(sorry about the slightly shyte sound quality on this one)
HY lecture notes:
Pulmonary capillary wedge pressure (PCWP) is exceedingly HY on the USMLE and remains a concept students struggle with.
PCWP = left atrial pressure.
If you stick a catheter tip through the venous system all the way up to the right atrium, then right ventricle, then pulmonary arteries, until it can’t go any farther within a pulmonary capillary, any pressure waves it senses at the distal tip of the pulmonary capillary must be coming from the left atrium, since the LA is just distal to the pulmonary circulation. So if LA pressure is up for whatever reason, so is PCWP.
High PCWP occurs if there is any left heart pathology. Not just in cardiogenic shock, but also in mitral stenosis (increased afterload on LA –> so more pressure in LA –> higher PCWP) or mitral regurg (higher preload in LA –> higher pressure –> higher PCWP), as well as any cause of left ventricular hypertrophy (if the LV is experiencing a pressure or volume overload, that effect will back up to the LA).
In aortic stenosis –> higher afterload on LV (concentric hypertrophy) –> therefore higher afterload on LA –> higher PCWP.
In aortic regurg –> higher preload on LV (eccentric hypertrophy) –> therefore higher preload on LA –> higher PCWP.
It should also be noted that it is very rarely stated that the left atrium “hypertrophies”; the LV will hypertrophy; the LA will dilate. So any pathology at the level of the mitral valve or later (LV, aortic valve, or aorta) will cause left atrial dilatation and increased PCWP.
Most pleural effusions are due to either congestive heart failure (CHF) or malignancy.
CHF = left heart failure + right heart failure.
The most common cause of right heart failure is left heart failure.
Therefore in CHF, PCWP is high –> this increased pulmonary pressure leads to pulmonary hypertension (where there is actual hypertrophy of the tunica media of the pulmonary arteries) –> backs up to the RV –> right heart failure.
So let’s say you’ve got fluid in the lungs, but they say PCWP is normal –> that means the fluid in the lungs can’t be due to left-heart origin because, if it were, PCWP would have to be high.
ARDS (acute respiratory distress syndrome) presents with normal PCWP. ARDS is when proteinaceous fluid leaks out into the alveoli bilaterally and is frequently due to sepsis, trauma, or pancreatitis. So USMLE Qs like to say, e.g., 44M alcoholic with abdominal pain gets dyspnea with bilateral infiltrates + his PCWP is normal –> Dx = ARDS.
In ARDS, pO2/FiO2 must be less than 300. It is treated with low-tidal-volume mechanical ventilation (prevents barotrauma) + permissive hypercapnia (we allow CO2 to be slightly elevated [>44 mmHg] to prevent barotrauma).
Hypovolemic shock: low VR, low CO, high TPR, low PCWP
Distributive shock (anaphylactic, septic, neurogenic): high VR, high CO, low TPR, normal PCWP
Cardiogenic shock: low VR, low CO, high TPR, high PCWP
After giving fluids, give:
Anaphylactic –> IM epinephrine
Septic –> Norepinpehrine
Cardiogenic –> Dobutamine (beta-1 agonist) or Dopamine
