Anaesthesia 2014, 69, 387–398
Correspondence Re-inventing the piston pump Smart, Primrose, Peters and Speirits [1] are to be commended on a useful invention that may, as they state, facilitate the easy rapid infusion of fluids in resource-poor settings. However, there is nothing new under the sun. O’Callaghan and Singh described an near-identical creation in Anaesthesia in 2009 [2], also involving the combination of one-way valves and a three-way tap to speed the administration of fluids compared with a three-way tap alone. Dr Smart and his colleagues have taken their experiment in a different direction by comparing their device to a pressure infuser, rather than to the more conventional rotating three-way tap system as O’Callaghan and Singh did. It is nonetheless intriguing to note the independent invention of the same device by separate two groups of clinicians several years apart. A. Maddock Forth Valley Royal Hospital Larbert, UK Email: [email protected]
L. Li Ninewells Hospital Dundee, UK No external funding and no conflicts of interest declared. Previously posted on the Anaesthesia correspondence website: www.anaesthesia correspondence.com.
References 1. Smart CM, Primrose CW, Peters AL, Speirits EJ. The properties of an improvised piston pump for the rapid delivery of intravenous fluids. Anaesthesia 2014; 69: 111–7. 2. O’Callaghan E, Singh S. Use of R-Lock one-way valve for rapid fluid administration using a three-way tap. Anaesthesia 2009; 64: 811–2. doi:10.1111/anae.12646
Is flow really laminar in a piston pump infusion? Smart and colleagues [1] should be congratulated for describing a simple and effective way of delivering resuscitation fluids to patients at a fast rate. However, they assert that their analyses demonstrate laminar flow during their experiments, which may not be the case. There are theoretical reasons why laminar flow might not exist in
this model. For example, the fluid may not behave in a Newtonian manner, the tube may not be a perfect cylinder of constant, circular cross section and even if it was, it may not have been at least 20 times the diameter of the tube, only after which fully laminar flow develops (i.e. within only the distal half of the 16-G cannula in the study). Furthermore, empirical studies in similar models have shown that fully laminar flow doesn’t exist at flow rates much lower than those used by Smart et al. [2, 3]. Calculations using the data presented suggest that laminar flow wasn’t present. The mean pressure in the system shown in Smart et al.’s Fig. 4 was around 250 mmHg. This is approximately 33 kPa. The external diameter of the cannula used was 1.8 mm. The internal diameter is not given [4], but approximates to 1.35 mm [5]. Applying Poiseuille’s law with these values gives a flow of about 390 ml.s 1, more than ten times the actual measured flow of 25 ml.s 1 or 6.25 ml.s 1, depending on whether the caption of Fig. 4 is correct that 2000 ml flowed in 80 s or the text is correct that 500 ml
A response to a previously published article or letter must be submitted via the dedicated correspondence website at www.anaesthesiacorrespondence.com, following the guidance there and using the online form (not uploaded as a Word attachment). Please note that a selection of this correspondence will be reproduced (possibly in modified form) in the Journal. Correspondence on new topics should be submitted as an email attachment to [email protected]. Copy should be prepared in the usual style of the Correspondence section. Authors must follow the Guidance for Authors at wileyonlinelibrary.com/journal/anae, including completion and submission of an Author Declaration Form. © 2014 The Association of Anaesthetists of Great Britain and Ireland
387
Anaesthesia 2014, 69, 387–398
flowed in 80 s. These flows also give a Reynolds number of about either 2000 or 8000, suggesting that laminar flow was unlikely or very unlikely. The fact that a rectangular hyperbola emerged is used as evidence that laminar flow was present, but this is merely an artefact of the method chosen. The reciprocal of any straight line through the origin is a rectangular hyperbola, and only a straight line through the origin can result from linear regression with a fixed, zero intercept. There was no justification for the choice of a fixed, zero intercept: there may be a zeroflow (positive) pressure, for example. No evidence, such as a residual plot or correlation coefficient, is given that the regression line is actually a good fit for the data, or that some other curve doesn’t fit better. The authors have described a clinical device that could be very useful. I am likely to use their idea when the need arises. However, it is unlikely that flow was laminar in their experiment, nor is it in clinical situations. Indeed, if laminar flow did exist in clinical intravenous infusions their device would be redundant. D. McPherson University Hospital Southampton, Southampton, UK Email: [email protected] No external funding and no competing interests declared. Previously posted on the Anaesthesia correspondence website: www.anaesthesia correspondence.com.
References 1. Smart CM, Primrose CW, Peters AL, Speirits EJ. The properties of an improvised piston pump for the rapid delivery 388
Correspondence
2.
3.
4.
