A few things to consider it that respect Geoff - 1) the operator is on hand to ensure that the suction unit vacuum is not damaging to the patient, 2) newer units such as the LSU regulate vacuum to a preset level, using a vacuum regulator, which can be preset by the operator and, 3) most suction units indicate the vacuum in the suction tubing/container on a gauge. There's also the consideration that the risk of potential damage due to excessive vacuum is possibly outweighed by the benefits of suctioning (considering emergency situations in particular).
In practice Yankauer suction-catheter handpieces are available with a finger-controlled leak, bypassing the path of the aspirate with air, to allow fine control of the vacuum applied. This effectively limits peak applied vacuum (given a particular suction displacement, resistance to flow of aspirate/air in the suction tubing/filter, viscosity of aspirate and peak vacuum attainable) to somewhat less than the peak vacuum that can be attained at the vacuum source or that vacuum preset by the regulator.
Only under no-flow conditions, without a vacuum regulator fitted, or a vacuuum regulator that is preset by the operator to the maximum setting, possibly inappropriately, should the peak vacuum attainable by any suction unit be applied to the patient, in theory. This assumes that the blockage/restriction, leading to excessive vacuum applied to the patient, is at the suction-catheter tip itself, i.e. is in contact with patient tissue, since any blockage within the suction unit/tubing will result in zero or much reduced suction at the suction-catheter tip.
To be effective suction units need to have more than adequate vacuum and displacement (flow rates) to overcome the resistance in the suction tubing/filter/vacuum source, the viscosity of aspirate and, initially, remove the volume of air in the collection bottle as fast as possible, to enable removal of aspirate from the patient quickly. Manufacturers provide the features to allow this to be done safely but it's up to the operator to be responsible and to make sure they're using these appropriately in my opinion.
