1.3 Hemodynamic Monitoring & Waveform Analysis
Key Takeaways
CVP normal is 2-6 mmHg and reflects right-heart preload; it rises with right heart failure, fluid overload, and tension pneumothorax.
PCWP normal is 6-12 mmHg and estimates left-atrial pressure; a value above 18 mmHg signals cardiogenic pulmonary edema.
Cardiac output normal is 4-8 L/min and cardiac index 2.5-4.0 L/min/m2; the four classic shock profiles are distinguished by CVP, PCWP, CO, and SVR together.
SvO2 normal is 60-80%; a low value means oxygen delivery is failing to meet demand (low CO, anemia, hypoxemia, or high consumption).
Pulse oximetry reads falsely HIGH in carbon monoxide poisoning because carboxyhemoglobin is counted as oxyhemoglobin — use a CO-oximeter.
SpO2 can be normal despite dangerously low oxygen content in severe anemia, because it measures percent saturation, not total content.
Capnography normal ETCO2 is 35-45 mmHg; a sudden rise during CPR signals return of spontaneous circulation, while a flat-line means esophageal tube or no circulation.
Ventilator graphics reveal auto-PEEP (expiratory flow not returning to zero before the next breath) and patient-ventilator asynchrony.
Why RTs Read Hemodynamics
Mechanical ventilation, PEEP (positive end-expiratory pressure), and oxygen therapy all alter cardiac filling and output, so the TMC expects therapists to interpret hemodynamic numbers and connect them to a clinical state. Learn the normal value, then learn the one or two conditions that move it.
Core Parameters
| Parameter | Normal | Elevated In | Low In |
|---|---|---|---|
| CVP (right-heart preload) | 2-6 mmHg | RV failure, fluid overload, tension pneumothorax, tamponade | Hypovolemia |
| PAP | 25/10 mmHg (mean ~15) | Pulmonary hypertension, PE, ARDS, hypoxic vasoconstriction | Hypovolemia |
| PCWP (left-atrial estimate) | 6-12 mmHg | Left heart failure (>18 = cardiogenic edema), mitral disease | Hypovolemia |
| CO | 4-8 L/min | Early sepsis, exercise, fever | Cardiogenic/hypovolemic shock |
| CI | 2.5-4.0 L/min/m2 | (CO indexed to BSA) | Same as CO |
| SVR | 800-1200 dynes-sec/cm5 | Cardiogenic shock, hypovolemia (compensatory) | Warm septic shock, anaphylaxis |
| SvO2 | 60-80% | High delivery / low demand | Shock, low CO, anemia, hypoxemia |
The Four Shock Profiles
| Shock | CVP | PCWP | CO | SVR |
|---|---|---|---|---|
| Cardiogenic | High | High | Low | High |
| Hypovolemic | Low | Low | Low | High |
| Septic (warm) | Low | Low | High | Low |
| Obstructive (tamponade/tension PTX) | High | High/equalized | Low | High |
The pump-versus-tank logic resolves most stems: a failing pump backs blood up (high filling pressures, low CO), an empty tank drops every pressure, and distributive/septic shock vasodilates (low SVR) while CO is high early.
Pulse Oximetry — and Its Lies
Noninvasive SpO2 estimates arterial saturation from light absorption at two wavelengths. Normal is 95-100%; in COPD the target is often 88-92% to avoid blunting the hypoxic drive and to limit CO2 retention.
| Pitfall | Effect on SpO2 |
|---|---|
| Carbon monoxide poisoning | Falsely HIGH — carboxyhemoglobin reads as oxyhemoglobin |
| Methemoglobinemia | Pins toward ~85% regardless of true value |
| Severe anemia | Normal SpO2 despite low oxygen CONTENT |
| Poor perfusion / hypothermia / vasoconstriction | Erratic or absent signal |
| Dark nail polish, motion | Artifact, unreliable reading |
The distinction the NBRC tests: SpO2 reports the percentage of hemoglobin saturated, not the total oxygen carried. A burn-unit patient with SpO2 99% can be profoundly hypoxic from CO poisoning — confirm with a CO-oximeter and treat with 100% oxygen.
Capnography (ETCO2)
End-tidal CO2 measures exhaled CO2 and normally runs 35-45 mmHg, a few mmHg below PaCO2.
