Dear Colleagues
In mailing 03, I'd like to provide a little more information about the relationship between pressure, mechanical stress, shear and hemolysis.
Firstly, some comments from Jean-Yves of Belgium.
"Although measurement of arterial and venous pressures on a machine do NOT ALWAYS detect too high and/or too low pressures within a specific area in the circuit a sudden change in pressures of 20% should always alarm the operator to be cautious. A common problem is that when a pressure alarm occurs a new window limit is allowed by each new start up of the machine loosing track of what is going on. After all it is the difference between the arterial negative pressure and the venous pressure that will damage the RBC-membrane causing hemolysis. If a RBC is stressed one moment by minus 200 mmHg and further on in the circuit by plus 200 mmHg a potential of 400 mmHg is acting constantly on the RBC membrane. In the case of single needle mode this is even more important. Than you create not only an alternating stress by the switching of arterial and venous pump but also a much higher min/max pressure for a same mean QB! One should not forget that if they dialyse in single-needle mod!
us the chance to induce hemolysis is much higher than in double-needle modus because of the needed double pump speed (a mean QB of 300 ml/min results in peak flows of 600 ml/min by both the arterial and venous pump) increasing the risk towards shear stress ... once more a reason to choose a flow-adapted needle- or catheter size."
I think these are important points, especially given Susan Hanson's comments about high blood flow dialysis in the last mailing. I was also sent these insightful comments from Hans-Dietrich Polaschegg (currently on holiday somewhere)
"...A limited comment about mechanical hemolysis. If anybody is interested in a review on hemolysis please look up my paper in
'Replacement of Renal Function by Dialysis' 5th edition.
- Mechanical hemolysis is caused by shear, not by pressure. The pressure indicated by dialysis machines is a combination of static (height differences) and dynamic pressure. Shear is proportional to dynamic pressure but not identical. The pressure drop in a long catheter can be substantial but may not cause hemolysis while the same pressure drop in a needle may cause hemolysis.
- Hemolysis means that erythrocytes are destroyed and free hemoglobin can be detected in the blood stream.
- It is however possible (and has been documented) that erythrocytes get damaged but not hemolysed (sub-lethal damage) and are taken out of circulation by the spleen. The result is a drop of hematocrit a few hours after the event.
- Surface roughness has an influence. New papers about these effects were published within the last few years because of the importance of surface finish for rotary blood pumps (in heart-assist devices). Dialysis needles are siliconised. Without this layer the risk of clotting and hemolysis would be much higher.
- A common cause of hemolysis in dialysis is an obstruction between the blood pump and the dialyser (kink). It is, however, unclear if the resulting hemolysis is produced in the kink or in the blood pump. Blood pump rollers are spring loaded and give way when the pressure increases above a certain threshold. This effect would cause hemolysis."
Thanks to both Jean-Yves and Hans for these comments. If anyone can provide more details I'm sure it would of interest to JC members. I'd like to appeal again for information on how incidents like those described in the paper would be reported and acted upon in your country.
As well as this, I think it would be useful to hear from anyone with more information about hemolysis and blood volume (monitors). Can these really be used as a predictor of low level hemolysis? If you have any ideas, experience or thoughts on the subject please let me know. I'm sure there must be many nurses with clinical experience of hemolysis during dialysis. It would be great if you could share your experiences with those less clinical (like myself) so we can better understand what the signs, symptoms and actions might be in such situations.
I look forward to your comments.
Best regards
Gareth Murcutt
EDTNA/ERCA Journal Club Manager
JC Housekeeping:
The current paper (May 07) can be seen at nephrologynursing.net/cur...efault.htm
In mailing 03, I'd like to provide a little more information about the relationship between pressure, mechanical stress, shear and hemolysis.
Firstly, some comments from Jean-Yves of Belgium.
"Although measurement of arterial and venous pressures on a machine do NOT ALWAYS detect too high and/or too low pressures within a specific area in the circuit a sudden change in pressures of 20% should always alarm the operator to be cautious. A common problem is that when a pressure alarm occurs a new window limit is allowed by each new start up of the machine loosing track of what is going on. After all it is the difference between the arterial negative pressure and the venous pressure that will damage the RBC-membrane causing hemolysis. If a RBC is stressed one moment by minus 200 mmHg and further on in the circuit by plus 200 mmHg a potential of 400 mmHg is acting constantly on the RBC membrane. In the case of single needle mode this is even more important. Than you create not only an alternating stress by the switching of arterial and venous pump but also a much higher min/max pressure for a same mean QB! One should not forget that if they dialyse in single-needle mod!
us the chance to induce hemolysis is much higher than in double-needle modus because of the needed double pump speed (a mean QB of 300 ml/min results in peak flows of 600 ml/min by both the arterial and venous pump) increasing the risk towards shear stress ... once more a reason to choose a flow-adapted needle- or catheter size."
I think these are important points, especially given Susan Hanson's comments about high blood flow dialysis in the last mailing. I was also sent these insightful comments from Hans-Dietrich Polaschegg (currently on holiday somewhere)
"...A limited comment about mechanical hemolysis. If anybody is interested in a review on hemolysis please look up my paper in
'Replacement of Renal Function by Dialysis' 5th edition.
- Mechanical hemolysis is caused by shear, not by pressure. The pressure indicated by dialysis machines is a combination of static (height differences) and dynamic pressure. Shear is proportional to dynamic pressure but not identical. The pressure drop in a long catheter can be substantial but may not cause hemolysis while the same pressure drop in a needle may cause hemolysis.
- Hemolysis means that erythrocytes are destroyed and free hemoglobin can be detected in the blood stream.
- It is however possible (and has been documented) that erythrocytes get damaged but not hemolysed (sub-lethal damage) and are taken out of circulation by the spleen. The result is a drop of hematocrit a few hours after the event.
- Surface roughness has an influence. New papers about these effects were published within the last few years because of the importance of surface finish for rotary blood pumps (in heart-assist devices). Dialysis needles are siliconised. Without this layer the risk of clotting and hemolysis would be much higher.
- A common cause of hemolysis in dialysis is an obstruction between the blood pump and the dialyser (kink). It is, however, unclear if the resulting hemolysis is produced in the kink or in the blood pump. Blood pump rollers are spring loaded and give way when the pressure increases above a certain threshold. This effect would cause hemolysis."
Thanks to both Jean-Yves and Hans for these comments. If anyone can provide more details I'm sure it would of interest to JC members. I'd like to appeal again for information on how incidents like those described in the paper would be reported and acted upon in your country.
As well as this, I think it would be useful to hear from anyone with more information about hemolysis and blood volume (monitors). Can these really be used as a predictor of low level hemolysis? If you have any ideas, experience or thoughts on the subject please let me know. I'm sure there must be many nurses with clinical experience of hemolysis during dialysis. It would be great if you could share your experiences with those less clinical (like myself) so we can better understand what the signs, symptoms and actions might be in such situations.
I look forward to your comments.
Best regards
Gareth Murcutt
EDTNA/ERCA Journal Club Manager
JC Housekeeping:
The current paper (May 07) can be seen at nephrologynursing.net/cur...efault.htm
