8 Integrated Hemodynamics Practice Questions

Pressure, flow, and cardiac output

Hemodynamics describes the relationship between pressure and blood flow. Cardiac output and blood pressure reflect interacting factors, so a single pressure reading does not fully describe perfusion. Use calculations to organize the available data, then interpret them alongside the patient’s condition, measurement quality, and other clinical findings.

Calculate cardiac output and index

Cardiac output depends on heart rate and stroke volume. Cardiac index adjusts cardiac output for body surface area, making the estimate relative to that area.

For a patient with a heart rate of $110$ beats per minute, a stroke volume of $50$ milliliters per beat, and a body surface area of $2.0$ square meters:

This calculation alone does not establish whether tissue perfusion is adequate. Interpret the estimate with the patient’s condition and other findings.

Estimate pressure and interpret perfusion

Pulse pressure is systolic blood pressure minus diastolic blood pressure. Mean arterial pressure can be estimated from diastolic pressure and one-third of pulse pressure:

This is an approximation, especially when heart rate is high or rhythm is irregular. For a blood pressure of $86/50$ millimeters of mercury, pulse pressure is $86-50=36$ millimeters of mercury, and estimated mean arterial pressure is $50+\frac{1}{3}(36)=62$ millimeters of mercury.

Mean arterial pressure estimates average arterial pressure over the cardiac cycle; it is not a complete measure of organ blood flow. In adults with septic shock receiving vasopressors, an initial target of $65$ millimeters of mercury is recommended, while the individual target depends on clinical context. A low estimated pressure together with worsening alertness and cool extremities is concerning for inadequate perfusion and calls for prompt clinical escalation and reassessment.

Assess filling and fluid response

Preload describes ventricular filling and myocardial stretch before contraction. A central venous pressure measurement is not a direct measure of circulating blood volume and does not prove that cardiac output will increase with fluid. Static filling pressures alone are poor guides to fluid responsiveness.

For example, a patient with suspected infection has a central venous pressure of $3$ millimeters of mercury. After a fluid bolus, the pressure rises to $11$ millimeters of mercury, but measured cardiac output does not change and new crackles are heard. These findings do not show improved output, and the crackles raise concern for worsening congestion. Do not use the initial low pressure alone to justify more fluid; reassess the patient and escalate the change in condition.

When appropriate, dynamic assessment can help determine whether flow increases with a reversible change in venous return. A passive leg raise with measurement of cardiac output or stroke volume is one such approach.

Interpret resistance and afterload

Systemic vascular resistance can be estimated from the pressure gradient between mean arterial pressure and central venous pressure relative to cardiac output:

Use pressures in millimeters of mercury and cardiac output in liters per minute; the result is in dyn·s/cm⁵. With mean arterial pressure of $58$ millimeters of mercury, central venous pressure of $8$ millimeters of mercury, and cardiac output of $7.2$ liters per minute:

This is a low-resistance pattern despite the relatively high cardiac output. Marked vasodilation is one possible explanation for hypotension, but a single calculation should not be used to diagnose a shock type. Low blood pressure does not necessarily mean low cardiac output because arterial pressure is influenced by both output and vascular resistance.

Afterload is the load a ventricle must overcome to eject blood. Stroke volume is influenced by preload, contractility, and afterload. Intense systemic vasoconstriction can raise arterial pressure and increase afterload; the increased ejection load can reduce stroke volume and, in turn, cardiac output. Blood pressure alone does not show whether forward flow is adequate.

Verify arterial-line measurements

A sudden lower arterial-line pressure after repositioning may reflect a measurement problem rather than physiologic deterioration, particularly if the patient appears unchanged and a cuff measurement is higher. Before acting on a discrepant reading, re-level the transducer to the appropriate reference point, verify that the system is zeroed as required, and inspect the arterial waveform and tubing. Compare with a properly obtained cuff measurement and reassess the patient.

A transducer above heart level can produce falsely low pressures; one below heart level can produce falsely high pressures. Repositioning can change the transducer’s relationship to the heart, so verify measurement quality before interpreting a sudden reading change as a physiologic change.

Recognize deterioration beyond blood pressure

Clinical deterioration and impaired perfusion can occur even when blood pressure has changed little. A rising heart rate, falling urine output, new confusion, and cool, clammy skin together are concerning and call for prompt reassessment and escalation through the appropriate clinical response pathway.

Compensatory changes can maintain blood pressure while flow or tissue perfusion worsens. Consider trends in mental status, urine output, skin perfusion, heart rate, and other clinical findings together; no single blood pressure value rules out deterioration. Real patient concerns require prompt assessment and escalation according to local protocols.

• Cardiac output (CO)

• Cardiac index (CI)

• Mean arterial pressure (MAP)

• Systemic vascular resistance (SVR)