2.1 Hemodynamic Monitoring & Values

Why Hemodynamic Monitoring Matters

Hemodynamic monitoring measures how well the circulation delivers oxygen to tissue — the balance between oxygen delivery (DO2) and oxygen consumption (VO2). On the CCRN, cardiovascular content is 13% of the Clinical Judgment blueprint, and the most common item style hands you a set of numbers (cardiac output, SVR, wedge pressure, CVP) and asks you to name the disorder or select the next intervention. Memorizing the normal ranges — and, more importantly, the pattern each disorder produces — is the single highest-yield skill in this domain.

The Arterial Line

An arterial line (A-line) gives continuous, beat-to-beat blood pressure and a site for arterial blood gas sampling. The radial artery is the usual site; an Allen test confirms collateral ulnar flow first. The waveform's dicrotic notch marks aortic-valve closure. Two setup rules are tested repeatedly:

1. Level the transducer to the phlebostatic axis — the 4th intercostal space at the mid-anteroposterior (mid-chest) diameter, which approximates the right atrium. A transducer placed too high falsely lowers the reading; too low falsely raises it (roughly 7.5 mmHg per 10 cm of error).
2. Zero to atmosphere so ambient pressure registers as 0.

The square-wave (fast-flush) test validates the system's dynamic response. An optimally damped system snaps upward during the flush, then shows 1-2 sharp oscillations before settling to baseline. An overdamped trace (clot, air bubble, kink, loose connection) returns slowly with no oscillation and falsely lowers the systolic and narrows the pulse pressure. An underdamped trace shows many oscillations and falsely raises the systolic. Classic trap: assess the patient before treating a number — a dampened MAP in an awake, warm, conversant patient is artifact, not shock.

Central Venous Pressure (CVP)

CVP, measured from the distal port of a central line in the superior vena cava/right atrium, estimates right-ventricular preload. Normal CVP is 2-6 mmHg. It rises with volume overload, right-heart failure, cardiac tamponade, tension pneumothorax, and high PEEP; it falls with hypovolemia and vasodilation. CVP is a weak stand-alone predictor of fluid responsiveness — trends and dynamic measures outperform any single value.

The Pulmonary Artery (Swan-Ganz) Catheter

Threaded RA → RV → pulmonary artery, the PA catheter reports right- and left-sided pressures plus cardiac output:

• Pulmonary artery pressure (PAP): systolic 15-25, diastolic 8-15, mean 10-20 mmHg.
• Pulmonary artery wedge/occlusion pressure (PAWP/PCWP): the balloon is inflated to "wedge"; it reflects left-atrial pressure / LV preload. Normal 6-12 mmHg. Inflate no more than 1.5 mL for under 8-10 seconds to avoid pulmonary artery rupture or infarction.

Normal Hemodynamic Values

Cardiac output (CO) = stroke volume x heart rate, normal 4-8 L/min. Cardiac index (CI) indexes CO to body surface area, normal 2.5-4.0 L/min/m2; a CI below 2.2 signals inadequate perfusion. Systemic vascular resistance (SVR), normal 800-1200 dynes/sec/cm-5, is LV afterload: it climbs (compensatory) in hypovolemic and cardiogenic shock and falls in distributive shock. Stroke volume variation (SVV), normal < 10-13%, is a dynamic marker of fluid responsiveness valid only in sedated, fully mechanically ventilated patients in sinus rhythm — an SVV > 13% predicts that a fluid bolus will raise cardiac output.

Mean Arterial Pressure (MAP)

MAP is the driving pressure for organ perfusion, estimated as [SBP + 2(DBP)] / 3. The threshold to memorize: keep MAP >= 65 mmHg. Surviving Sepsis and most shock protocols target 65; higher targets have not improved outcomes and increase harm. Below roughly 60 mmHg, coronary, renal, and cerebral autoregulation collapses.

Mixed Venous Oxygen Saturation (SvO2 / ScvO2)

SvO2, drawn from the PA catheter tip (true mixed venous blood), reflects the DO2/VO2 balance. Normal SvO2 is 60-75%. ScvO2 (central venous, from the SVC) runs a few points higher, roughly 65-80%. A low SvO2 (< 60%) means tissues are extracting more oxygen — from low CO, low hemoglobin, low SaO2, or high demand (fever, shivering, pain). A high SvO2 (> 80%) suggests impaired extraction, classically late/severe sepsis (microcirculatory shunting and mitochondrial dysfunction) or cyanide toxicity. Reading SvO2 alongside lactate and MAP tells you whether resuscitation is working better than any single pressure.

Oxygen Delivery and Consumption

The purpose of every number above is to protect oxygen delivery (DO2), which equals cardiac output x arterial oxygen content. Arterial oxygen content is driven mostly by hemoglobin and SaO2, so anemia and hypoxemia impair delivery just as much as a falling cardiac output. Oxygen consumption (VO2) rises with fever, shivering, seizures, pain, and work of breathing. When delivery cannot meet demand, tissues extract more oxygen (SvO2 falls) and lactate climbs — the earliest laboratory signal of shock. Serial lactate clearance (a falling lactate over hours) is a validated resuscitation endpoint on the CCRN, often more useful than any single pressure.

Static Versus Dynamic Assessment of Fluid Responsiveness

A frequent exam theme: static filling pressures (CVP, PAWP) poorly predict who will respond to a fluid bolus. Dynamic measures are superior. Stroke volume variation (SVV) and pulse pressure variation (PPV) above roughly 13% predict fluid responsiveness, but only in sedated, mechanically ventilated patients in sinus rhythm. The passive leg raise is a reversible "self-bolus" valid even in spontaneously breathing or arrhythmic patients: raising the legs autotransfuses about 300 mL of venous blood, and a >10-15% rise in stroke volume or cardiac output predicts the patient will respond to fluids.

Common Monitoring Traps

• Treating the number, not the patient. A sudden MAP drop with a dampened waveform in a warm, awake patient is artifact — re-level, re-zero, and run a square-wave test before titrating pressors.
• Forgetting to re-level after position changes. Every bed height or patient reposition requires re-leveling the transducer to the phlebostatic axis.
• Over-inflating the PA balloon or leaving it wedged, which risks pulmonary artery rupture.