Guide — Critical Care
Hemodynamic Monitoring Basics
Hemodynamic monitoring turns perfusion into numbers — pressures, flows, and a venous oxygen saturation that together say whether the circulation is keeping up with the tissues. This guide covers the core parameters, how oxygen delivery ties in, and the bedside pitfalls the RT needs to read alongside the gas.
10 min read · Critical Care
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Hemodynamic monitoring exists to answer one question in a critically ill patient: is the circulation delivering enough oxygen to the tissues? The numbers let the team assess perfusion, preload, afterload, contractility, and oxygen delivery, and then decide among the three levers — fluids, vasopressors, and inotropes — rather than guessing.
For the respiratory therapist, these values are not someone else’s data. The arterial blood gas, the venous oxygen saturation, and the capnogram all sit inside the same oxygen-delivery picture the hemodynamic monitor is tracking, and positive-pressure ventilation actively changes several of the readings. Reading them together is what makes the assessment coherent.
Key Concepts
A handful of parameters carry most of the bedside meaning. Knowing the normal range and what each one reflects is the foundation everything else builds on.
| Parameter | Normal | What It Reflects |
|---|---|---|
| MAP | 70–100 mmHg (target ≥ 65 in shock) | Organ perfusion pressure |
| CVP / RAP | 2–6 mmHg | Right-heart preload |
| PCWP (wedge) | 8–12 mmHg | Left-heart preload surrogate |
| Cardiac output | 4–8 L/min | Total blood flow per minute |
| Cardiac index | 2.5–4.0 L/min/m² | CO indexed to body surface area |
| SVR | 800–1200 dynes·s·cm⁻⁵ | Left-ventricular afterload |
| SvO₂ (mixed venous) | 60–80% | Global oxygen supply/demand balance |
| ScvO₂ (central venous) | ≥ 70% | Central venous surrogate for SvO₂ |
Several of the most useful numbers are derived rather than measured directly:
Systemic vascular resistance
SVR = [(MAP − CVP) / CO] × 80
Quantifies left-ventricular afterload from the pressure gradient and flow.
Oxygen delivery
DO₂ = CaO₂ × CO × 10
Ties arterial oxygen content to cardiac output — the supply side of the balance.
SvO₂ falls when delivery is inadequate or when demand rises, which is what makes it such a sensitive readout of that supply-demand balance.
These numbers come from a range of methods. An arterial line gives continuous blood pressure and a port for ABG sampling. A central venous catheter provides CVP and a central venous oxygen saturation (ScvO₂). A pulmonary artery (Swan-Ganz) catheter adds pulmonary artery pressures, the wedge, thermodilution cardiac output, and a true mixed venous SvO₂. Less-invasive options — arterial waveform analysis, pulse pressure variation for fluid responsiveness in a fully ventilated patient, the passive leg raise, and echocardiography — increasingly substitute for the indwelling catheter.
Assessment & Findings
The parameters earn their keep when read as patterns. A few combinations point cleanly toward a physiology and its treatment.
- Low CVP + low cardiac output = hypovolemia. An underfilled circulation — the pattern that responds to volume.
- High wedge + low cardiac output = left-ventricular failure. The left heart is full but not moving blood forward — a pump problem, where inotropy and afterload reduction matter more than fluid.
- Low SVR + high or normal cardiac output = distributive (septic) shock. The vasculature has lost its tone; the flow is there but the pressure is not — the setting for vasopressors.
- Low SvO₂ = inadequate delivery or excessive demand. The tissues are extracting more oxygen than usual — a prompt to look at cardiac output, hemoglobin, arterial saturation, and the patient’s metabolic load.
RT Priorities / Interventions
Much of the hemodynamic picture is directly relevant to the RT, and several pieces of it move with the ventilator the RT controls.
- Read the ABG inside the oxygen-delivery picture. Arterial oxygen content is the supply side of DO₂; interpret the gas alongside cardiac output and hemoglobin rather than in isolation.
- Use SvO₂ and ScvO₂ as a feedback loop on your interventions. They reflect the DO₂/VO₂ balance the RT influences through oxygenation and ventilation — improving oxygenation or reducing work of breathing should move them in the right direction.
