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:
During CPR the ETCO2 abruptly rises from 12 mmHg to 38 mmHg and stays elevated. This MOST likely indicates:
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:
On a ventilator flow-time scalar, the expiratory flow tracing does NOT return to baseline (zero) before the next breath begins. This finding indicates: