Catheter path, balloon, and the zero
A balloon-tipped pulmonary-artery catheter is floated from a central vein (usually RIJ) through the RA → RV → PA. The distal lumen sits in a PA branch. Inflating the balloon occludes that branch so the distal port sees pulmonary-capillary / left-atrial pressure (PCWP). Proximal ports read RA (CVP) and, on some catheters, RV.
Every number is only as good as the transducer. Zero at the phlebostatic axis: 4th intercostal space, mid-axillary / mid-chest, patient supine. A 10 cm height error is about 7.5 mmHg — enough to misclassify pulmonary hypertension or invent hypovolemia. If every chamber is impossibly low and the MAP is fine, re-zero before you treat.
Read pressures at end-expiration (the moment pleural pressure is closest to zero). Digital means average the whole cycle and will lie on a swinging ventilator.
Normal PA-catheter waveforms
The console in this lab redraws four traces as the physiology changes. Learn the pictures here, then open a case and name the chamber before you believe the digital mean. A Swan is only as honest as the waveform it is sitting in.
What each port is for
Four lumens, four jobs. The balloon is only inflated to get to the wedge and to confirm it — not to live there.
| Port | Sits in | What you read | What it estimates |
|---|---|---|---|
| Proximal / RA | RA | CVP: a, x, v, y | RV preload. Not LV preload. |
| RV (if present) | RV | RV sys / EDP | RVEDP should ≈ RA. Peak ≈ PA sys. |
| Distal, balloon down | PA branch | PA sys / dias / mean, notch | RV afterload. PAD ≈ PCWP if PVR is low. |
| Distal, balloon up | Occluded PA | PCWP: delayed a / v | LA pressure ≈ LVEDP if the MV is open. |
How the four traces line up on one beat
Read them against the ECG in your head, not as four independent graphs.
- a wave follows the P wave — atrial kick. On RA it fills the RV. On PCWP it fills the LV.
- c wave (often tiny) is the AV valve bulging back into the atrium as the ventricle starts to contract.
- x descent is atrial relaxation plus the floor of the atrium being pulled down. Preserved in tamponade.
- v wave is the atrium filling while the AV valve is closed (ventricular systole). Giant v = the valve is not competent (TR on RA, MR on PCWP).
- y descent is the atrium emptying when the valve opens. Blunted in tamponade. Brisk in constriction and after a giant v.
PCWP is an RA-like tracing, delayed by transit through the pulmonary veins: a after QRS, v after T. If your “wedge” still has a dicrotic notch, you are in the PA.
Numbers that have to agree
| Check | Should be | If it is not |
|---|---|---|
| RV peak vs PA sys | Nearly equal | Pulmonic stenosis, or you are not in the PA |
| RVEDP vs RA | Nearly equal | Catheter whip, wrong port, or not end-expiratory |
| PCWP vs PAD | PCWP ≤ PAD | Overwedge, still in PA, or a giant v you averaged |
| PCWP vs LVEDP | Close if no MS | Mitral stenosis (PCWP > LVEDP), stiff LV |
- RA / CVP
- RV
- PA
- PCWP
Abnormal PCWP — the tracing that changes therapy
Most of the harm from a Swan comes from believing a bad wedge. The left-atrial tracing is where volume, MR, tamponade, occult HFpEF, and artifact all pretend to be each other. Name the shape before you name the mean.
A one-pass read of every wedge
- Is it even a wedge? a/v morphology, no dicrotic notch, distal sat ≥95%, balloon in a distal PA. A leftover PA tracing is the commonest fake.
- Is it overwedged? Damped, too high, often above PAD. Deflate, pull a centimeter, reinflate gently.
- Read a and v, not the mean. Giant v is MR (or a stiff LA). Giant a is MS, a stiff LV, or a cannon wave. No a is AF.
- Read the y descent. Blunted = tamponade (or MS). Brisk / dip-and-plateau = constriction or a giant v emptying.
- Compare to PAD and RA. PCWP stays at or below PAD; a wedge above PAD is almost never real. PCWP ≈ RA and a blunted y is equalization. PCWP low + RA high is the right heart (PE, RV infarct).
| PCWP picture | Mean | The tell | First move |
|---|---|---|---|
| Normal | 6–12 | a ≈ v, y present, ≤ PAD | Believe it |
| High, preserved a/v | 18–30 | Both waves up, wet lungs | Unload / diurese |
| Low, small a/v | <6 | Dry, or the zero is on the floor | Fill — after you re-zero |
| Giant v | Mean lies low | v to 40–50, brisk y | Afterload ↓. Echo. Surgery if papillary. |
| Giant a / cannon a | Mean middling | a ≫ v | MS, stiff LV, or AV dissociation |
| No a wave | Variable | Irregular v only | AF. Do not invent an a. |
| Overwedge | Falsely high | Damped, often > PAD | Deflate, withdraw, retry |
| Still in PA | Looks like PAD | Notch still there, mixed-venous sat | You never wedged |
| Tamponade | Equals RA | y gone, x kept | Drain. Do not fill for long. |
| Constriction | Equals RA | Deep y, square root | Not tamponade. Imaging / peel. |
| Occult HFpEF | 12 at rest | Jumps on 500 mL or bike | Do not start Group-1 therapy |
Two more ways a wedge lies
- Not end-expiration. Spontaneous breathing: inspiration pulls the number down — read the high point (end-expiration). Positive-pressure ventilation: inspiration pushes the number up — read the low point (end-expiration). Digital means average the swing and will misclassify PH or invent hypovolemia on a 15 cmH₂O PEEP.
- Resting 12, stressed 28. Occult HFpEF. A pretty resting wedge does not clear the left atrium. Supine bike past ~25 mmHg, or a 500 mL fluid challenge that jumps PCWP above 18 with little CO gain, is the same disease. One bolus is a test. The second is a treatment you did not mean to write. Do not start Group-1 therapy on a resting tracing and a story.
Looks like X, is actually Y
| You see | Could be | What splits them |
|---|---|---|
| High mean wedge | LV failure | Preserved a/v, wet, high PAD ≈ PCWP |
| High mean wedge | Overwedge | Damped, > PAD, sat not arterial |
| High mean wedge | Giant v averaged | Look at the paper. v ≫ mean. |
| High mean wedge | Still in PA | Notch present, mixed-venous sat |
| Giant peak on wedge | MR v wave | a still there, no notch, brisk y |
| Giant peak on wedge | Cannon a | Tracks the P wave, not systole |
| Equalized diastolic | Tamponade | y blunted, pulsus, post-op / bleed |
| Equalized diastolic | Constriction | Deep y, Kussmaul, pericardial knock / Ca²⁺ |
| Low wedge | Hypovolemia | RA also low, dry exam |
| Low wedge | PE / RV failure | RA high, tall PA, PAPi < ~1 |
| Low wedge | Transducer on the floor | Every chamber low, MAP fine |
Confirm every surprising wedge
A real wedge has three votes. Need two of three if the number is going to change therapy:
- Waveform — a/v morphology, not a damped flat line and not a leftover PA tracing.
- Saturations — blood from the distal port should look arterial (typically 95% or higher). A mixed-venous sat means you are still in the PA.
- Fluoro / CXR — balloon in a distal PA, not overinflated into a tiny branch (overwedge) and not still in the main PA.
Overwedge: the balloon sees itself. The number is too high, often above PAD, and the tracing is damped. Deflate, withdraw a centimeter, reinflate gently. Treating a fake wedge of 34 with diuretics creates real hypovolemia.
Cardiac output: thermodilution vs Fick
Thermodilution injects cold (or room-temp) indicator through the RA port and reads the PA thermistor. Average three decent curves. It fails when the indicator recirculates or never mixes — severe TR wrecks TD, as do shunts, very low flow, and an injection in the wrong port.
Fick is VO₂ / (CaO₂ − CvO₂). Use a measured VO₂ when you can; assumed VO₂ is a guess. In a left-to-right shunt, TD over-reads pulmonary flow (Qp). Know which cardiac output you are quoting.
This lab recomputes the derived set on every tick:
- SVR = (MAP − RA) / CO × 80
- CI = CO / 2.0 (BSA assumed 2.0 m²)
- CPO = MAP × CO / 451
- PAPi = (PAsys − PAdias) / RA
- PVR = (mPAP − PCWP) / CO ← Wood units
Four shock profiles
Name the column before you pick a drip. Filling pressures, flow, and SVR do most of the work; the waveform tells you when the column is a lie.
| RA / PCWP | CO | SVR | First lever | |
|---|---|---|---|---|
| Hypovolemic | Both low | Low | High | Fill (and blood) |
| Cardiogenic | PCWP high | Low | High | Inotrope / unload |
| Distributive | Normal-low | High | Low | Pressor + source |
| Obstructive | RA high, PCWP low* | Low | High | Relieve the obstruction |
*Tamponade equalizes RA and PCWP — it is obstructive with a different tracing. Massive PE keeps the wedge low.
Forrester: cold-wet vs warm-dry
Forrester plotted CI against PCWP after infarct. The bedside version:
- Warm-dry — CI adequate, wedge low. Leave them alone, or they are empty.
