow the same pump physics but operate under different pressure conditions.
- Displayed flow and effective systemic perfusion are not always equivalent and require clinical corroboration.
References
- Rich JD, Burkhoff D. HVAD flow waveform morphologies: theoretical foundation and implications for clinical practice. ASAIO J. 2017;63:526–535. DOI: 10.1097/MAT.0000000000000557. PMID: 28323662.
- Belkin MN, Kagan V, Labuhn C, Pinney SP, Grinstein J. Physiology and clinical utility of HeartMate pump parameters. J Card Fail. 2022;28:845–862. DOI: 10.1016/j.cardfail.2021.11.016. PMID: 34980564.
- Hosseinipour M, Gupta R, Bonnell M, Elahinia M. Rotary mechanical circulatory support systems. J Rehabil Assist Technol Eng. 2017;4:2055668317725994. DOI: 10.1177/2055668317725994. PMID: 31186935.
- Tagusari O, Yamazaki K, Litwak P, et al. Effect of pressure-flow relationship of centrifugal pump on in vivo hemodynamics: a consideration for design. Artif Organs. 1998;22:399–404. DOI: 10.1046/j.1525-1594.1998.06157.x. PMID: 9609348.
- Rosenbaum AN, Antaki JF, Behfar A, Villavicencio MA, Stulak J, Kushwaha SS. Physiology of continuous-flow left ventricular assist device therapy. Compr Physiol. 2022;12:2731–2767. DOI: 10.1002/cphy.c210016. PMID: 34964115.
- Zakhary B, Vercaemst L, Mason P, Lorusso R, Brodie D. How I manage drainage insufficiency on extracorporeal membrane oxygenation. Crit Care. 2020;24:151. DOI: 10.1186/s13054-020-02870-1. PMID: 32295631.
- Su Y, Liu K, Zheng JL, et al. Hemodynamic monitoring in patients with venoarterial extracorporeal membrane oxygenation. Ann Transl Med. 2020;8:792. DOI: 10.21037/atm.2020.03.186. PMID: 32647717.