as shown in fig1 , 11 two - mm sized droplets may be evenly surgically grafted inside a 42 droplet sized recipient tissue . the recipient site is depicted as being distensible enough such that the interstitial tissue pressure does not increase significantly at the 53 droplets size and the meticulously inserted grafts may be expected to survive completely . however , squeezing another 10 droplets would likely result in 2 problems : 1 — confluence ; as droplets coalesce to become larger than 2 mm and end up with central necrosis ; and 2 — as the recipient tissue can no longer stretch and distend to accommodate the size increase , the interstitial pressure will rise to choke capillary circulation and prevent the neovascularization required for the graft to survive and cause additional necrosis . fig1 thus illustrates that measurement of interstitial pressure can be used to guide a surgeon during grafting to achieve reliably desirable results . as illustrated in fig2 , compliant tissue can accept more graft before a critical pressure limit is reached : beyond a certain % volume change , compliance drops precipitously , the curve becomes nearly vertical and minimal additional grafting causes the pressure to dramatically increase . tissues have different volume to pressure compliance curves , with subcutaneous being the most compliant , muscle intermediate , and scarred radiated tissue being the worst . by pre - expanding of the recipient , such as through use of the brava ® external expansion device 6 , 7 , the tissue compliance curve shifts to the right to thereby allow for greater amounts of graft . this device and method is the subject of previous patent filings by the inventor . as illustrated in fig3 , there is a dramatic loss of fat graft survival when the last drop causes the interstitial pressure to exceed its maximally tolerable levels and graft survival to “ fall off the cliff ” ( vertical line ). the larger and the more compliant the recipient the further to the right this vertical limit has been found to shift . the use of the invention may be more fully understood by referring now to its use in a grafting procedure as depicted in fig4 . as depicted therein , a needle is inserted in the tissue to be grafted and the needle is connected through watertight sterile tubing to a pressure transducer . the transducer measures the equilibrium fluid pressure inside the tubing into which a minimal amount of physiologic fluid is slowly injected through a slow pumping mechanism . the transducer is connected to an electronic monitor that can display the measured pressure . to prevent hydrostatic pressure from affecting the reading , the pressure transducer is preferably level with the needle insertion site in the tissue . to that effect , a laser pointer with a leveling bubble is incorporated in the measuring device . as an alternative to the leveling device , some electronic monitors can be zeroed regardless of the relative position of the needle insertion to the pressure transducer . after flushing the line to eliminate air bubbles and ensure a continuous water column , the “ zero ” pressure is set with the needle at the level of the tissue insertion point , with its lumen still exposed to the atmosphere . the needle is then inserted in the tissue and the equilibrium pressure read on the monitor display . multiple samplings are often required as the pressure is not always homogenously distributed between the recipient site tissue compartments . although there are multiple such devices on the market , an example of the pressure transducer that can be used is the edwards ® device commonly used in the intensive care units to monitor arterial and central venous pressures . more particularly , as shown in fig4 , the measurement device is preferably “ zeroed ” with needle / catheter ( 1 ) just outside the skin at the level of the insertion . the needle is inserted in 4 - 6 random locations to sample the pressure in the various regions as the fibrous nature of the recipient does not necessarily allow for equilibration of the pressure in all areas . the inventor considers it important to sample and to make sure there is no area of excess pressure which would be undesirable . once the catheter / needle ( 1 ) is inserted , the interstitial tissue fluid pressure is transmitted through the tubing ( 2 ) to the pressure transducer ( 3 ) and displayed on the electronic display monitor ( 6 ) connected to it via cable ( 5 ). unless the monitor can electronically adjust to the “ zero ” ( 7 ) the pressure transducer is preferably brought to be substantially at the level of the tissue to be monitored with , for example , the help of a leveling device and a laser pointer ( 4 ). measuring compartmental pressure is known in the surgical art . a number of methods and devices have been advocated for that purpose . however , most if not all the clinically available devices are aimed at diagnosing a dreadful pathological condition known as compartment syndrome . this is a clinical situation where a feed - forward loop mechanism of tissue edema leads to increase interstitial tissue pressure within a confined tissue compartment leading to decrease tissue perfusion and ischemic injury which in turn leads to further edema , further pressure rise and further circulatory compromise and injury . the capillary lumen collapse and circulation come to a grinding halt when extrinsic interstitial pressure on the capillary wall is higher than the blood pressure inside the capillary . compartment syndrome typically happens in muscles with a well - defined , tight enveloping fibrous fascia sheath . as a result of strenuous exercise , vascular compromise , or reperfusion injury after ischemia , the muscles swell in the