5.
of intravenous fluids. Anaesthesia 2014; 69: 111–7. McPherson D, Adekanye O, Wilkes AR, Hall JE. Fluid flow through intravenous cannulae in a clinical model. Anesthesia and Analgesia 2009; 108: 1198–202. Philip BK, Philip JH. Characterization of flow in intravenous infusion systems. IEEE Transactions on Bio-Medical Engineering 1983; 30: 702–7. Becton, Dickinson and Company. BD VenflonTM Pro Safety Shielded IV Catheter. http://www.bd.com/europe/saf ety/en/products/infusion/bdv_prosafe ty.asp (accessed 14/2/2014). Terumo Europe N.V. Versatus Intravenous Catheter Product Range. http://www. terumo-europe.com/hospital/products/ catheter_polyurethane.php (acc-essed 14/2/2014). doi:10.1111/anae.12653
A reply The piston pump assembly described by O’Callaghan and Singh [1] is indeed exactly the same as ours [2]. We were unaware of their earlier description of this technique and have not accredited them with its prior publication. We would like to apologise to them for this oversight. The concept of the piston assembly is clearly very simple, replicating the design of pumps that have existed since the 3rd century BC. We are disappointed therefore, but not surprised, that the technique has previously been described, but had found to our knowledge no evidence of its adoption a priori, certainly in the West of Scotland. We believe our description of the device’s self-refilling properties in concert with a blood warmer circuit are hitherto unpublished, and will lend to the ergonomic appeal of the device.
We are grateful to Dr McPherson for his very interesting analysis, and would agree with several points he has made. To clarify, we would like to point out that first, our observations about the nature of flow through the cannula are based on Experiment 3 in our paper [2] (Fig. 5), and second, each of the four individual lines within each group in Fig. 4 represents a single 500-ml bag, not 2000-ml. We would agree with McPherson that a Reynolds number of 2000 will be exceeded within the cannula, though we arrive at a different value. Assuming a density of 1.0046 kg.m 3, and a viscosity of 0.001002 Pa.s, we calculate that the Reynolds number will remain less than 2000 for a 1.35-mm tube, unless the flow exceeds 1 126 ml.min (which it clearly does here). The analysis of the flow mechanics was post hoc, the primary intention being calculation of upstream peak pressure. The experimental setup was thus designed to interfere minimally with the normal function of the device. For this reason, we opted to measure pressure at the already engineered ‘Y’ in the extension, rather than introduce flow restriction with a measuring port above the cannula [3]. The ‘Y’, being > 80 cm upstream of the cannula in this setup, alters the calculated downstream resistance from 370 Pa.s.ml 1 for the cannula alone, to anything up to 2400 Pa.s.ml 1, if the internal diameter of the tubing is taken as 2 mm (manufacturer’s specifications were not available at short notice). This may explain the laminar appearance of the pressure
© 2014 The Association of Anaesthetists of Great Britain and Ireland
Correspondence Re-inventing the piston pump Smart, Primrose, Peters and Speirits [1] are to be commended on a useful invention that may, as they state, facilitate the easy rapid infusion of fluids in resource-poor settings. However, there is nothing new under the sun. O’Callaghan and Singh described an near-identical creation in Anaesthesia in 2009 [2], also involving the combination of one-way valves and a three-way tap to speed the administration of fluids compared with a three-way tap alone. Dr Smart and his colleagues have taken their experiment in a different direction by comparing their device to a pressure infuser, rather than to the more conventional rotating three-way tap system as O’Callaghan and Singh did. It is nonetheless intriguing to note the independent invention of the same device by separate two groups of clinicians several years apart. A. Maddock Forth Valley Royal Hospital Larbert, UK Email: [email protected]
L. Li Ninewells Hospital Dundee, UK No external funding and no conflicts of interest declared. Previously posted on the Anaesthesia correspondence website: www.anaesthesia correspondence.com.
References 1. Smart CM, Primrose CW, Peters AL, Speirits EJ. The properties of an improvised piston pump for the rapid delivery of intravenous fluids. Anaesthesia 2014; 69: 111–7. 2. O’Callaghan E, Singh S. Use of R-Lock one-way valve for rapid fluid administration using a three-way tap. Anaesthesia 2009; 64: 811–2. doi:10.1111/anae.12646
Is flow really laminar in a piston pump infusion? Smart and colleagues [1] should be congratulated for describing a simple and effective way of delivering resuscitation fluids to patients at a fast rate. However, they assert that their analyses demonstrate laminar flow during their experiments, which may not be the case. There are theoretical reasons why laminar flow might not exist in
this model. For example, the fluid may not behave in a Newtonian manner, the tube may not be a perfect cylinder of constant, circular cross section and even if it was, it may not have been at least 20 times the diameter of the tube, only after which fully laminar flow develops (i.e. within only the distal half of the 16-G cannula in the study). Furthermore, empirical studies in similar models have shown that fully laminar flow doesn’t exist at flow rates much lower than those used by Smart et al. [2, 3]. Calculations using the data presented suggest that laminar flow wasn’t present. The mean pressure in the system shown in Smart et al.’s Fig. 4 was around 250 mmHg. This is approximately 33 kPa. The external diameter of the cannula used was 1.8 mm. The internal diameter is not given [4], but approximates to 1.35 mm [5]. Applying Poiseuille’s law with these values gives a flow of about 390 ml.s 1, more than ten times the actual measured flow of 25 ml.s 1 or 6.25 ml.s 1, depending on whether the caption of Fig. 4 is correct that 2000 ml flowed in 80 s or the text is correct that 500 ml
