ABGs and Heart Failure

emtb2rn, BSN, RN, EMT-B · Oct 25, 2006

You're not missing anything. Pulse ox's are contraindicated in smokers (O2 levels can be falsely elevated) and can return incorrect readings in folks with anemia and impaired cardiopulmonary function, among other things, but I believe those are relevant to your question.

km5v6r, EdD, RN · Oct 26, 2006

Ok school was a long time ago (before oximetry was available even) but here goes. Sats are going to tell you if the pt is receiving enough oxygen but not how well he is able to use the oxygen or get rid of the CO2. ABG's or VBG's are also necessary to monitor the pH and pCO2. The pt may have a sat of 100% with a pO2 >200 but if he has a pH of 6 and a pCO2 of 150 he is still in deep, deep trouble. And yes I have seen these types of numbers in real pt's. Many times we do use pulse oximetry on vent pts but vent management; determining the proper mode of ventilation, the proper rate etc; depends on knowing both the pH and the pCO2. Think about what happens when you shift the oxyhemaglobin curve in a pt whose pH is either to high or to low. Also as a smoker this pt is at risk for COPD and CO2 retention.

Megsd, BSN, RN · Oct 26, 2006

Thanks guys!

Daytonite, BSN, RN · Oct 26, 2006

the answer lies in the working of the heart and it's hemodynamics. icu was not my field, but i do have some links that might help you out in finding the answers. i can tell you that the pulse oximetry gadgets are not all that accurate. for serious blood gas monitoring an arterial sampling needs to be taken. the pulse oximetry gizmos work on a mathematical formula that is programmed into their mini computer chip. however, if the patient's heart rate is irregular at all, it can throw the oximetry reading off. also, pulse oximetry measures peripheral saturation in a finger. after open heart surgery the physicians want to see what kind of oxygen perfusion the cardiac tissue is getting. that would best be assessed with an arterial sampling of blood.

http://www.ccmtutorials.com/rs/oxygen/index.htm - all about oxygen, a tutorial about oxygen with a section on "how pulse oximeters work" talks about the downside of the use of the gadget. you will find your answer here.

http://learn.sdstate.edu/craigg/coutput.html - cardiac output. the relationship of heart rate, stroke volume, preload, afterload and contractility on cardiac output along with the affect of the drugs used to treat these various things.

http://learn.sdstate.edu/craigg/mentele.html - this is a student case study on a patient with chf

http://classes.kumc.edu/son/nurs420/unit4/hemomon.html - a tutorial on hemodynamic monitoring from the university of kansas school of nursing

NurseguyFL · Oct 29, 2006

km5v6r and emtb2rn hit the nail on the head. Pulse Oximetry is an unrealiable method of monitoring this patient's oxygenation. The patient's hx of smoking and anemia are just two of many reasons I can think of that make pulse oximetry unsuitable in this case. Long term smokers are prime candidates for COPD, and this is made worse by anemia because there aren't enough red blood cells to transport and distribute the O2.

A pulse ox monitor doesn't always work well on patients with anemia, and the level of innaccuracy can vary, depending on where on the patient's body the monitor is attached. If, for example, this is a patient with poor peripheral circulation r/t diabetes or any type of cardiovascular disease, and the extremities are cool, you may get a poor pulse ox reading or no reading at all if you attach the monitor to a finger. On the other hand, you may get a 100% saturation reading from a pulse ox monitor (which technically means that the patient is receiving adequate O2), but the real question is: how well is that O2 being utilized by the body? Another problem with pulse oximetry is that it doesn't tell you if the patient is blowing off enough CO2, and if he's retaining that CO2 that's just another problem waiting in the wings. You need arterial blood gases to ontain this information.

You also have to pay close attention to the patient. A sat monitor can continue to show readings in the high 90s---even when the patient has rapid, labored respirations, and is beginning to turn blue. This actually happened in a hospital where I used to work, and the nurse kept the patient there and kept turning up the O2 (on a COPD-er :uhoh3: ) until the patient coded---and died!

Nrs_angie, BSN, RN · Oct 30, 2006

Hi there,

In responses to this post, I see no one else has mentioned acid-base balance. ABGs are most commonly used to assess the acid-base balance. The respiratory, cardiovascular, and renal systems work hand in hand to maintain this balance. For people with chronic renal failure, it might be helpful to see how well they are compensating. The anemia and smoking history are also correct, they will alter a pulse ox reading. However I did find a source for you that mentions the use of ABG's with CABG. Schnell, Van Leeuwen, & Kranpitz (2003) state "This group of tests is used to assess conditions such as ashtma, chronic obstructive pulmonary disease (COPD), embolism (e.g., fatty or other embolsim) during coronary arterial bypass surgery, and hypoxia". The authors also noted that ABGs are generally indicated for patients on ventilators or being weaned from ventilation (Schnell, Van Leeuwen, & Kranpitz, 2003). Ignatavicius & Workman (2002) also note that, a client is placed on the bypass machine intraoperatively, to provide oxygenation and ciruculation during an induced cardiac arrest. The client is also cooled to a hypothermic state during the procedure to reduce myocardial oxygen demand. In the postoperative care of a client who has undergone CABG, after surgery the client is transported to a post- open heart surgery unit where they will be placed on a ventilator for 3 to 6 hours. The nurse also monitors for complications of CABG that include hypothermia, hypertension, hypotension, and fluid / e-lyte imbalance, among a few other things. The nurse monitors the body temperature post-operatively and maintains rewarming procedures. If the client is rewarmed too quickly, this may lead to metabolic acidosis and hypoxia (Ignatavicius & Workman, 2002).

A pulse oximeter will not monitor for a disturbance in acid-base balance.

So far this is all I have found... I will get back to you when I get more info. Hope this is helpful!

Angie

I am working on a case study for a pt with CAD and CHF who has just undergone a CABGx3. He has a history of chronic renal insufficiency, anemia, diabetes, smoking, hypertension. After surgery he has a Swan-Ganz catheter in place for hemodynamic monitoring, is intubated and put on a ventilator. ABGs are drawn. The question I am supposed to answer is:
"Why are ABGs necessary in the case of this patient? List two reasons why it would be inappropriate to use pulse oximetry on this patient to assess his oxygen saturation status"
I have looked in 3 textbooks and 3 sets of lecture notes and we have not covered anything about why a pulse oximetry would be inappropriate. The only things I can think of include possibly the anemia showing a low SaO2 that wouldn't reflect the PaO2 (but that I'm not sure about), or his smoking history lowering his SaO2 without lowering the PaO2. Or maybe that we want to monitor his ability to ventilate, which SaO2 won't necessarily show, since ventilation includes both O2 and CO2 levels.
Am I on the right track here? What am I missing? Thanks in advance for your help!