Retroperitoneal Bleeding

This can present immediately in the cath lab as well as, as you've seen, in a delayed timing. It's a 'rare' complication but since PCI in all it's permutations occurs over a million times a year, it happens in thousands of cases. There are risk groups for this event that should be known to nurses caring for these patients. 

A couple of case reports that are instructive to your questions:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3617546/

I have experienced one patient with a retroperitoneal bleed. She had abdominal pain, nausea, and hypotension. It was diagnosed approximately 2 hours after the procedure. The cause was that when the interventional cardiologist accessed the artery, he went through it. So, the actual site to me looked fine, but the patient was bleeding on the back side of the artery which was internal.

Even though it appears that the provider held pressure for an extended amount of time - it might not have been enough for this particular patient.
 

During PCIs, the patients receive blood thinners (ie. Heparin, Angiomax) so taking the sheath out too soon can cause these RPBs. 
 

It was a great use of your assessment skills, seeing his BP drop and trending the H/H. Hindsight is sometimes 20/20 in these situations. What could be done next time, wait another day or so before the provider removes the sheath.

Other safety measures include:

Noting Platelet Count, aPTT, and PT/INE prior to removal of any catheter/sheath.

This this was more than 5 years back so I don't remember all of the specifics of this case: 

I had a patient in ICU: Septic shock, intubated, on propofol, fentanyl, levophed, vasopressin, a heparin gtt, ABX, electrolyte protocols... She had a PICC line and A-line (upper only, no fem insertions that I remember) placed a few days prior but those were the most invasive procedures she had received. 

The night I was assigned to take care of her I was told during report that goals were to improve BPs, wean sedation, and then pressors if tolerated. Strangely no matter how low we went with sedation, the patient's bp kept going down, steadily during day shift and then through my night shifts. Labs were borderline but OK when I assumed care. 

The night while trying to reduce sedation the patient kept grimacing whenever I touched her abd and it was like she kept wanting to hold her abd or point to it. The patient was obese, and abd was rounded however not "hard." I had to keep increasing pressors while reducing sedation but nothing was fixing the BP and the patient appeared to be in pain. Concerned that something was up, I drew her labs early from her A-line. Several labs were criticals and were all over the place, including a significant H&H drop (hgb was like 7-8s, now only in the 6s). Was ordered to redraw labs by MD just in case the H&H and anything else was wrong. While waiting for results, the MD's were thinking to start CRRT (as she was trending that way anyway). The patient didn't tolerate it though, nearly coded when we initiated it, so we stopped it right away. 

Her second H&H was LOWER than the first I had drawn (hgb 4.6 I believe) and we initiated mass transfusion. We also took her to CT and showed a spontaneous RP bleed. Hep gtt stopped. OR consulted. 

I stayed until 10AM trying to help oncoming nurse stabilize this patient. I still don't know what happened nor why she developed this RP bleed like this. 

also related specifically to cardiac cath procedures: if it was femoral access and was considered a "high stick" that can certainly lead to problems with controlling bleeding