- Confirms intubation: a sustained waveform proves tracheal placement; a flat trace means esophageal intubation or no pulmonary blood flow.
- CPR quality / ROSC: ETCO2 reflects pulmonary perfusion during compressions; an abrupt sustained rise (e.g., 12 to 38 mmHg) signals return of spontaneous circulation.
- Rising ETCO2: hypoventilation, fever/sepsis, rebreathing.
- Falling ETCO2: hyperventilation, falling cardiac output, pulmonary embolism, or a circuit leak.
Ventilator Waveform Analysis
Scalars (pressure, flow, volume vs time) and loops detect problems numbers miss. Auto-PEEP (air trapping) appears when the expiratory flow tracing fails to return to zero before the next breath begins — common in COPD and high rates; the fix is to lengthen expiratory time, lower rate, or treat bronchospasm. A sharp negative pressure deflection before a breath signals patient effort and possible trigger asynchrony, while a pressure-time scalar that dips during a mandatory breath indicates flow starvation (inspiratory flow set too low).
Reading the Numbers as a System
The TMC rarely asks for a single normal value; it gives a constellation and expects a diagnosis. The pump-versus-tank framework resolves the shock table above, but two further integrations are worth drilling. First, SvO2 and cardiac output move together when oxygen demand is stable: a falling SvO2 with a falling cardiac output confirms that delivery is the limiting factor, whereas a high SvO2 in a septic patient reflects impaired tissue oxygen extraction rather than abundant delivery.
Second, PEEP raises intrathoracic pressure, which falsely elevates CVP and PCWP and can lower venous return and cardiac output; the exam expects you to recognize that a rising CVP after a PEEP increase may be an artifact of pressure transmission rather than true fluid overload.
Oxygen Content vs Saturation — the Core Distinction
The single most-tested monitoring concept is that saturation is not content. Arterial oxygen content (CaO2) is driven mostly by hemoglobin: CaO2 = (1.34 x Hgb x SaO2) + (0.003 x PaO2). The dissolved term is trivial, so halving the hemoglobin halves the oxygen delivered even at a perfect SpO2 of 100%. This is why a severely anemic patient, a carbon-monoxide-poisoned firefighter, and a patient in low-output shock can all read a reassuring pulse oximetry value while tissues starve.
When a stem pairs a normal SpO2 with a low hemoglobin, a smoke exposure, or a low cardiac output, the correct interpretation is inadequate delivery despite the number, and the recommendation is to treat the underlying defect — transfuse, give 100% oxygen, or support the circulation.
Capnography Waveform Shapes
Beyond the value, the shape of the capnogram carries information the exam tests. A normal waveform has a sharp upstroke, a flat alveolar plateau, and a sharp downstroke. A shark-fin (upsloping) plateau indicates obstructive airflow such as bronchospasm or a kinked tube, mirroring the wheeze heard on auscultation. A sudden drop to zero means a disconnect, complete obstruction, or esophageal placement, while a gradual fall suggests declining cardiac output or developing hypovolemia. Reading the trace alongside the number lets the therapist localize the problem to ventilation, perfusion, or equipment.
ICU hemodynamics: CVP 14 mmHg, PCWP 22 mmHg, CO 3.2 L/min, SVR 1800 dynes-sec/cm5. This profile is MOST consistent with:
Hypovolemic shock
Early (warm) septic shock
Cardiogenic shock
Anaphylactic shock
During CPR the ETCO2 abruptly rises from 12 mmHg to 38 mmHg and stays elevated. This MOST likely indicates:
The endotracheal tube has migrated into the esophagus
A malfunctioning capnograph
Development of a tension pneumothorax
Return of spontaneous circulation
A firefighter pulled from a structure fire is alert but has an SpO2 of 99% on room air. The therapist should recognize this reading as:
Accurate and reassuring
Irrelevant to the patient's oxygenation
Falsely low because of soot on the probe
Potentially falsely elevated from carbon monoxide poisoning
On a ventilator flow-time scalar, the expiratory flow tracing does NOT return to baseline (zero) before the next breath begins. This finding indicates:
A leak in the ventilator circuit
Correct, well-matched ventilator settings
Inadequate humidification
Air trapping (auto-PEEP)
Sections you finish are checked off in the contents.