Arterial Lines & Mean Arterial Pressure

NU2ICU V3 logoARTERIAL LINE CARE , MONITORING AND MAP

 Arterial Catheter and the Pressure Transducer

OVERVIEW

  • an arterial catheter connected to a pressure transducer

 

arterial line setup

Image –  http://www.snehalmayekar.com/what-is-arterial-line/

INDICATIONS

  • blood pressure monitoring (systolic, diastolic, mean and pulse pressure)
  • arterial blood sampling

Specific indications

  • labile blood pressure
  • anticipation of haemodynamic instability
  • titration of vasoactive drugs
  • frequent blood sampling
  • morbid obesity (unable to fit an appropriately sized NIBP cuff)

Consists of;

  • arterial line
  • non-distensible rigid-walled, fluid filled tubing
  • pressure transducer and automatic pressure flushing system
  • pressure bag and automated slow infusion   (20dpm or 3mL/h) of pressurised normal saline
  • electronic transducer / amplifier with display

Sites for insertion

  • arm – radial / brachial
  • leg – femoral / dorsal

Other sites in emergencies only

 

QUESTIONwhy rigid tubing? 

 

Mechanism

  • fluctuations of vascular pressure cause a pulsation of the saline column
  • wave form built up by analysis of simple wave forms
  • wave forms differ slightly depending on where the cannula is inserted

Calibrating (‘zeroing’)

  • ensure the transducer pressure tubing and flush solution (0.9% Saline) are correctly assembled and free of air bubbles
  • place transducer at level of the right atrium
  • ‘off to patient, open to air (atmosphere)’
  • press ‘zero’ -> sets atmospheric pressure as zero reference point
  • whenever patient position is altered the transducer height should be altered

QUESTIONSo what is “zero”?

artline 1

Image – http://www.derangedphysiology.com/main/core-topics-intensive-care/haemodynamic-monitoring/Chapter%201.1.5/normal-arterial-line-waveforms

ACCURACY AND MEASUREMENT ERRORS

 Conditions that must be met to ensure accuracy

  • cannula properly placed within the lumen of an unobstructed artery
  • cannula not kinked or blocked
  • cannula connected by short, rigid, wide-bore tubing to the transducer
  • no air bubbles in tubing or transducer
  • transducer is leveled and zeroed to desired point (ie. right atrium / phebostatic axis)
  • monitor calibrated accurately

Common sources of error

  • bubbles in catheter-transducer system
  • clotting in arterial catheter
  • elastic walls causes increased damping so increases discrepancy with noninvasive readings
  • cannula won’t flush – kinked, clotted, tissued

 

QUESTIONwhy do bubbles make the system inaccurate?

Information other than blood pressure can be obtained

  • pulse rate and rhythm
  • effects of dysrhythmia on perfusion
  • ECG lead disconnection
  • continuous cardiac output using pulse contour analysis
  • specific wave form shapes can be diagnostic
    – e.g. slow rising = Aortic Stenosis, pulsus alternans = tamponade
  • pulse pressure variation (suggests fluid responsiveness if greater than 10mmHg)
  • steeper upstroke of pulse pressure = increased contractility
  • area under upstroke = SV
  • steep downstroke = low SVR

Advantages of using MAP rather than SBP/DBP

  • least dependent on measurement site or technique (whether invasive or not)
  • least altered by damping
  • determines tissue blood flow

 

COMPLICATIONS

    • pain
    • thrombosis and distal ischaemia
    • infection
    • increased diagnostic blood loss and anemia
    • retrograde air embolism
    • inadvertent drug/air injection
    • haematoma
    • retroperitoneal haematoma (femoral)
    • vessel damage may lead to stricture and prevent future AV fistula formation for haemodialysis
    • arterial dissection

 

Mean Arterial Pressure (MAP)

Mean Arterial Pressure or Organ Perfusion Pressure

Many methods of calculation

EASIEST is SBP – DBP + 1/3 of difference (difference is known as pulse pressure)

so if SBP 135 and DBP 60 the difference is 75 divided by 3 = 25 and then add back to DBP so 60 + 25 = MAP of 85mmHg

 

MAP 1Or

MAP 2

 

In ICU targeted MAP is usually 65 – 70mmHg or as ordered on the ICU chart

 

Arterial Line care

Greatest risk to patient is system opening or dislodgement – EXSANGUINATION.  These patients can loose deadly amounts of blood quickly and quietly.

Arterial lines are always sutured in, well dressed and kept clearly on view at all times – not covered by blankets etc.  Check every patient coming in with an arterial line that it is sutured, if not please report it.

The insertion site should be observed closely every shift for signs of leakage, inflammation or infection.  ALL INVASIVE LINES MUST BE CLEARLY LABELLED.

Arterial blood sampling is always an aseptic technique.  Aspiration sites are cleaned with alcohol wipes pre and post use.

NURSING CARE

      • pressure bag (kept) inflated to 300mmHg
      • 0.9% saline present in flush bag changed @ 0800 hrs 24hrls
      • transducer at phlebostatic axis – see question
      • lines changed / redressed 7/7 & PRN

To decrease discarded blood amounts some ICU  use the SafeSet blood sampling system or similar

safeset picture

Image – Google image search 2/04/2016

QUESTION – where should artlines be labeled?

The Phlebostatic axis

Referencing and zeroing the haemodynamic monitoring system in a supine patient.   The phlebostatic axis is determined by drawing an imaginary vertical line from the fourth intercostal space at the sternal border to the right side of the chest (A)

A secondary imaginary line is drawn horizontally at the level of the midpoint between the anterior and posterior surfaces of the chest (B)

The phlebostatic axis is located at the intersection of points A and B
Phleb 1

and no matter whats the elevation of the bed head the axis remains constant

Phleb 2

Image – https://medical-dictionary.thefreedictionary.com/phlebostatic+axis

REMOVAL OF LINES

Arterial

      • refer to current Procedural Guidelines
      • medical order for removal
      • PPE
      • educate patient as indicated
      • gauze swab packet
      • dressing pack
      • 10mls 0.9% saline
      • chlorhexidine based cleaning agent
      • suture cutter
      • length of dressing tape ie leukplast
      • rubbish bin

Be prepared after removal to apply moderate pressure uninterrupted for around 5 minutes ( sometimes much more!)  When haemostasis achieved apply gauze swab folded into quarters and secure with tape

Observe effected limb at least twice 30 minutes apart post removal for ischaemia – reeducate patient, if able, to facilitate ongoing self-observation re bleeding / pain / tingling / numbness and dressing removal timing

If transferred to ward within 2 hours inform ward staff of risk of haemorrhage

 

Answer to why bubbles are bad? – air is compressable and may lead to incorrect readings and air may be flushed into artery leading to ischaemia / embolus
Answer to why rigid tubing? – because a more compliant tubing would absorb some or all of the pulsations and not transfer them to the transducer for accurate pressure readings
Answer to what is zero? – this allows a common reference point for calibration as room atmospheric pressure should be around 760mmHg at sea level – this becomes ‘zero’
Answer to where should arterial lines be labeled? – wherever an “access to blood” port occurs to decrease accidental injection into artery and near pressure bag to differentiate ig using pressurised CVC monitoring setup

 

REFERENCES

Oh’s Intensive Care Manual 7th Edition 2014 by Andrew Bersten, Neil Soni

More reading

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NU2ICU CVC

NU2ICU SHOCK