- Watch capnography as a perfusion monitor. End-tidal CO₂ tracks cardiac output and pulmonary perfusion; an abrupt fall in EtCO₂ can signal a drop in output before the blood pressure follows.
- Account for positive-pressure ventilation. It reduces venous return and alters every preload-dependent reading; factor the ventilator settings and PEEP into how you and the team interpret CVP, wedge, and output.
Common Pitfalls
- Chasing a single CVP value. A lone CVP is a poor predictor of fluid responsiveness — trend it and pair it with a dynamic measure rather than treating one number.
- Failing to zero and level the transducer. A transducer not zeroed and leveled at the phlebostatic axis gives systematically wrong pressures — the readings are only as good as the setup.
- Ignoring the effect of positive-pressure ventilation. PEEP and ventilator pressures inflate transduced values and lower venous return; readings interpreted as if the patient were breathing spontaneously will mislead.
- Over-relying on the wedge. The PCWP is a surrogate with real limitations and a procedural risk; it is one data point among several, not the final word on left-heart filling.
Board Exam Pearls
- Memorize the normals cold: CVP 2–6, wedge 8–12, CI 2.5–4, SvO₂ 60–80%, SVR 800–1200, and MAP ≥ 65 as the shock target.
- Know the SVR formula — SVR = [(MAP − CVP) / CO] × 80 — and what a high or low value implies about afterload.
- A low SvO₂ means inadequate delivery or excessive demand; reason from cardiac output, hemoglobin, arterial saturation, and metabolic load.
- Level the transducer at the phlebostatic axis — a classic stem detail that turns a plausible reading into a wrong one when it is omitted.
FAQ
What is a normal mixed venous oxygen saturation and what does a low value mean?
Normal mixed venous oxygen saturation (SvO₂) is 60–80%, measured from the pulmonary artery, and it reflects the global balance between oxygen delivery and consumption. A low SvO₂ means the tissues are extracting a greater fraction of the oxygen carried to them — either because delivery is inadequate (low cardiac output, low hemoglobin, or low arterial saturation) or because demand has risen (fever, shivering, agitation, work of breathing). It is one of the better single bedside indicators that the supply-demand balance has tipped against the patient.
What does the wedge pressure estimate?
The pulmonary capillary wedge pressure (PCWP), obtained by inflating the balloon at the tip of a pulmonary artery catheter, is a surrogate for left-atrial pressure and therefore for left-ventricular preload — normal is 8–12 mmHg. A high wedge with a low cardiac output points toward left-ventricular failure or volume overload, whereas a low wedge with a low output suggests hypovolemia. It estimates left-heart filling, not the right heart, which is what central venous pressure reflects.
Why is a single CVP reading a poor guide to fluids?
Central venous pressure (normal 2–6 mmHg) is a static number influenced by venous tone, right-ventricular compliance, intrathoracic pressure, and tricuspid disease — so any one value correlates poorly with whether a patient will actually increase cardiac output in response to a fluid bolus. Modern practice favors dynamic measures of fluid responsiveness — pulse pressure variation in a fully ventilated patient, a passive leg raise, or echocardiography — and uses CVP as a trend alongside the rest of the picture rather than a stand-alone trigger to give fluid.
How does positive-pressure ventilation affect hemodynamic readings?
Positive-pressure ventilation, and PEEP in particular, raises intrathoracic pressure, which reduces venous return and can lower preload-dependent readings and cardiac output. It also inflates the measured CVP and wedge because those transduced pressures include the surrounding intrathoracic pressure. The practical consequences: interpret CVP and wedge in the context of the ventilator settings, read end-expiratory values, and remember that the same cyclic pressure swings are what make pulse pressure variation a useful index of fluid responsiveness in a passively ventilated patient.
Put it to work
The venous saturation and the oxygen-delivery picture only make sense next to the gas. Run an ABG through the interpreter and correlate the arterial oxygen content and SvO₂ with what the numbers are telling you about delivery.
Open the ABG Interpreter →Related Resources
Sources
- Cecconi M, De Backer D, Antonelli M, et al. Consensus on circulatory shock and hemodynamic monitoring. Task force of the European Society of Intensive Care Medicine. Intensive Care Med. 2014;40(12):1795-1815.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Hemodynamic monitoring chapter.