CVP readings

  1. Hi everone.
    When reading the CVP via a monitor. First you need to zero the transducer, to set it to atmospheric pressure. On our monitors it says "offset to, lets say, 5mmHg". My question is, when you eventually record the actual CVP do you take the zeroing reading into consderation? Or is the value on the monitor the value you record?

    For example:
    When zeroing, the transducer is offset to 5mmHg. The monitor then records 10mmHg as the CVP. Is the CVP 10mmHg or do you use the offset value. So that the CVP is actually 5mmHg.
    Also what are the lowest levels a CVP could record. I.e could it record minus levels?

    I hope this all makes sense to you.
    Any help here would be appreciated

    Thanks

    sazza
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  2. 24 Comments

  3. by   lovp
    Is your patient flat in the bed when you are zeroing your CVP line and reading your monitor?

    Kelly RN
  4. by   sazza
    Quote from lovp
    Is your patient flat in the bed when you are zeroing your CVP line and reading your monitor?
    Hi lovp. The patient was not flat. More like lying at 30 degrees in bed. No more than 45 degrees.
  5. by   babynursewannab
    Standard practice for us is ensuring the pressure bag is at set at 300. The line has been zeroed and the transducer is leveled with the phlebostatic axis (4th ICS at midaxillary line) regardless of pt head elevation.

    At that point, what is on the monitor is what is written down.
  6. by   pricklypear
    I'm trying to remember if our monitors ever say "offset". I always leave the transducer open until the # reads 0 when zeroing. I'm thinking maybe you're not waiting long enough when you zero. Or it could just be your monitor telling you how much it had to compensate during zeroing - in which case you would use the CVP reading as it is displayed. Pts don't have to be flat when zeroing or taking a reading - the transducer just needs to be level.
  7. by   zambezi
    Quote from pricklypear
    I'm trying to remember if our monitors ever say "offset". I always leave the transducer open until the # reads 0 when zeroing. I'm thinking maybe you're not waiting long enough when you zero. Or it could just be your monitor telling you how much it had to compensate during zeroing - in which case you would use the CVP reading as it is displayed. Pts don't have to be flat when zeroing or taking a reading - the transducer just needs to be level.
    This is what I was thinking...
  8. by   sazza
    Thanks for clearing that up. I think it would be less confusing if the monitor did not display the offset value in the first place.
    With regards to the CVP values. Can you have a minus value? If so how low can it go (in mmHg and cmH20)?
  9. by   dawngloves
    I have seen a - reading, but only if I'm zeroing the transducer or there is air in the line. So in therory I guess it could.Not sure what that would mean if it was real. Couldn't be good!
  10. by   papawjohn
    Hey Y'all

    I've never worked with a monitor that displayed "offset" so didn't join in but have thought a bit about the peculiarity of "negative" numbers on the CVP monitor. Here's my thoughts....

    If the number was really 'negative', it would mean the Right Heart is actually 'sucking' the venous return in from the Superior Vena Cava. Which I think we agree it doesn't. So how does a carefully zeroed and leveled monitor show a minus pressure? I mean--my monitors sometimes did and I wondered if I was nuts!!

    Remember that the CVP reflects intra-thoracic pressure. We know that adding PEEP to the Vent will increase pressures in the whole chest, ok? So imagine a non-vented Pt with some shortness of breath, working pretty hard to breathe--think of someone you'd check for 'retractions'--and check his CVP. You'd expect negative intrathoracic pressures. (I would anyhow.)
    So negative numbers are possiably accurate WHILE HE'S INHALING. Then he exhales and you get positive numbers (a sort of respiratory equivalent to systolic/diastolic pressures.)

    Now remember that the CVP number is a MEAN pressure. Presumably, there is a systolic and a diastolic central venous pressure. So its possible there's decent venous return over the systolic part of the cardiac cycle but negative pressure over the diastolic and inhalation parts of the cardiac and respiratory cycles.

    So I quit thinking I was NUTS when I was sure that the CVP monitor was well set up and STILL giving negative numbers.

    Hope that helps
    Papaw John
  11. by   pricklypear
    I paid a little more attention to my monitors the other night, and they do say "transducer offset ___mmhg" (or whatever measurement, can't think of it right now!) after zeroing. I think it is just a reflection of how much the transducer had to compensate for air pressure during the zeroing process.
  12. by   azrn22
    The way I understand it...if you have a negative CVP reading and your transducer is in correct position and has been zeroed, your Pt is volume depleted. Can someone else confirm this.
  13. by   papawjohn
    Hey AZM22

    Gosh yes!!!! CVP numbers are low numbers (by memory: 0-6?) but never negative if the Pt has adequate blood volume in his Superior Vena Cava. Blood is a heavy, thick fluid and will have pressure against the walls of the SVC if volume is adequate.

    I really meant that in the daily business of taking care of critically ill Pt's in ICUs with CVP lines--it is reasonable that sometimes the monitors will show a negative number. That if they did the monitors were NOT screwed up and the nurses were NOT crazy.

    But--you're correct--that CVP should not, really cannot--be negative pressure. If the CVP were REALLY negative, the blood would flow backwards from the right atrium to the Superior Vena Cava. We all hope it doesn't.

    Papaw John
  14. by   SEOBowhntr
    John,
    I've seen a couple patients who had negative CVP's, but they were EXTREMELY DRY!!! And yes, their Right Heart did literally suck the blood. I was drawing blood on one and didn't clamp the tubing and when changing syringes, watched it literally suck the blood right back in. Nothing several liters of saline didn't cure. I've also had a Post CABG one night with a CVP reading of 23mmHg and a BP of 74/34, any ideas where he shut down his graft??? Severe RV Hypokinesis, required 4 days of a 1:1 Balloon pump, and then an additional 3 days of weaning, and he ACTUALLY made it out of the hospital!!! I couldn't believe he lived!!!

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