tight unyielding compartment and the interstitial tissue pressure rises to further compromise the circulation and worsen the situation until total necrosis ensues . this is often an emergency situation requiring an emergency operation called fasciotomy where the enveloping fascia is surgically incised to release the tightness and drop the interstitial pressures to the normal range while the tissue relieved from the non - compliant fascia freely swells . it is generally accepted that an emergency surgical fasciotomy should be performed when the interstitial tissue pressure comes close to 30 mmhg or comes within 30 mmhg of the diastolic arterial blood pressure . the nascent neovasculature responsible for graft survival , however , is much more fragile and the inventor has recognized in his work that the critical pressure cut off point to stop grafting should be significantly lower . also pointing to a much lower cut off point is the fact that grafting is in itself an edema causing traumatic process which will subsequently lead to an additional increase in pressure . on the other hand , depending upon the osmolarity of the graft and its water content , a certain degree of fluid shifts is likely to also alter the interstitial pressure . it is recognized that depending upon the measurement technique used , the normal physiologic range of tissue interstitial pressure goes up to 4 - 6 mmhg . the inventor &# 39 ; s clinical data points to the fact the pressure after grafting should not exceed 7 - 12 mmhg , depending upon the water content of the graft . this invention advocates monitoring of the interstitial tissue pressure during the grafting process and stopping when the pressure reaches the dangerous critical upper limit of 7 - 12 , depending upon the water content of the graft ( more dilute grafts can tolerate a higher pressure as excess fluid resorbtion is bound to bring the pressure down .) a number of methods and devices have been advocated in the prior art to measure interstitial pressure . but in general terms these fall into the following categories or a combination thereof : 1 — those that measure the driving pressure required to inject very small amounts of additional fluid inside the tissue . they require the insertion of a hollow needle or cannula inside the tissue and have it connected by a closed fluid channel to a pressure transducer . the non - compressible fluid channel then transmits the pressure from the cannula aperture exposed to the interstitial space to the pressure transducer . of these , the stryker ® device is the most popular . unfortunately the commercially available version is limited by the fact it is designed for use in the clinic , not in the operating room . while the needle to be inserted in the tissue and the internal fluid channels are sterile , the monitor and the pressure transducers are not and therefore without modifications cannot be handled in the sterile surgical field of tissue grafting . as an alternative to the stryker device , commercially available sterile cannulas can be connected via long tubings from the sterile surgical to commercially available sensitive pressure transducers . 2 — those that use an inserted wick to collect the interstitial fluid and directly measure its pressure . the wick measurement is considered more cumbersome and while it is commonly used in research settings where it can also measure colloid oncotic pressure , it is not practical for clinical use . 3 — those that use external bladders or other means of measuring pressure - volume changes such as plethysmography . while these methods require no needle insertion and are less invasive , they are generally less precise . as a research tool , implanted chambers with compliant bladder walls have also been used . it is clear that there are many described methods for measuring interstitial tissue pressure . they vary in their practicality and their precision and also in the type of pressure they measure . some measure the purely mechanical pressure , while others take into account the colloid osmotic pressure . a person expert in the art , utilizing the teaching of this disclosure and invention , would clearly be able to substitute a measuring technique for another and as long as the method is standardized , still use that measurement method to determine that the grafting capacity of the recipient has been reached . while all of these can be modified to monitor the recipient interstitial tissue pressure rise during the grafting process , the inventor presently favors the first method as the compromise between precision , practicality and cumbersomeness . however , others may have different preferences depending on their experiences and familiarity with other devices . various changes may be made without departing from the invention , as would be apparent to those of skill in the art , including that the inserted needle can be sharp or blunt , have end holes or side holes and be of different bore sizes . as long as the measurement procedure is standardized these will not significantly affect the end point result . similarly , variation and modifications of the measurement apparatus will not affect the final outcome that accurate measurement of the pressure is crucial to the success of the technique .
a device comprising a kit and a method for using the kit are disclosed that warn a surgeon that the maximal grafting capacity of a recipient tissue has been reached . the increase in interstitial pressure is measured at the recipient site using any of several pressure sensor devices during the grafting process to alert the surgeon to stop grafting once a critical level has been reached , or a sudden increase in pressure is detected , indicative of a maximum graft size .