Measuring Hemodynamics

Hemodynamic instability causes a mismatch between oxygen delivery and demand, leading to organ failure. Hemodynamic instability can often be managed with regular clinical examination and monitoring of vital signs (heart rate, blood pressure, oxygen saturation, and respiratory rate) and urine output.  However, when the patient does not improve or deteriorates further, invasive hemodynamic monitoring is needed to guide fluid management and vasopressor/inotropic support. 

Clinical Assessment
A clinical examination is the fastest and least invasive hemodynamic monitor available.

• A patient with inadequate global perfusion often presents with signs of organ dysfunction, such as tachypnea, tachycardia, confusion, weak peripheral pulses, skin mottling, and oliguria.
• Capillary refill time (CRT) can be rapidly tested and is the time required for blood flow (and color) to return to the distal capillaries after the release of fingertip compression sufficient to cause blanching of the skin (10 seconds of compression time). The upper limit of normal is 3 seconds in adults; a longer CRT indicates reduced capillary perfusion.

Non-invasive Monitoring
Electrocardiogram (ECG)

• Heart rate is an important determinant of cardiac output (CO = HR X SV).
• A 12-lead ECG confirms cardiac rhythm and provides baseline information on ST segments and T waves.
• Continuous monitoring of heart rate, rhythm and ST segments allows early recognition of hypovolemia and myocardial ischemia.
• Tachyarrhythmias are a common finding in certain shock states. Bradycardia and/or heart block may indicate cardiogenic shock.

Blood pressure (BP)

• The definition of hypotension (low BP) is patient-specific and interpreted in the context of the patient’s usual BP.
• Hypotension is a common feature of most shock states.
• Mean arterial blood pressure (MAP) is an approximation of organ perfusion pressure.
• Severely elevated BP, especially if acute, is associated with increased vascular resistance and may be associated with inadequate tissue perfusion, for example hypertensive encephalopathy or acute renal failure.

Pulse oximetry (SpO₂)

• Continuous SpO₂ monitoring enables rapid detection of even a small reduction in arterial oxygen saturation, which is an integral part of oxygen delivery.
• The SpO₂ signal is often inaccurate in the presence of altered skin perfusion. The inability to measure SpO₂ is itself an indicator of abnormal peripheral perfusion.

Echocardiography

• An echocardiogram provides visualization of the cardiac chambers, valves, pericardium, and overall cardiac function.
• It allows for measurement of left ventricular ejection fraction (LVEF) and estimates of SV and CO based on measurement of LV outflow tract (LVOT), LVOT velocity and heart rate. 

Fluid responsiveness

• Fluid resuscitation is a critical component of the treatment of hemodynamically unstable patients. Although rapid optimization of volume status has been shown to improve outcomes, volume overload is associated with increased morbidity and mortality.
• A fluid challenge is necessary to determine whether fluid administration will benefit the patient.
• Fluid responsiveness is frequently defined as an increase in cardiac output (≥ 10% from baseline) with a fluid challenge (250-500 mL administered over 10-15 minutes).
• An alternative to a fluid challenge is to perform a passive leg raise (PLR) maneuver. This produces an ‘autotransfusion’ of blood from the venous compartments in the abdomen and lower limbs.
  • Position the patient in the semi-recumbent position with the head and torso elevated at 45 degrees.
  • Obtain a baseline blood pressure measurement.
  • Lower the patient's upper body and head to the horizontal position and raise and hold the legs at 45 degrees for one minute.
  • Obtain subsequent blood pressure measurement.
  • The expected response to this maneuver in those that are fluid responsive is a 10% or greater increase in cardiac output (CO). Although not considered a validated measure, we often use blood pressure as a surrogate marker of CO in evaluating response to the PLR.
  • Only patients who are fluid responsive after a fluid bolus or passive leg raise should receive additional fluids.

Invasive Monitoring
Intra-arterial blood pressure

• Arterial cannulation (usually the radial artery) allows for accurate continuous blood pressure measurement. Arterial line BP monitoring is the standard of care for patients on vasopressor/inotrope infusions.
• Arterial lines facilitate frequent blood draws for blood gases or other lab studies.

Central venous pressure (CVP)

• The CVP is the blood pressure in vena cava/right atrium; normal range is 2-6 mm Hg.
• The CVP reflects right ventricular (RV) function and venous return to right side of heart.
• It is measured via a catheter positioned in the vena cava.

Pulmonary artery pressure (PAP)

• PAP is the blood pressure in pulmonary artery; normal systolic PAP range is systolic 15-30 mm Hg and normal diastolic PAP 5-15 mm Hg.
• It may be measured during right heart catheterization or via introduction of a catheter into the pulmonary artery (i.e., Swan Ganz Catheter).

Mixed venous oxygen saturation (SvO₂)

• SvO₂ reflects the balance between oxygen delivery and oxygen consumption (VO₂).
• It depends on arterial blood saturation (SaO₂), the balance between VO₂ and CO, and hemoglobin (Hgb) levels.
  • Normal SvO₂ is greater than or equal to 70% (drawn from a pulmonary artery catheter).
  • Central venous oxygenation (ScvO₂) is normally greater than or equal to 65% (drawn from a central venous catheter).

Hemodynamic Values
	Definition	Calculations and Normal Ranges
Cardiac output (CO)	The volume of blood pumped through the heart per minute (L/min)	Normal range is 4-8 L/minute.   Calculation CO = Stroke Volume (SV) X Heart Rate (HR)
Cardiac index (CI)	CO adjusted for body surface area (BSA)	Normal range is 2.8-4.2 L/min/m2.   Calculation CI = CO/BSA
Central venous pressure (CVP)	The blood pressure in the vena cava and an estimate of right atrial pressure; used to assess preload and volume status	Normal range is 2-6 mm Hg.
Mean arterial blood pressure (MAP)	Systolic blood pressure + (2 x diastolic blood pressure)/3	Normal range is 70-105 mm Hg.
Right atrial pressure (RA)	This pressure reflects venous return to the right atrium and right ventricular end-diastolic pressure	Normal range is 0-7 mm Hg.
Right ventricular pressure (RV)	Measured during catheter insertion	Normal RV systolic pressure is 15 to 25 mm Hg.
Pulmonary artery pressure (PA)	Used to diagnose pulmonary artery hypertension	Normal PA systolic pressure is 15 to 25 mm Hg. Normal mean PA pressure is 10 to 20 mm Hg.
Pulmonary capillary wedge pressure (PCWP)	Reflects left atrial pressure and left ventricular end-diastolic pressure (left ventricular preload)	Normal range is 6 to 15 mm Hg.
Systemic vascular resistance (SVR)	The amount of resistance the heart must overcome to open the aortic valve and push the blood volume out into the systemic circulation	Normal range is 800 to 1200 dynes-sec/cm-5.
Pulmonary vascular resistance (PVR)	Reflects the resistance the blood must overcome to pass into the pulmonary vasculature	Normal is < 250 dynes-sec/cm-5.

Novel Cardiac Output Monitoring Devices

The gold standard for cardiac output monitoring is using periodic measurements derived from a pulmonary artery catheter (PAC). However, there are safety concerns with PACs (e.g., infection, pneumothorax, pulmonary artery rupture) and evidence suggests that there is no mortality benefit. Devices for minimally invasive cardiac output monitoring using arterial pressure tracings and pulse-contour analysis or chest bioreactance have been developed. Esophageal doppler monitoring utilizes a flexible trans-esophageal doppler ultrasound probe to estimate cardiac output and stroke volume. These techniques perform better to monitor trends in cardiac output as opposed to providing absolute cardiac output values.