A response to a previously published article or letter must be submitted via the dedicated correspondence website at www.anaesthesiacorrespondence.com, following the guidance there and using the online form (not uploaded as a Word attachment). Please note that a selection of this correspondence will be reproduced (possibly in modified form) in the Journal. Correspondence on new topics should be submitted as an email attachment to [email protected]. Copy should be prepared in the usual style of the Correspondence section. Authors must follow the Guidance for Authors at wileyonlinelibrary.com/journal/anae, including completion and submission of an Author Declaration Form. © 2014 The Association of Anaesthetists of Great Britain and Ireland
387
Anaesthesia 2014, 69, 387–398
flowed in 80 s. These flows also give a Reynolds number of about either 2000 or 8000, suggesting that laminar flow was unlikely or very unlikely. The fact that a rectangular hyperbola emerged is used as evidence that laminar flow was present, but this is merely an artefact of the method chosen. The reciprocal of any straight line through the origin is a rectangular hyperbola, and only a straight line through the origin can result from linear regression with a fixed, zero intercept. There was no justification for the choice of a fixed, zero intercept: there may be a zeroflow (positive) pressure, for example. No evidence, such as a residual plot or correlation coefficient, is given that the regression line is actually a good fit for the data, or that some other curve doesn’t fit better. The authors have described a clinical device that could be very useful. I am likely to use their idea when the need arises. However, it is unlikely that flow was laminar in their experiment, nor is it in clinical situations. Indeed, if laminar flow did exist in clinical intravenous infusions their device would be redundant. D. McPherson University Hospital Southampton, Southampton, UK Email: [email protected] No external funding and no competing interests declared. Previously posted on the Anaesthesia correspondence website: www.anaesthesia correspondence.com.
References 1. Smart CM, Primrose CW, Peters AL, Speirits EJ. The properties of an improvised piston pump for the rapid delivery 388
Correspondence
2.
3.
4.
5.
of intravenous fluids. Anaesthesia 2014; 69: 111–7. McPherson D, Adekanye O, Wilkes AR, Hall JE. Fluid flow through intravenous cannulae in a clinical model. Anesthesia and Analgesia 2009; 108: 1198–202. Philip BK, Philip JH. Characterization of flow in intravenous infusion systems. IEEE Transactions on Bio-Medical Engineering 1983; 30: 702–7. Becton, Dickinson and Company. BD VenflonTM Pro Safety Shielded IV Catheter. http://www.bd.com/europe/saf ety/en/products/infusion/bdv_prosafe ty.asp (accessed 14/2/2014). Terumo Europe N.V. Versatus Intravenous Catheter Product Range. http://www. terumo-europe.com/hospital/products/ catheter_polyurethane.php (acc-essed 14/2/2014). doi:10.1111/anae.12653
A reply The piston pump assembly described by O’Callaghan and Singh [1] is indeed exactly the same as ours [2]. We were unaware of their earlier description of this technique and have not accredited them with its prior publication. We would like to apologise to them for this oversight. The concept of the piston assembly is clearly very simple, replicating the design of pumps that have existed since the 3rd century BC. We are disappointed therefore, but not surprised, that the technique has previously been described, but had found to our knowledge no evidence of its adoption a priori, certainly in the West of Scotland. We believe our description of the device’s self-refilling properties in concert with a blood warmer circuit are hitherto unpublished, and will lend to the ergonomic appeal of the device.
We are grateful to Dr McPherson for his very interesting analysis, and would agree with several points he has made. To clarify, we would like to point out that first, our observations about the nature of flow through the cannula are based on Experiment 3 in our paper [2] (Fig. 5), and second, each of the four individual lines within each group in Fig. 4 represents a single 500-ml bag, not 2000-ml. We would agree with McPherson that a Reynolds number of 2000 will be exceeded within the cannula, though we arrive at a different value. Assuming a density of 1.0046 kg.m 3, and a viscosity of 0.001002 Pa.s, we calculate that the Reynolds number will remain less than 2000 for a 1.35-mm tube, unless the flow exceeds 1 126 ml.min (which it clearly does here). The analysis of the flow mechanics was post hoc, the primary intention being calculation of upstream peak pressure. The experimental setup was thus designed to interfere minimally with the normal function of the device. For this reason, we opted to measure pressure at the already engineered ‘Y’ in the extension, rather than introduce flow restriction with a measuring port above the cannula [3]. The ‘Y’, being > 80 cm upstream of the cannula in this setup, alters the calculated downstream resistance from 370 Pa.s.ml 1 for the cannula alone, to anything up to 2400 Pa.s.ml 1, if the internal diameter of the tubing is taken as 2 mm (manufacturer’s specifications were not available at short notice). This may explain the laminar appearance of the pressure
© 2014 The Association of Anaesthetists of Great Britain and Ireland