- Warm-wet — CI adequate, wedge high. Diurese / vasodilate.
- Cold-dry — CI low, wedge low. Fill carefully, then inotrope. RV infarct lives here.
- Cold-wet — CI low, wedge high. The classic cardiogenic box. Unload. Do not give a liter for the kidneys.
CPO and PAPi
Two dimensionless-feeling numbers that are not dimensionless, and that decide devices.
Cardiac power output (Fincke, SHOCK trial): CPO = MAP × CO / 451, in watts. A left ventricle that cannot deliver 0.6 W is in trouble. CPO is also low in empty hypovolemia — it is a product, not a diagnosis. Read it with the wedge.
PA pulse-pressure index (Korabathina): PAPi = (PAsys − PAdias) / RA. The RV that cannot generate a pulse pressure over a high RA has a PAPi below 1.0. That is the number people use when they talk about RV MCS.
Both down at once is biventricular crash. Stacking drips is not a plan.
PH classification (ESC/ERS 2022)
Pulmonary hypertension is mPAP above 20 mmHg. The wedge and the PVR decide the compartment:
| PCWP | PVR | What it is | |
|---|---|---|---|
| Pre-capillary | ≤15 | >2 WU | Group 1 / 3 / 4 pattern |
| Isolated post-cap (Ipc) | >15 | ≤2 WU | Left heart — diurese |
| Combined (Cpc-PH) | >15 | >2 WU | Left heart plus a pre-cap component |
Confirm the wedge before you classify. A pulmonary vasodilator on a high LA pressure can flood the lung. Diurese Cpc-PH first; the residual PVR is the pre-capillary piece.
Giant v waves and acute MR
The digital mean PCWP averages away the story. A v wave to 40–50 mmHg after an infarct is acute MR (or, less often, a stiff LA) until echo disagrees. Afterload reduction shrinks the regurgitant fraction; a pure pressor enlarges it. Papillary-muscle rupture is a surgical disease — the catheter only names it.
Tamponade equalization
RA = RVEDP = PAD = PCWP is an equalized diastolic plateau. On the RA tracing the y descent is blunted (x is often preserved). Pulsus, a vanishing pulse pressure, and a climbing CVP after cardiac surgery do not wait on a formal lab. Fluids are a bridge of minutes. Drain the pericardium.
Exercise RHC and the fluid challenge
A resting PCWP of 12 does not clear the left atrium. Occult HFpEF is diagnosed when the wedge rises on stress:
- Supine bike — PCWP rising past ~25 mmHg is the usual teaching cutoff.
- 500 mL fluid challenge — when the patient cannot pedal. PCWP jumping above 18 with little CO gain is the same disease. One bolus is a test; the second is a treatment you did not mean to write.
Do not start Group-1 therapy on a pretty resting tracing and a story.
O₂ sat run
Sample SVC, IVC, RA, RV, and PA. A step-up of about ≥7% from the cavae into the RA is the classic atrial-level left-to-right screen (ASD, anomalous pulmonary return). A PA sat in the mid-80s without fever is not “great cardiac output” — it is arterial blood arriving early. Thermodilution then quotes Qp, not systemic CO.
High-output failure
High CO, low SVR, high filling pressures: the heart is overworking into a low-resistance circuit. Drivers to name out loud — ESLD, large AV fistula, thyrotoxicosis, beriberi, severe anemia, sepsis-overlap. Stop the fluids. Echo is hyperdynamic, not empty. An inotrope on a 9 L heart is not a plan.
When to escalate MCS
Think about a device when:
- CPO stays below 0.6 W after you have unloaded and inotroped.
- PAPi stays below 1.0 and the RA is climbing (RV MCS / biventricular plan).
- Lactate and urine are still going the wrong way on two drips.
- You are about to add a third pressor to a wet, cold patient.
The catheter does not pick Impella versus VA-ECMO versus a surgical pump. It tells you which ventricle has left the building.
How to work a case in this lab
Same order every time — it is the order that keeps people alive at 03:00.
- Look at the patient: warm or cold, wet or dry, urine, lactate.
- Look at the tracing: RA a/x/v/y, PA notch, wedge morphology. Confirm any number that would change the drug.
- Name the profile in one sentence (empty, cold-wet, vasoplegic, obstructive, pre-cap, artifact…).
- Do the matching move. Harmful clicks treat the wrong chamber.
- Untreated tamponade, massive PE, or a crashed CPO/PAPi will keep dropping MAP and climbing lactate until you fix the cause.
Guided mode forces three decisions. Free play is the same physiology without the multiple choice. The debrief is the attending in the doorway.