If the GFR was low, this would indicate that the pt was in some type of renal failure/ renal insuff, acute tubular necrosis (ATN). One of the things that the kidneys do is produce bicarbonate. Pts who are on dialysis are often on PO bicarb (if it isn't properly corrected during dialysis). If the kidneys were damaged, the bicarb gtt would replace what would normally be produced in the body. Renal pts are also at risk for becoming acidotic. The gtt helps combat that.
Your PH is determined by your hydrogen concentration (acid). Renal pts can lose the ability to pass H+ in the urine. You retain H+, making you more acidic. So, a bicarb gtt will act as a buffer to the retained hydrogen molecules.
Sodium bicarbonate itself act as a protector to the kidneys. By alkalizing your urine, it can reduce damage to the kidneys from contrast-induced nephropathy. If you pt was going to the cath lab, then they are going to receive contrast. The gtt helps combat that too. Nephrologists hate contrast.
With ATN, cells are sloughing off in the blood and the bicarb gtt can protect it with alkalinization and proper flushing.
FYI...Bicarb gtts also help lower potassium levels by shifting K+ back into the cell. Renal pts can have issues with hyperkalemia, but cardiac pts should have a potassium above 4 (for proper electrical cycles of the heart). If a pt is on a bicarb gtt, you should always check a K+ while they are on it. At least every 12 hrs. If the bag has 100 meq or 150 meq of bicarb, the K+ level should be checked every 8 or even 6. Bicarb makes potassium drop rapidly. It's great for someone with hyperkalemia, but if the level was normal initially, be ready to call the doc for some K+ replacement at some point.